In this work, we present the results of a structural and geological investigation carried out along the 123-km-long Kur Fault, the frontal structure of the Kura Fold-and-Thrust Belt (Greater Caucasus, Azerbaijan). For the first time, field surveys and paleoseismological trenching revealed a fault plane that reaches the surface along this regional structure, exposing a clear tectonic contact where Lower Pleistocene deposits overthrust Holocene sediments. This observation is crucial, as it demonstrates that the frontal fault of the Greater Caucasus is a capable fault, despite the lack of strong historical earthquakes reported in the area.

In addition to the tectonic contact described above, the Lower Pleistocene deposits exposed in the trench are cut by numerous fault planes, allowing us to reconstruct a stress tensor indicating a purely compressive regime, characterised by reverse dip-slip motion and a horizontal σ₁ oriented NNE–SSW. The orientation of this σ₁ is parallel to both the GPS velocity vectors and the P-axes of available focal mechanisms, suggesting that a NNE–SSW compressional stress field has remained stable from the Pleistocene to the present day. This σ₁ direction is also orthogonal to the regional strike of the Kur Fault (WNW–ESE) and matches the orientation observed at the trench site.

In the same area of the trench site, we identified three distinct river terraces associated with the Kura River. The uppermost and oldest terrace is currently uplifted to 37 m above the modern river level and has been dated to 10 kyr using the OSL method; it displays a tilting of about 5°, consistent with the kinematics of the Kur Fault. The most recent and lowest terrace lies 4–6 m above the present river level, also indicating recent uplift and tilting of the palaeoterraces as a result of active tectonics along the Kur Fault.

Geological evidence from the trench site, combined with uplift data from the river terraces, indicate an average Holocene shortening rate that is greater than the value inferred from GPS measurements. Additionally, the exposed fault plane corresponds to a ~31-km-long segment of the Kur Fault which, based on empirical scaling relationships, is capable of generating an earthquake of approximately M 6.8.

The work was carried out entirely through field data collected during two dedicated campaigns within the framework of the NATO Project G5907 – Science for Peace and Security Programme, which focuses on geohazard assessment around the Shamkir Hydroelectric Power Station (https://shamkirproject.unimib.it/).

How to cite: Piccio, G., Pánek, T., Pasquarè Mariotto, F., Břežný, M., Bracchi, V. A., Dell'Era, E., Panzeri, L., Galli, A., Babayev, G., and Tibaldi, A.: A Surface-Breaking Capable Fault in the Greater Caucasus: New Evidence from the 123-km-Long Kur Fault, Azerbaijan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-441, https://doi.org/10.5194/egusphere-egu26-441, 2026.

EGU26-441

Download Close

The Philippine fault system is characterized by primarily sinistral fault segments and traverses the entire Philippine archipelago. On the populous Luzon Island, the northern segment of this active fault system poses significant seismic hazards, as evidenced by the 1990 M_W 7.7 Luzon earthquake in central Luzon and the 2022 M_W 7.0 Abra earthquake in northwestern Luzon. However, the precise location and characteristics of the fault traces along some of the fault’s segments still remain poorly understood, such as the Abra River fault system (ARFS) in the Abra Province. Therefore, this study aims to identify and characterize the active fault traces of the ARFS on the basis of tectonic geomorphic features related to strike-slip faulting using a 5-m resolution DEM, augmented by field investigations.

Based on geomorphic manifestations and results from our field investigations, we identified at least three major sinistral fault traces of the ARFS along the Abra River valley. Although our mapping results are generally consistent with the published map by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the new mapping provides better constraints and information for several fault segments that were previously uncertain. Along the fault traces, numerous offset channels, offset alluvial fans, and offset bedrock ridges indicate that the ARFS exhibits primarily left-lateral motion. During field investigation, we found two fault zone outcrops aligned with offset geomorphic features with vertical fault plane and horizontal slickensides, consistent with strike-slip faulting of the ARFS. Flexural scarps and pressure ridges that deform Quaternary fluvial sediments show that these ARFS traces are active. The predominantly sinistral motion of the ARFS is not consistent with the focal mechanism of the 2022 Abra earthquake, which is characterized by reverse motion on a gently dipping fault plane. This suggests the ARFS is not the seismogenic fault of the 2022 event, and the accumulated strain along this structure may have not yet been fully released within the time period of written history. As a result, the ARFS poses a great seismic hazard for the area, and it is necessary to further understand its earthquake behavior and paleoseismic characteristics.

How to cite: Chen, C.-C., Shyu, J. B. H., and Ramos, N. T.: Identification of active fault traces of the Abra River fault system, northwestern Luzon, Philippines, from tectonic geomorphic features, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7388, https://doi.org/10.5194/egusphere-egu26-7388, 2026.

EGU26-7388

Download Close

The southern Vienna Basin, a Miocene pull-apart structure formed along the sinistral Vienna Basin Transfer Fault System (VBTFS) extending from the Eastern Alps to the Western Carpathians, exhibits negative flower structures with strike-slip and branching normal faults. Miocene basin subsidence and sedimentation produced up to 5 km thick sedimentary sequences overlying the pre-Neogene basement. Quaternary and recent tectonic activity, documented by instrumentally recorded and historical seismic events as well as focal mechanisms of selected earthquakes, in addition to paleoseismological data showing evidence for prehistoric earthquakes of magnitudes up to ~ 6.8, confirms ongoing sinistral motion and normal faulting.

We present here a comprehensive overview of the seismotectonics of the Vienna Basin, integrating earthquake information, such as high-precession relocation of hypocenters, focal mechanisms and historical earthquake information together with fault information from industrial seismic campaigns, geological mapping and geomorphological studies.

The seismological characteristics are presented based on the Austrian Earthquake Catalog (AEC) of GeoSphere Austria. Suitable earthquakes that occurred after 2006 were relocated using two methods. For further analysis, the most accurate available locations were combined to obtain a complete picture of earthquake distribution. Five existing focal mechanism solutions of earthquakes were recalculated and further used to determine the recent stress field. The fault information is compiled into two datasets attributed with information on fault activity, kinematics, and displacement: surface fault traces and fault traces at the base of the Vienna Basin. The faults of both datasets are sorted into fault systems in order to correlate the fault information from both datasets.

This newly compiled seismotectonic dataset allows a systematic study comparing earthquake occurring in more than 5 km depth and faults, documented either at depths of 1-3 km by industrial seismic campaigns or at the surface by geological and geomorphological mapping in order to re-evaluate the most seismically active faults in the Vienna Basin. Despite the wealth of available information, uncertainties remain in the data, as well as additional ambiguities arising from the combination of geological and seismological data.

How to cite: Apoloner, M.-T., Hintersberger, E., Salcher, B., Decker, K., Klaus, T., and Weginger, S.: Re-evaluation of seismogenic faults in the southern Vienna Basin from seismogenic depth to the surface, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21328, https://doi.org/10.5194/egusphere-egu26-21328, 2026.

EGU26-21328

Download Close

We present a high-resolution geological reinterpretation of the central Coastal Range of Taiwan based on newly produced LiDAR-assisted geological maps integrated with targeted field verification. LiDAR-derived digital elevation models (DEMs) overcome limitations imposed by poor exposure and dense vegetation and allow systematic mapping of stratigraphic boundaries and fault geometries in this key segment of the Taiwan subduction–collision system. The new maps reveal several previously unrecognized structural features. The Tuluanshan volcanic sequence contains laterally continuous, thick shear zones expressed by aligned geomorphic lineaments and systematic topographic offsets. These shear zones demonstrate significant internal deformation of the volcanic rocks, indicating that the Tuluanshan Formation actively accommodated strain rather than behaving as a rigid volcanic block. Along the western margin of the central Coastal Range, normal faults are commonly observed and consistently occur adjacent to contractional structures. Their spatial association with a major west-verging fault suggests that extension postdated major thrusting and records post-thrust extensional deformation, potentially driven by gravitational collapse or internal reorganization of the Coastal Range wedge. LiDAR-based mapping also significantly refines the distribution of the Lichi Mélange. Mélange boundaries are sharply delineated, and exotic blocks within the Lichi Formation are systematically documented, providing new constraints on mélange formation and transport and underscoring its structural importance in the collision zone. In addition, several previously unrecognized north–south–trending thrust faults are identified, separating sedimentary basins from the Tuluanshan volcanic sequence and defining fundamental tectonic boundaries that segment deformation within the central Coastal Range. These results demonstrate the critical role of LiDAR-based geological mapping in resolving complex structural relationships and provide new constraints on deformation processes during arc–continent collision in Taiwan.

How to cite: Chan, Y.-C., Hsu, Y.-C., Chao, P.-L., Pai, T.-Y., Sun, C.-W., Chen, C.-T., and Hu, J.-C.: LiDAR-Based Geological Mapping of the Central Coastal Range, Taiwan: New Constraints on Fault Systems and Arc Deformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3241, https://doi.org/10.5194/egusphere-egu26-3241, 2026.

EGU26-3241

Download Close

Quantifying fault evolution in time and space is essential for characterising earthquake hazards and understanding landscape evolution. For a complete picture of Quaternary fault evolution, slip rates calculated from geomorphic strain markers bridge the gap between long-term rates from geological data and contemporary data from seismicity or geodesy. Here, we present a high-resolution record of fault slip rates from the Sierra de Aconquija (SdA), northwest Argentina, based on ¹⁰Be surface exposure dating of boulders and cm-scale topographic data derived from drone-based photogrammetry. Located at the broken foreland of the southern-central Andes, the SdA overlies a transition zone from dipping to flat-slab subduction and therefore provides an opportunity to investigate how complex slab interactions at depth manifest in upper-crustal fault slip. Coalesced alluvial fans have been deposited on the western flank of the SdA, which preserve at least seven aggraded depositional units up to ~300 ka and display scarps associated with east-dipping, range-bounding, reverse faults. We present 55 new cosmogenic ¹⁰Be surface exposure ages from boulders deposited on the fan surfaces. These ages extend an existing fan chronology of 43 ages to cover ~55 km along strike. To measure fault slip, we flew 54 drone surveys to collect photogrammetry data from which we generated 14, centimetre-scale, digital-elevation models using structure-from-motion techniques. Preliminary slip rates span 0.06–2.22 mm/yr. Our data indicate that a fault strand propagated outward from the range front around ~200 ka, which exhibits comparable average slip rates to a parallel strand at the range front. The slowest rates of ~0.06 mm/yr are from the end of this outbound strand and the fastest rates of 1.23–2.22 mm/yr are at the southern end of the Aconquija fault, where deformation is focused on a single range-front strand. Long-term slip rates decrease around a pronounced bend in the fault, suggesting rupture segmentation and ongoing fault linkage. Overall, late Quaternary deformation along the western SdA is evolving both outwards from the range front, and southwards along the range front. This pattern supports existing models of landscape evolution and drainage divide migration linked to Quaternary slip on predominantly east-dipping faults. Ongoing work aims to integrate these findings into a broader context of tectonic and landscape evolution in the Andean foreland.

How to cite: Hughes, A., Schildgen, T., D'Arcy, M., Crawford, H., Wittmann, H., and Brune, S.: Late Quaternary fault evolution at the Sierra de Aconquija, Argentina, characterized from 10Be and drone-based topographic data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7851, https://doi.org/10.5194/egusphere-egu26-7851, 2026.

EGU26-7851

Download Close

The Italian Apennines are among the Mediterranean areas with the highest seismic hazard. Geodetic data show that the belt is experiencing slow deformation rates (3-4 mm/yr, D’Agostino, 2014; Carafa et al., 2020), with a prevalent SW-NE extension. Tectonic activity is expressed by well-exposed normal-fault planes dissecting the carbonate ridges. To infer the activity of these faults, several investigations using morphotectonic, paleoseismological, geophysical, and field survey techniques have been applied, leading to a robust literature in which the active structures are characterized and parametrized. Also, in recent times strong earthquakes with extensional kinematics struck the belt (e.g., 2009 L’Aquila, Mw 6.1; 2016 Norcia, Mw 6.5).

This work focuses on the sector between the central and southern Apennines, the Abruzzo-Molise region boundary (AMB), bordered to NW and SE by well-known active normal fault systems with opposite dip (SW-dipping and NE-dipping, respectively). AMB is characterized by a seismic gap and a complex lithological arrangement composed of prevalent flysch-like and clayey-marls outcrops, whose thickness reaches 2.5 km, which hamper the recognition of active faults at the surface. In a recent study, taking advantage of morphotectonic and remote sensing analysis, Battistelli et al. (2025) highlighted the presence of an organized strip of slope instabilities that could represent the surface expression of unknown normal faults, possibly active from the Late Quaternary to present. The structures align with the fault systems outcropping at the AMB border and define a 10 km wide corridor marked by subtle evidence of recent tectonic activity, such as linear scarps and crest offsets (Castel di Sangro-Rionero Sannitico corridor, CaS-RS).

With this contribution, we made a step forward to constrain the aforementioned lineaments also in the subsurface by interpreting two commercial seismic reflection profiles (that cross-cut the CaS-RS) calibrated by two deep well (ViDEPI Project). Three geological cross sections were also drawn to cross-check the subsurface with the available geological and structural maps.

Seismic line interpretation and time-to-depth conversion pointed out normal faults that align well with the lineaments highlighted by Battistelli et al. (2025), and thus also the presence of minor extensional structures that do not seem to directly affect the topography. The estimated fault offsets range between 100 and 400 m, and increase moving from NW to SE. Tentatively assuming an age of 120-750 kyr for these offsets, the resulting fault slip rates range from 0.1 to 0.9 mm/yr.

In this peculiar geo-lithological context, we propose that faulting can be strongly influenced by the mechanical stratigraphy, producing, at the shallower structural levels, a wide area marked out by diffuse and partly off-fault deformation (sensu Ferrill et al., 2017). A complementary interpretation envisages the possibility that the CaS-RS corridor could represent a linkage zone, between fault systems with opposite dip, whose evolutionary stage has not yet led to well-developed normal fault structures and related basins.

Battistelli et al., 2025. https://doi.org/10.3390/rs17142491

Carafa et al., 2020. https://doi.org/10.1029/2019JB018956

D'Agostino, 2014. https://doi.org/10.1002/2014GL059230  

Ferrill et al., 2017. https://doi.org/10.1016/j.jsg.2016.11.010

How to cite: Battistelli, M., Carafa, M. M. C., Brozzetti, F., and Ferrarini, F.: Faults like to hide: subsurface evidence of poorly known and possibly active normal faults at the border between central and southern Apennines (Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11802, https://doi.org/10.5194/egusphere-egu26-11802, 2026.

EGU26-11802

Download Close

The Rioni foreland basin system lies between the Greater and Lesser Caucasus orogens and is 
located in the far-field part of the Arabia-Eurasia collision zone. Deformation of the Rioni 
double flexural foreland basin was controlled by the action of two opposing orogenic fronts, 
the Lesser Caucasus retro-wedge to the south and the Greater Caucasus pro-wedge to the 
north (e.g., Alania et al., 2022; Banks et al., 1997; Tibaldi et al., 2017).  

Recent GPS and earthquake data indicate that the Rioni foreland basin is still tectonically 
active (e.g., Sokhadze et al., 2018; Tibaldi et al., 2020). Historical and instrumental seismic 
activity is concentrated along the frontal thrusts located along the northern and southern 
borders of the Greater and Lesser Caucasus orogens, and in the core of this foreland basin. 
All the focal mechanism solutions within the study area have a reverse and thrust fault 
kinematics (Tibaldi et al., 2020; Tsereteli et al., 2016). 

Fault-related folding and wedge thrust folding theories (Shaw et al., 2005) were employed in 
the interpretation of seismic reflection profiles and the construction of regional structural 
cross-sections across the Rioni foreland basin. Seismic profiles and structural cross-sections 
show that most earthquakes in the Rioni foreland basin occur at depths of 5-10 km.  In the 
Rioni foreland basin, fault planes do not necessarily reach the surface, and some active 
structures can be regarded as blind thrust faults, fault-bend and fault-propagation folds, 
duplexes, and these structures are mainly located at the frontal part of the Lesser Caucasus 
retro-wedge and the Greater Caucasus pro-wedge. 

How to cite: Merkviladze, D., Giorgadze, A., and Kvavadze, N.: Active structures in the Rioni foreland basin, Georgia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9518, https://doi.org/10.5194/egusphere-egu26-9518, 2026.

EGU26-9518

Download Close

Actively deforming orogens are significant seismic hazard zones, especially in areas with steadily growing populations and infrastructure. An essential and yet oftentimes poorly understood attribute for a coherent hazard and risk assessment is whether the responsible tectonic fault systems are subject to permanent, creeping deformation or episodic, seismogenic rupture processes. In the southern Bolivian Subandes recent regional geodetic surface velocities measurements indicate that the décollement beneath the eastern orogen is the primary contributor to its lateral and vertical growth. Its surface manifestation is the Mandeyapecua Thrust Fault System (MTFS), which marks the active front of the Subandean fold-and-thrust belt in the Chaco foreland basin of Bolivia. Despite significant surface offsets within Quaternary landforms its geomorphic features and tectonic activity remain poorly understood. This study focuses on its longest fault segment – the ~300 km-long Mandeyapecua Fault (MF) located between 19° and 21°S. To evaluate its role in accommodating Quaternary deformation we used high-resolution DEMs, field-based mapping, and morphometric analyses, to document uplifted terraces, drainage anomalies, and fault-related landforms indicative of Quaternary tectonic activity. Electrical Resistivity Tomography surveys at two key sites reveal near-surface structures consistent with blind thrusting and folding. Where faults have reached the surface, the expressions of scarps suggest that the Mandeyapecua Fault (MF) may be segmented. Geochronological data along the front indicate fault activity during the past 12,000 years, with ruptures possibly spanning ~100 km, but the complex, distributed surface deformation indicates that the MF might not fit a standard thrust-fault model.

How to cite: Patyniak, M., Arnous, A., Alvarellos, V., Jagoe, L., Williams, A. M., Guerra Colque, J. M., Rosales Sadud, O. A., Preusser, F., Arrowsmith, J. R., Bookhagen, B., and Strecker, M.: Quaternary Faulting and Fault-Related Geomorphology along the Orogenic Retro Arc Wedge-Front Structure of the Central Andes: The Mandeyapecua Thrust System, Southeastern Bolivia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9200, https://doi.org/10.5194/egusphere-egu26-9200, 2026.

EGU26-9200

Download Close

Northern Germany is commonly regarded as a low seismicity area, but a number of historic earthquakes with intensities of up to VII have occurred in this region during the last 1200 years. The Aller Valley fault system, with a length of about 250 km, is one of the major fault systems in northern Germany. It strikes NW-SE and extends from the Magdeburg area over Wolfsburg across Lower Saxony to the area of Bremen and Oldenburg, close to the border to the Netherlands. This fault was highly active in the Mesozoic. Reflection seismic profiles of the petroleum industry show that during the Triassic it was a normal growth fault, which was inverted during Late Cretaceous compression. In addition, a large number of earthquakes have occurred close to the Aller Valley Fault system between AD997 and 1576.

We carried out seven, high-resolution, shear(S)-wave reflection seismic profiles accompanied by georadar in an area of the Aller Valley Fault system near Lehringen in Lower Saxony. Shear waves propagate up to twelve times slower than P-waves in unconsolidated sediments, making it the ideal tool to investigate the near-surface. The geological map displays a rhomboidal outcrop of Eemian sediments in this area, which we hypothese is a pull-apart basin.

The S-wave seismic profiles image a number of Eemian and Weichselian depocentres at depths of 10-30 m that are progressively displaced north-eastwards by a series of steep to vertical faults that propagate from depth. The georadar data provide a high-resolution imaging of the upper 5 m of the Weichselian sediments and support the findings of the seismics. In some georadar profiles, fault structures in the Weichselian sediments are imaged, indicating that the faults must still have been active after sedimentation. OSL-dating of a hand drill core has substantiated the geological interpretation. We postulate that the recent fault activity is due to glacial isostatic adjustment.

How to cite: Tanner, D., Brandes, C., Polom, U., Igel, J., Winsemann, J., and Tsukamoto, S.: Geophysical evidence of neotectonic activity on the Aller Valley Fault system in northern Germany, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5531, https://doi.org/10.5194/egusphere-egu26-5531, 2026.

EGU26-5531

Download Close

Taiwan lies within the active arc–continent collision between the Philippine Sea Plate and the Eurasian margin. In the Western Foothills, the foreland basin has been incorporated into the fold-and-thrust belt, with a deformation front clearly defined by onshore topography. However, the seaward extension of these structures remains poorly constrained. Neglecting faults that traverse the coastline can lead to a significant underestimation of seismic hazards. To characterize these potential seismogenic sources, we utilize high-resolution multichannel seismic reflection profiles acquired by a GI-gun system to understand the Holocene subsurface structure and quantify deformation parameters in the western offshore of Taiwan.

This study interprets key regional stratigraphic markers, including the unconformity formed during the last glacial period, to characterize fault-related folds in the offshore domain. Additionally, we developed a shallow 3-D velocity model based on semblance velocity to assess structures down to approximately 1 km depth. To provide robust evidence across the study area, we integrated offshore fault interpretations and strata offsets with onshore outcrop and borehole data. This integration allowed us to quantify fault orientation, length, dip, and vertical displacement.

Seismic interpretation shows that strata overlying thrust faults with asymmetric anticlines indicate fault-propagation folds, accompanied by noticeable uplift above the Last Glacial Maximum Unconformity. Eight major NE–SW trending thrust faults identified within the offshore deformation front likely extend more than 20 km when linked with onshore segments. Additionally, the long-term uplift rates estimated from seismic profiles are consistent with geochronological constraints from borehole data. These segment-scale fault parameters at the western offshore deformation front establish crucial parameters for offshore seismic hazard assessment and risk-informed development in northwestern Taiwan.

How to cite: Chang, S.-P., Fan, C.-J., Hsu, H.-H., Chen, Y.-P., Lin, Y.-X., Han, W.-C., and Chen, S.-C.: Structural Architectures and Distribution of Active Faults in Taiwan Strait, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17262, https://doi.org/10.5194/egusphere-egu26-17262, 2026.

EGU26-17262

Download Close

Active fault mapping is an essential tool for predicting future surface ruptures. However, many earthquakes occur along unknown or partially mapped faults, even in “well-mapped” seismically active regions. This phenomenon is particularly evident in Southern California, as demonstrated by several surprising events, including: Ridgecrest 2019, El Mayor Cucapah 2010, Hector Mine 1999, Landers 1992, and Kern County 1952. Following these earthquakes on unmapped faults, it is often possible to find evidence suggesting pre-earthquake ruptures with paleoseismological studies. Thus, gaps in fault mapping may result from a lack of visible surface ruptures or from subtle signs that are challenging to identify. Recognizing these faults, despite weak signals in the landscape, is crucial for better predicting future shallow earthquakes and their potential impacts on human infrastructure. To understand why evidence of surface ruptures may disappear in certain fault sections, it is essential to learn how these ruptures develop following an earthquake.

The advent of very high-resolution satellite imaging, combined with image correlation techniques, presents new opportunities for characterizing the morphology of co-seismic surface ruptures. This study aims to investigate whether a systematic relationship exists between pre-earthquake fault mapping and the characteristics of observed co-seismic surface ruptures. Specifically, we search to determine whether faults mapped before a rupture exhibit statistically different co-seismic displacements or near-field deformation characteristics compared to unmapped faults, and whether ruptures lacking clear pre-event geomorphological expression display distinct signatures. We begin by analyzing the co-seismic surface rupture of the 2019 Ridgecrest earthquake and comparing the rupture characteristics with pre-event fault mapping obtained from the USGS database. This analysis will then be extended to the 1992 Landers and 1999 Hector Mine earthquakes to evaluate the robustness and generality of the observed patterns across multiple large strike-slip events. For each earthquake, we construct dense datasets sampled along the surface ruptures, integrating morphological information derived from 2-meter resolution digital elevation models (DEMs) and displacement measurements obtained through 1-meter image correlation. We employ an unsupervised machine learning approach, specifically a hierarchical clustering, to group rupture segments based on their similarities across various parameters.

This methodology enables us to identify distinct classes of surface rupture behavior and evaluate how their distribution relates to pre-existing faults across different earthquakes. Our analyses reveal a strong correlation between the presence of pre-seismic geomorphic signal and lithology, as well as the intensity of co-seismic displacement. We found that more erosion-prone sediments and regions with smaller co-seismic displacement tend to show limited geomorphic expression prior to the earthquake. Additionally, some subtle pre-earthquake geomorphic signals can indeed be detected and mapped using very high-resolution satellite imagery. One initial approach to enhance fault mapping practices would be to utilize very high-resolution imagery, particularly in arid and sedimentary regions.

How to cite: Rocamora, I., Hollingsworth, J., Giffard-Roisin, S., Pousse-Beltran, L., and Ben-Zion, Y.: Assessing Active Fault Mapping Gaps in Southern California Using Co-Seismic Surface Rupture Characteristics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19302, https://doi.org/10.5194/egusphere-egu26-19302, 2026.

EGU26-19302

Download Close

The Castle Mountain fault (CMF) is a major active fault in south-central Alaska that poses a significant seismic hazard to the Anchorage and Matanuska-Susitna Valley urban areas. Previous studies of the CMF have reached conflicting conclusions regarding its kinematics, slip behavior, and earthquake rupture history. Earlier paleoseismic, geomorphic, and geodetic studies suggested that the CMF is predominately right-lateral with slip rate values ranging from 0.07 - 3.0 mm/yr, while more recent work suggests that the CMF accommodates reverse dip-slip motion of <0.3 mm/yr (based on the long-term bedrock rate). Early studies were constrained by limited methodologies and data, such as low-resolution topographic maps. In this study, we apply modern geomorphic and geophysical methods at several sites along the CMF to reassess interpretations of its slip sense and better constrain the number and timing of past earthquake ruptures. We have completed geomorphic mapping using high-resolution digital elevation models (DEMs) and collected two electrical resistivity tomography (ERT) profiles at one of two designated study sites. The well-defined CMF scarp resolved in lidar DEMs allows precise placement of ERT profiles across the fault. The two profiles spanned 80 meters across the fault scarp. ERT probes measured resistivity at 5m-spacing for a deeper profile and 2m-spacing for a more detailed profile closer to the surface. Relative fault displacements along strike of the CMF will be analyzed and measured using statistical analyses of scarp heights.  Preliminary results indicate that the ERT profiles can distinguish different geologic units and fault features such as fault planes, fracture zones, and stratigraphic offsets that have strong lateral resistivity contrasts. Based on geomorphic features observed in the DEMs, our preliminary findings suggest that past earthquakes on the CMF involved predominantly reverse slip. These features include hanging-wall-grabens, south-facing scarps, folded surfaces, and left-stepping en echelon scarps superimposed on the larger scarp. To better define the slip-rate history and geometry of the CMF, we plan to collect additional ERT profiles across the scarp where it displaces various fluvial terraces. We will also describe sediment cores and soil profiles. Samples from the cores and profiles will be collected for optically stimulated luminescence and radiocarbon dating. Our results will be compared with previous interpretations and observations in the field to help resolve long-standing discrepancies in interpretations of CMF behavior and improve regional seismic hazard assessments.

How to cite: Berrien, L., Harrichhausen, N., Witter, R., Koehler, R., and Munk, J.: Resolving the Deformation Style and Slip Behavior of the Castle Mountain Fault, South-Central Alaska, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15452, https://doi.org/10.5194/egusphere-egu26-15452, 2026.

EGU26-15452

Download Close

The Greater Caucasus is a typical active double wedge orogen that accommodates the crustal shortening due to far-field effects of the collision between the Arabian and Eurasian plates. Our study area is the western part of the Greater Caucasus pro-wedge, represented by the central and northern parts of the Rioni Foreland basin and the southern slope of the Greater Caucasus. Here, we present a new structural model based on interpreted seismic profiles, regional structural cross-sections, and earthquake focal mechanisms. From SSW to NNE, serial structural cross-sections reveal: (1) basement-involved thrust faults and thick-skinned fault-bend folds, and (2) thin-skinned structures expressed as duplexes and imbricate fault-propagation folds. The dominant compressional structural styles are controlled by multiple detachment horizons.

According to the presented serial structural cross-sections, the Enguri HPP dam is located on top of the triangle zone. Major basement-involved thrusts produce first-order thick-skinned fault-bend folds, which move southward, creating second-order fault-propagation folds and duplexes in the sedimentary cover. Preexisting, basement-involved extensional faults inverted during compressive deformation produced basement-cored uplifts that transferred thick-skinned shortening southward onto the thin-skinned structures detached above the basement.

The correlation of earthquake hypocenters and focal mechanisms with faults interpreted from 3D structural models enables the identification of active structures. Five potentially active thrust faults are recognized within the study area. Four of these structures are south-vergent thrusts, whereas one corresponds to an out-of-sequence thrust.

Acknowledgments. This work was funded by Shota Rustaveli National Science Foundation (SRNSF) (grant# FR-23-8896).

How to cite: Kvavadze, N., Alania, V., Enukidze, O., Magalashvili, A., Razmadze, A., and Merkviladze, D.: Seismically active thrust faults and wedge structures beneath the western Greater Caucasus orogen pro-wedge, Georgia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9533, https://doi.org/10.5194/egusphere-egu26-9533, 2026.

EGU26-9533

Download Close

The Alboran Basin is transected from southern Spain to northern Morocco by the active left-lateral Al Idrissi Fault Zone, whose southern termination corresponds to the Bokkoya fault system. These faults accommodate the oblique convergence between the African and Eurasian plates and the extrusion of the Betic–Rif block, generating recurrent seismicity. The Bokkoya Fault Zone lies between offshore segments that ruptured during the 1994–2004 seismic crises (Mw 5.9 and 6.3) and the 2016 and 2021 events (Mw 6.4 and 5.5). The ANR-funded ALBANEO project aims to constrain the long-term behaviour of this currently low-seismicity segment by reconstructing its activity over the last ~120 ka, with implications for regional seismic hazard assessment.

This study integrates a multi-proxy dataset from the ALBACORE marine campaign (https://doi.org/10.17600/18001351), including multibeam bathymetry, seismic reflection and sub-bottom profiles, piezocone penetration tests (CPTu), and sediment cores. Data were collected along ~20 km of the Bokkoya fault segment, from the Small Al Idrissi Volcano to Al Hoceima Bay.

Deformation is distributed across localized and diffuse fault segments with both vertical and horizontal offsets. Fault architecture evolves from north to south, controlled by relay zones and step-overs, up to the Moroccan coastline where the fault system terminates. Individual segments are on average ~5 km long, with maximum cumulative horizontal offsets of ~3 km over 1 Ma and vertical offsets of up to 32 m over the last 120 ka.

Paleoseismological analysis highlights major tectonic events during the Last Glacial Maximum (LGM). In the Bokkoya fault system, seismic reflection data calibrated with sediment cores and CPTu measurements indicate late- to post-LGM fault sealing on some segments, as well as in-situ disrupted seismic facies dated to the LGM. This facies is interpreted as the result of seismically induced soft-sediment deformation.

Moreover, a chaotic sedimentary facies observed between 8 and 10 m depth in core ALB_CL56 correlates with increased sediment strength derived from CPTu data and is dated between 20.9 and 20.3 ka. This facies extends over ~30 km² on sub-bottom profiles and is interpreted as a mass-transport deposit (MTD), likely triggered by a coeval seismic event. The source area is identified on the eastern shelf of the Bokkoya fault system, where submerged headscarps are observed. During the LGM (~18–24 ka), sea level was approximately 120 m lower, exposing the shelf by up to ~40 m.

The MTD and the in-situ disrupted seismic facies likely represent paleoseismic archives, consistent with recent studies documenting LGM-aged seismic events on the Bokkoya fault (Vidil et al., 2025). However, disentangling climatic forcing (sea-level changes and post-LGM warming) from tectonic triggering remains challenging. The spatial distribution of seismic clusters and paleo-fault activity suggests an immature segmentation of the plate boundary, with important implications for regional seismic hazard.

How to cite: d Acremont, E., Vidil, L., Emmanuel, L., Lafuerza, S., Caroir, F., Leroy, S., Latni, E. M., and Rabaute, A. and the ALBANEO-ALBACORE team: Multi-proxy evidence of activity of the Bokkoya fault system during the Last Glacial Maximum (LGM), Alboran sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18578, https://doi.org/10.5194/egusphere-egu26-18578, 2026.

EGU26-18578

Download Close

This study focuses on the investigation of the geological and neotectonic conditions of the Amyntaio Basin, with a particular emphasis on the recent geological formations that host the region’s primary aquifers. Understanding the stratigraphy and tectonic structure is considered essential, as these formations serve as the primary water source for domestic, agricultural, and industrial requirements. The basin fill consists of thick Neogene and Quaternary sediments deposited unconformably over the Mesozoic basement. Dominating these deposits is the lignite-bearing series, while the overlying Quaternary formations are distinguished into the lower coarse-grained Proastio Formation, characterized by conglomerates and sands, and the upper finer-grained Perdikka Formation, which primarily includes marls and clays. Significant importance is attributed to modern alluvial deposits, which cover most of the basin and directly influence the hydrogeodynamic system. In the central part of the basin, near Lake Chimaditida, up to six-meter-thick layers of peat and organic silt occur, while the northwestern sector is dominated by the extensive alluvial fan of Sklithro.

Structural mapping revealed a dense fault fabric compatible with the current extensional stress regime of the area, dominated by normal faults striking NE-SW to ENE-WSW. The primary structures include the Vegoritida fault zone, which terminates at the northern boundary of the Amyntaio mine, the Chimaditida fault, which is likely connected to the Vegoritida system, and the Anargyroi fault, which defines the southern margin of the sub-basin. The combination of these structures creates a second-order tectonic graben where Lake Chimaditida has developed, while the Amyntaio mine area is situated within a fault transfer zone. Within the mine itself, the tectonic fabric consists of smaller normal faults following the same primary orientation, creating a complex horst and graben system.

One of the main conclusions of this study is the systematic geographical distribution and geometry of the mapped ground fissures. The orientation of these fissures coincides with the primary direction of the regional neotectonic fabric, specifically following the trends of the Petres-Sklithro and Anargyroi fault systems. Their kinematics align with the general tectonic extensional regime, suggesting a clear genetic relationship between active faults and surface ruptures. In certain areas, such as the one west of the mine, between the settlements of Anargyroi and Valtonera, the traces of the mapped faults practically coincide with the observed fissures. Furthermore, the alignment of these outcropping structures, between Valtonera and Rodona, confirms the existence of the Valtonera Fault. This structure constitutes an integral part of the neotectonic fabric and is identified as the primary factor responsible for the magnitude of the ground deformation phenomena within the settlement.

How to cite: Kranis, H., Skourtsos, E., Filis, C., Andreadakis, E., Kapourani, E., Roumpos, C., Kostaridis, P., and Louloudis, G.: Geological Structure and Neotectonic Fabric of the Amyntaio Basin, NW Greece: The Correlation Between Fault Systems and Ground Fissures, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21084, https://doi.org/10.5194/egusphere-egu26-21084, 2026.

EGU26-21084

Download Close

We have presented a new structural model of the Mtskheta 1275 (Mw=6.5) historical earthquake epicentral area. The Mtskheta historical earthquake is located in the frontal part of the Lesser Caucasus orogen pro-wedge (Alania, V., et al., 2023). The frontal part of the Lesser Caucasus orogen is characterized by moderate seismic activity (e.g., Tsereteli et al., 2016). From the determination of the deep structure of the Mtskheta 1275 (Mw=6.5) historical earthquake epicentral area, we use seismic reflection profiles. Seismic reflection profiles show north-vergent duplexes, and structural wedge at the triangle zone beneath the thrust front monocline and is represented by Cretaceous-Neogene strata. In the southern part of the Kura foreland basin, the Oligocene-Lower Miocene strata have been deformed and uplifted by passive-back thrusting at the triangle zone. Based on the new structural model, it has been suggested that the Mtskheta 1275 (Mw=6.5) historical earthquake was related to structural wedge. The results of our subsurface interpretations have important implications for how this fold-and-thrust belt formed, in addition to the effect of structural style on active tectonics in the retro-wedge of the Lesser Caucasus orogen.

Acknowledgments. This work was funded by Shota Rustaveli National Science Foundation (SRNSF) (grant# FR-23-8896).

How to cite: Razmadze, A., Kvavadze, N., and Shikhashvili, T.: Structural model of the Mtskheta 1275 (Mw=6.5) historical earthquake epicentral area using seismic profiles, Georgia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9794, https://doi.org/10.5194/egusphere-egu26-9794, 2026.

EGU26-9794

Download Close

In the Alboran Sea, oblique convergence between the African and Eurasian plates has driven the development of the active Al Idrissi-Bokkoya sinistral strike-slip fault system since ~1 Ma. Several moderate-magnitude earthquakes (Mw > 6) have been recorded along different segments of this fault system, highlighting its ongoing activity. This study investigates the dynamics of this nascent plate boundary by identifying seismic events recorded in sedimentary archives.

We focus on the Bokkoya transtensive fault system, which offsets the Small Al Idrissi Volcano and extends over ~20 km along strike. Sedimentation in this area is strongly influenced by the circulation of Deep Mediterranean Water masses, resulting in contourite deposition, and is likely punctuated by mass-movement processes triggered by seismic events.

A multidisciplinary dataset was acquired during the ALBACORE oceanographic campaign (R/V Pourquoi pas?, 2021), conducted within the framework of the ANR ALBANEO project, which aims to characterize the dynamics and seismic hazards of this emerging plate boundary. The dataset includes two 18 m-long Calypso sediment cores (ALB_CL54 and ALB_CL53) located directly above and within the subsiding basin of the main Bokkoya Fault. Analyzes include Multi-Sensor Core Logging (MSCL), X-Ray Fluorescence (XRF), Total Organic Carbon (TOC, Rock-Eval), and stable Isotope analyses (δ¹³C and δ¹⁸O), complemented by multibeam bathymetry and seismic reflection/sub-bottom profiler data.

Radiocarbon-calibrated δ¹⁸O records allow sedimentary sequences to be dated back to ~45 ka, encompassing major cold climatic intervals such as the Younger Dryas, Heinrich Stadial 1, and the Last Glacial Maximum (LGM). The mean sedimentation rate within the subsiding basin is approximately 35 cm.kyr^-1. Comparison of sedimentary successions across different fault compartments reveals pronounced contrasts during the LGM (at ~20-21 ka), when core ALB_CL54 -penetrating the fault plane- records an exceptionally high sedimentation rate (> 200 cm.kyr^-1), an absence of bioturbation within contouritic deposits, and a distinct coupled δ¹⁸O- δ¹³C (up to ~3 ‰) anomaly not observed in the adjacent core ALB_CL53, located in the fault zone.

The restriction of the isotopic anomaly to ALB_CL54 points to a localized, transient tectonic event involving the rapid expulsion of hot fluids along the fault zone., which temporarily served as a preferential fluid drainage pathway. The absence of a similar isotopic record in ALB_CL53 suggests limited lateral fluid dissipation, consistent with a brief, high-intensity fluid release occurring during a cold climatic period associated with low sea level. These results demonstrate that coupled δ¹⁸O and δ¹³C anomalies in planktonic foraminifera constitute a robust geochemical marker of tectonic events in marine sediments, providing a complementary tool to highlight episodes of fault activity beyond the resolution of sedimentological observations.

How to cite: Vidil, L., Emmanuel, L., d'Acremont, E., Lafuerza, S., Leroy, S., and Caroir, F. and the ALBANEO-ALBACORE: Planktonic foraminiferal δ¹⁸O-δ¹³C anomalies reveal earthquake-triggered transient fluid flow along the active Bokkoya strike-slip fault, Alboran Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18550, https://doi.org/10.5194/egusphere-egu26-18550, 2026.

EGU26-18550

Download Close

The Tanlu Fault Zone, extending approximately 2400 km in a NNE orientation, constitutes the most extensive fault system in eastern China and has undergone complex multi-stage tectonic evolution since the Mesozoic. Notwithstanding substantial geological and geophysical investigations, the shallow crustal deformation characteristics across distinct segments and their intrinsic relationship with seismicity distribution remain inadequately constrained. This study presents high-resolution three-dimensional S-wave velocity and azimuthal anisotropy models for the Suqian and Zhangbaling segments, utilizing Rayleigh wave dispersion data extracted from ambient noise recordings acquired by dense portable seismic arrays comprising 238 and 192 short-period seismometers, respectively.

Ambient noise cross-correlation techniques were employed to extract inter-station Rayleigh wave empirical Green's functions, with phase velocity dispersion curves measured across the period range of 0.5 to 8.8 seconds. The DAzimSurfTomo direct inversion method was applied to jointly determine three-dimensional isotropic shear-wave velocity structures and azimuthal anisotropy distributions within the shallow crust at depths of 0 to 8 kilometers. Checkerboard and recovery tests demonstrate that the obtained azimuthal anisotropy models possess reliable resolution capability in regions with adequate ray path coverage.

The inversion results reveal velocity structures corresponding to distinct tectonic units, with high-velocity anomalies associated with uplifted regions and low-velocity anomalies with sedimentary basins. The Suqian segment exhibits systematic depth-dependent variations in fast-wave directions: NNE orientations parallel to the fault strike at shallow depths of 1 to 3 kilometers, transitioning to NE orientations at greater depths of 4 to 8 kilometers, with anisotropic magnitude of 2% to 3%. The Zhangbaling segment displays pronounced anisotropic contrasts between tectonic units, characterized by NE fast-wave directions in the Zhangbaling uplift and NW directions in the Hefei Basin, reflecting fundamentally different deformation characteristics between the North China Plate and South China Plate. Notably, fast-wave directions at the fault-uplift interface exhibit complex depth-dependent variations, progressing from NE (1 to 3 kilometers) to NNE (3 to 6 kilometers) and reverting to NE (6 to 10 kilometers).

Pronounced spatial correlations between anisotropic structure and earthquake distribution indicate that pre-existing crustal fabric exerts primary control on contemporary rupture patterns, with seismicity concentrated at depths of 3 to 5 kilometers where anisotropic transitions occur. The two segments manifest contrasting deformation characteristics: distributed deformation in Suqian versus localized deformation along the sharp anisotropic boundary in Zhangbaling, reflecting distinct evolutionary stages and present-day tectonic regimes. These depth-dependent anisotropic patterns preserve signatures of multi-stage tectonic evolution encompassing early sinistral strike-slip motion, subsequent extensional deformation, and recent compressional tectonics, thereby providing novel constraints on deformation processes and the contemporary stress field within this significant intracontinental fault system.

How to cite: Zhang, B. and Yao, H.: Three-Dimensional Crustal Azimuthal Anisotropy Reveals Multi-Stage Deformation in the Suqian and Zhangbaling Segments of the Tanlu Fault Zone, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10885, https://doi.org/10.5194/egusphere-egu26-10885, 2026.

EGU26-10885

Download Close

The Gulang Ms 8.0 strong earthquake that occurred in 1927 claimed the lives of more than 40,000 people in the surrounding areas. Geological studies have shown that the occurrence of the Gulang earthquake did not reduce the seismic hazard of the Laohushan, Maomaoshan and Lenglongling faults, which are the westward extensions of the Haiyuan Fault. Based on the seismic moment accumulation rate, the existence of the Tianzhu Seismic Gap has been proposed. This seismic gap is potentially at risk of producing earthquakes of magnitude 7.0 or higher. Coupled with the frequent occurrence of small earthquakes in this area in recent years, it is regarded as being of considerable seismic danger. In the junction area of the Tianzhu Seismic Gap and its surrounding faults, we observed and collected dense broadband seismic array data for a period of more than 7 years. Through long-term continuous observations, the seismicity and the crustal S-wave velocity structure of the study area was obtained. The research results show that the current seismicity in the Gulang Seismic Zone distributes along the Wuwei-Tianzhu Fault with a southwestward trending feature, and does not extend to the Lenglongling Fault. This indicates that the seismogenic fault of the Gulang Earthquake may not include the Lenglongling Fault and the Jinqianghe Fault. In the Tianzhu Seismic Gap, seismicity distributes linearly along the Laohushan-Maomaoshan Fault, exhibiting obvious strike-slip fault characteristics. In terms of depth, seismic activities around the Maomaoshan Fault are concentrated in two intervals: the shallow layer above 10 km and the deep layer below 20 km, which also delineates the strong locking feature at the depth of 10–20 km beneath the Maomaoshan Fault. Obvious weak seismicity is also observed in the western segment of the Haiyuan Fault. The velocity structure results demonstrate that at the depth of the upper and middle crust, there are significant velocity differences on both sides of the Laohushan Fault, Maomaoshan Fault and Wuwei-Tianzhu Fault, and the seismic distribution is highly consistent with the boundary zones of these velocity differences. Beneath the Maomaoshan Fault and in the middle segment of the Laohushan Fault (at the upper and middle crust depth), there exist high-velocity anomalies distributed on both sides of the faults. These anomalies are inferred to be asperities that impede fault rupture, with a length of approximately 50 km and a width exceeding 20 km along the fault plane.

How to cite: Chen, J., Li, S., Guo, B., Chen, Y., and Yin, X.: Seismic Array Study of the Tectonic of the Tianzhu Seismic Gap and the Deep Characteristics of the Laohushan and Maomaoshan fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15087, https://doi.org/10.5194/egusphere-egu26-15087, 2026.

EGU26-15087

Download Close

Western Türkiye, situated on the westward-extruding Anatolian Plate, is one of the most actively deforming regions of the Eurasian-Arabian-African tectonic system. The shear regime of the North Anatolian Fault (NAF) Zone to the north and subduction of the Hellenic Trench to the south together drive significant N–S extension across western Türkiye. This extension is accommodated by major E-W-trending graben systems, including Gediz, Simav, and Menderes, making the region an excellent natural laboratory for studying stress transfer and seismic hazard. This tectonic setting, together with elevated heat flow and locally high crustal permeability, gives rise to a highly complex seismotectonic environment with multiple active fault systems in the Balıkesir–Sındırgı region. In 2025, two earthquakes (Mw 6.1 and Mw 6.0) ruptured the Balıkesir–Sındırgı segment of the Simav Fault Zone (SFZ) in this region, initiating an intense aftershock sequence characterized by Mw 3–4 events and several Mw ≥ 5 shocks. This short, spatially clustered sequence offers an opportunity to investigate the stress transfer, seismic productivity, and coseismic deformation in this complex extensional domain. In this study, to understand these processes better, both historical and instrumental period events are compiled and analyzed to describe the spatio-temporal distribution of earthquakes before and after the 2025 events. Coseismic Coulomb stress changes (ΔCFS) are computed for each mainshock, and the results are compared with the aftershock distribution. A regional ΔCFS analysis is also performed to assess cumulative loading on neighboring fault segments. To evaluate seismic productivity and magnitude–frequency characteristics, a- and b-values are estimated using the Gutenberg–Richter relationship, and spatial variations in b-values are compared with the ΔCFS models. Furthermore, Sentinel-1 SAR images are analyzed with the Interferometric Synthetic Aperture Radar (InSAR) technique to map coseismic deformation and to define the source geometry and slip behaviour. Finally, these results are discussed in conjunction with published seismic velocity, magnetotelluric, and geothermal studies, which together indicate a relatively thin and thermally elevated crust that may facilitate shallow normal/oblique faulting and efficient stress transfer.

How to cite: Gedik, M., Kaya Eken, T., Zoroğlu, Ç. S., and Özener, H.: Integrated Seismological and Geodetic Analysis of the 2025 Balıkesir–Sındırgı Earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-753, https://doi.org/10.5194/egusphere-egu26-753, 2026.

EGU26-753

Download Close

The 10 August 2025 Mw 6.0 Sındırgı earthquake occurred in one of the most tectonically complex regions of Inner Western Anatolia, where Aegean extension interacts with the westward extrusion of the Anatolian microplate. Despite initial reports indicating a NW–SE–oriented normal-faulting mechanism, the spatial distribution of aftershocks and early field observations point to a more intricate rupture behaviour. Rapid field investigations by the General Directorate of Mineral Research and Exploration (MTA) on 11 August 2025 revealed no evidence of surface rupture or localized coseismic deformation along the Sındırgı, Düvertepe or Gelenbe fault zones.

Full-waveform moment tensor inversion of the mainshock and 31 aftershocks yielded robust, well-constrained focal mechanisms. The mainshock exhibits a NW–SE striking oblique-reverse faulting mechanism, with ~90% double-couple content and a centroid depth of approximately 10 km. Aftershock mechanisms display a clear spatial partitioning: reverse and strike-slip components dominate south of the Sındırgı Segment, whereas normal faulting is prevalent to the north. The aftershock sequence further demonstrates a pronounced eastward migration pattern.

Statistical analysis of 6,711 earthquakes recorded between 20 July and 1 September 2025 indicates low regional b-values (0.60–0.70), suggesting elevated differential stress. Following the mainshock, b-values increase toward the eastern portion of the aftershock zone (0.75–0.80), reflecting evolving stress conditions. The Omori p-value (~0.18) indicates an unusually slow decay of aftershocks, consistent with a prolonged period of seismic activation. Stress tensor inversion of 32 focal mechanisms reveals a strike-slip–dominated regime with NE–SW–oriented maximum compression, in agreement with the regional tectonic pattern.

Integration of regional magnetotelluric (MT) profiles shows that the 2025 Sındırgı sequence coincides with deep, low-resistivity zones interpreted as thermally weakened or partially molten lithospheric domains beneath the Simav–İzmir–Balıkesir structural corridor. These MT-based lithospheric anomalies spatially correlate with previous major earthquake sequences, including the 2011 Simav and 2020 Akhisar events, implying a persistent lithospheric control on fault kinematics, stress localization and seismogenesis.

Overall, the 2025 Sındırgı earthquake sequence highlights the combined role of structural complexity and deep lithospheric processes in determining seismic behaviour in Inner Western Anatolia. The integration of seismological, geological and geophysical datasets provides a comprehensive framework for understanding rupture dynamics in this long-lived, active deformation zone.

Keywords :Sındırgı Earthquake Sequence; Moment tensor; Stress tensor inversion; b-value; Aftershock migration; Western Anatolia; Magnetotellurics; Active tectonics.

How to cite: Yalcin, H., Kürçer, A., Karayazı, O., Yalvaç, O., and Çal, Ç.: The 2025 Sındırgı Earthquake Sequence: Linking Fault Geometry, Stress Transfer and Deep Structure, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1240, https://doi.org/10.5194/egusphere-egu26-1240, 2026.

EGU26-1240

Download Close

Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques are largely used to detect coseismic deformation patterns associated with large-to-moderate magnitude earthquakes. In contrast, small earthquakes (M<5), although far more frequent and potentially crucial for understanding regional stress regimes and active faulting, generally produce weak surface deformation that is difficult to detect using remote sensing approaches.

In this work, we integrate relocated seismicity, observed DInSAR deformation, and the Okada elastic dislocation model to infer insights into the geometry and mechanics of the causative fault of the 2023 M_L 4.4 Umbertide extensional earthquake in Central Italy. The seismicity was relocated using the Non-Linear Earthquake Location Algorithm in combination with the three-dimensional velocity model developed specifically for the area.

We benefited of Sentinel-1 Line Of Sight (LOS) displacement maps generated over ascending and descending orbits through the EPOSAR service of the European Plate Observing System (EPOS) Research Infrastructure. These data were exploited to derive the vertical and east-west deformation components using a recently developed open-source Python tool capable of combining multiple LOS displacement maps. The results reveal up to ~2 cm of subsidence and ~1.5 cm of eastward motion in the epicentral area, suggesting the activation of a NE-dipping normal fault, consistent with the relocated seismicity distribution.

The focal mechanism parameters of this plane were adopted for the Okada modeling. According to the maximum-likelihood solution of the M_L 4.4 mainshock relocation, the source was modeled at 3.5 km of depth. The best-fitting solution between the modeled and observed deformation is a rectangular planar fault measuring 2.3 × 2.7 km (L × W), with a maximum slip of 20 cm.

Despite the earthquake’s limited magnitude and the surface deformation signal being partially affected by atmospheric disturbances, properly applied DInSAR techniques can provide a detailed estimation of surface displacement. The results demonstrate DInSAR’s ability to detect deformation induced by small-magnitude earthquakes in a seismically active region, with the potential to improve active fault mapping and seismic hazard assessment.

How to cite: Gaspari, R., Occhipinti, M., De Luca, C., Monterroso, F., Riva, F., Doukakos, I., Amorini, S., Cenci, G., Barchi, M. R., and Porreca, M.: Assessing the feasibility of DInSAR for detecting coseismic deformation in small-magnitude earthquakes: the 2023 ML 4.4 Umbertide earthquake (Central Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6444, https://doi.org/10.5194/egusphere-egu26-6444, 2026.

EGU26-6444

Download Close

The M_L 6.4 Dapu earthquake, which struck southwestern Taiwan on January 20, 2025, occurred in the western foothills belt, an area characterized by folds and thrust faults. The earthquake sequence exhibits intricate fault interactions, with recent observations suggesting a conjugate rupture pattern involving both east-dipping detachment faults and west-dipping basement structures. However, the detailed subsurface structure remains poorly constrained due to the lack of a high-resolution 3D velocity model in this region. To elucidate the seismogenic structure, we conducted a joint inversion of seismic arrival times and gravity data. We utilized a comprehensive dataset integrating: (1) long-term background seismicity recorded by the Central Weather Administration (CWA) from 2012 to 2020, (2) the 2025 Dapu mainshock and its aftershock sequence, and (3) dense gravity observation data in the study area. By incorporating gravity data, our model provides enhanced resolution for shallow crustal structures and density constraints that complement traditional seismic tomography. We focus on imaging the high-resolution 3D velocity and density structures to identify the specific lithological or structural boundaries governing the rupture. Furthermore, we investigate the temporal variations in seismic velocity structure before and after the mainshock to detect potential stress relaxation or fluid migration processes. In this presentation, we will demonstrate the correlation between the derived structural heterogeneity and the aftershock distribution, providing new physical constraints on the seismotectonics of the Dapu earthquake sequence.

How to cite: Kao, T.-W., Yen, H.-Y., and Lo, Y.-T.: Seismogenic Structure and Temporal Velocity Variations of the 2025 ML 6.4 Dapu Earthquake (Taiwan): Insights from Joint Inversion of Seismic and Gravity Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3754, https://doi.org/10.5194/egusphere-egu26-3754, 2026.

EGU26-3754

Download Close

The Hualien area, situated at the collision boundary between the Eurasian Plate and the Philippine Sea Plate, is the most seismically active region in Taiwan. Despite numerous studies, the detailed subsurface geometry and fault distribution remain incompletely resolved due to the complex tectonic interactions between the plates. This study aims to refine the 3D velocity structure using seismic data collected by the Central Weather Administration Seismological Network (CWASN) and the Taiwan Strong Motion Instrumentation Program (TSMIP) from 2012 to 2024. To handle the massive dataset and improve catalog completeness, we employed deep-learning algorithms—using EQTransformer (Mousavi et al., 2020) for phase picking and GaMMA (Zhu et al., 2022) for phase association. Subsequently, we applied the double-difference tomography method (TomoDD; Zhang and Thurber, 2003), incorporating gravity constraints to better resolve shallow velocities. We performed a sequential inversion to obtain high-resolution P- and S-wave velocity structures with a grid spacing of 5 km. Our preliminary static inversion results demonstrate high resolution in onshore regions and reveal critical structural features within the collision zone. These structural geometries are generally consistent with previous tomographic models (e.g., Huang et al., 2014), ensuring the reliability of our static velocity baseline. Building on this reliable static baseline (derived from 2012–2020 data), we further investigate temporal velocity variations (4D tomography) by integrating subsequent data from 2021–2024. By integrating the refined velocity models with relocated seismicity, we aim to provide a more detailed characterization of the complex subsurface structures and their spatiotemporal variations in this active collision zone.

How to cite: Hsu, C.-W., Yen, H.-Y., and Lo, Y.-T.: Investigating Spatiotemporal Variations of Subsurface Velocity Structure in the Hualien Area, Taiwan: Insights from AI-Enhanced Seismic Tomography (2012–2024), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3755, https://doi.org/10.5194/egusphere-egu26-3755, 2026.

EGU26-3755

Download Close

The Montello–Collalto area is located along the outer front of the eastern Southern Alps (Italy), within a fold-and-thrust belt that has been active since the Middle Miocene (Picotti et al., 2022). The region is characterized by a medium-to-high seismic hazard, as demonstrated by historically significant earthquakes such as the 1695 Asolo event (Mw 6.5). Despite this, the causative fault system remains poorly constrained, mainly because most tectonic structures, including the Montello thrust system, are buried beneath recent sediments, and the overall seismicity rate is generally low

At a depth of approximately 1.5 km within the Montello anticline, an Underground Gas Storage (UGS) facility is in operation. The site is continuously monitored by the National Institute of Oceanography and Applied Geophysics (OGS) through both regional and dedicated local seismic networks (Priolo et al., 2015). By collecting the seismological data acquired over the years from these two networks and other passive seismic experiments, Cipressi et al. (2025) recently compiled a new uniform seismic catalog for the area. It includes 4802 earthquakes (-0.9 ≤ ML ≤ 3.9) that occurred between 1977 and 2023, all relocated using the same code (NonLinLoc, by Lomax et al., 2001) and velocity model (Romano et al., 2019).

To better characterize the 3D seismic velocity structure of the area, a new velocity model was developed, based directly on laboratory measurements performed on rock samples representative of the local stratigraphic sequence. Through a fieldwork conducted in the study area, a total of 22 samples were collected and subjected to VP and VS measurements at ETH Zürich using the pulse-transmission method (Birch, 1960). Overall, the measurements were performed under varying confining pressures, during both loading and unloading phases, ranging from 5 MPa to 200 MPa to simulate different depth conditions.

The laboratory-derived values were scaled for better corresponding to the lithological volumes and implemented within a dedicated 3D geological model of the study area, based on the structural interpretation by Picotti et al. (2022) and constructed using Midland Valley’s 3D Move software. This approach allows for a detailed and physically constrained characterization of seismic velocities in the upper ~10 km of the crust, which represents the depth range most relevant for the UGS monitoring.

The newly developed 3D velocity model will be tested and validated by relocating the seismic events included in the updated seismic catalog (Cipressi et al., 2025). Through the analysis of the travel time residuals we will assess whether velocity models derived from geological and laboratory data can effectively constrain seismic velocities and improve earthquake locations. Ultimately, this approach may also help refine the current geological interpretation of the area and improve understanding of the seismic behaviour of the main seismogenic structures.

How to cite: Cipressi, G. M., Madonna, C., Picotti, V., and Romano, M. A.: Characterization of crustal physical properties in the Montello-Collalto area (eastern Southern Alps, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9399, https://doi.org/10.5194/egusphere-egu26-9399, 2026.

EGU26-9399

Download Close

The Hockai Fault Zone (HFZ) is a ~42 km-long intraplate fault system in eastern Belgium, located within the Rhenohercynian Zone of the Variscan orogenic belt. It has produced the strongest historical earthquake in the region (Mw 6.3, 1692 Verviers). It is also associated with clusters of slow-moving and reactivated landslides. Despite this relevance, the geometry of the HFZ, its terminations and its influence on near-surface mechanical properties remain insufficiently constrained.

In this study, we present a dense ambient noise dataset and ongoing quantitative modeling aimed at resolving site-specific seismic response and shallow subsurface structure. Multiple field campaigns were conducted along and across the HFZ using broadband (Guralp 6TD) and short-period (Lennartz LE-3D/5s) sensors. Single-station horizontal-to-vertical spectral ratio (HVSR) analysis was performed following established SESAME-type criteria, including time-window selection, stability tests, and frequency-dependent uncertainty assessment. The resulting HVSR curves display well-defined and spatially variable fundamental resonance frequencies, indicating strong lateral heterogeneity in near-surface conditions.

To quantitatively interpret these observations, HVSR curve inversion was initiated to derive 1D Vs models, constrained by local geological information. Preliminary results reveal pronounced impedance contrasts within the upper tens of meters, interpreted as the combined effect of weathered bedrock, sedimentary pockets, and fault-related damage zones. These velocity contrasts are expected to exert a first-order control on seismic amplification along the HFZ.

Ongoing HVSR inversions, constrained by local geology, reveal strong shear-wave velocity contrasts within the upper tens of meters, attributed to weathered bedrock, sedimentary pockets, and fault-related damage. This work demonstrates the effectiveness of passive seismic methods for site-response characterization in low-seismicity intraplate regions and provides new constraints relevant for seismic hazard and landslide assessment along the HFZ.

How to cite: Devi, S., Havenith, H.-B., Dorival, V., and Jordans, H.: Near-surface shear-wave velocity heterogeneity and site effects along the intraplate Hockai Fault Zone (Eastern Belgium) from ambient noise measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5215, https://doi.org/10.5194/egusphere-egu26-5215, 2026.

EGU26-5215

Download Close

Understanding the spatio-temporal evolution of seismicity is essential for unveiling the seismotectonic architecture of active regions, as it links earthquake occurrence with the geometry, kinematics, and origin of seismogenic processes.

We investigate persistent microseismic and moderate seismic activity in the central–northern Apennines (Italy) using the CLASS (Italian Absolute Seismic Catalogue), which is based on a 3D velocity model, and applying a template matching technique following seismic clustering. From an initial dataset of ~230.000 events, we analyse a subset of 69.875 seismic events (0.0 < M_L < 4.8) recorded between 2010 and 2023. Seismicity within the well-known 2016–2017 Amatrice–Visso–Norcia seismic sequence, the 2013–2015 Gubbio seismic activity and events classified as anthropogenic are excluded.

Seismic clusters are identified using the HDBSCAN algorithm, a hierarchical density-based clustering method that extends DBSCAN and is well suited for detecting clusters with variable density and shape in extensive spatial datasets. By introducing the temporal component, it is observed that HDBSCAN may produce clustering artefacts if applied to large datasets spanning long time intervals (14 years). To mitigate this effect, a Kernel Density Estimation is additionally applied to obtain more robust and well-defined spatio-temporal clusters. The analysis is performed by dividing the study area into six equal-area subregions and seven non-overlapping two-year time windows.

The resulting spatio-temporal clustering identifies 78 clusters, primarily classified as seismic swarms, distributed across the study area, with magnitudes up to M_L 4.8. Most clusters exhibit spatial patterns and focal mechanisms consistent with known active faults documented in the QUIN database (QUaternary fault strain INdicator). Conversely, three groups of clusters occur in the upper crust and align along an ~100 km-long arcuate trend between the foothills south of Bologna and the Apennines west of Pesaro. In this sector, lithological conditions may hinder fault outcropping, suggesting the presence of blind faults whose activity is expressed mainly at depth, near fault roots. These clusters refine the complex architecture of the extensional domain and may indicate previously unrecognized southwest-dipping blind normal faults, or structural complexities (e.g., synthetic or antithetic structures) within the basal detachment.

The envelope of the seismic clusters reveals that the front of the Apenninic extensional domain, hosting the most significant historical and instrumental earthquakes, extends eastward beyond the outcropping west–southwest-dipping normal faults. This finding has important implications for seismic hazard assessment in the densely populated foothill areas of the Northern Apennines and contributes to a better understanding of the architecture of low-angle normal fault systems.

How to cite: Di Gregorio, M., Vuan, A., Lelj, G., Talone, D., and Latorre, D.: Seismicity-Driven Insights into the Extensional Architecture of the Northern Apennines, Italy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14678, https://doi.org/10.5194/egusphere-egu26-14678, 2026.

EGU26-14678

Download Close

Resolving and characterizing the geometry and kinematics of blind thrusts is a primary challenge in active tectonic settings, notably where seismicity nucleates at depths beyond the resolution of industrial seismic reflection profiles and borehole data. In this context, the Emilian Thrust System represents a significant case study. As one of the three arcuate thrust fronts constituting the fold and thrust belt of the Northern Apennines (Italy), it exemplifies the complex interplay between deep-seated thrusting and shallower extension that drives crustal shortening in Plio–Quaternary basins. Despite this active deformation, the lack of surface constraints and the occurrence of seismicity at depths where standard geophysical imaging fails (20 – 25 km) create a critical knowledge gap.

This work aims to overcome these observational limitations by employing high-resolution microseismicity to decipher the hidden structural architecture of the arc. To address this, we performed a critical re-evaluation of the crustal velocity structure, as existing 1D and 3D regional models often provide discordant depth estimates, introducing significant uncertainties in hypocentral locations. By optimizing these models through the Velest algorithm, we were able to minimize depth location artifacts and better constrain the seismogenic volumes. Our new velocity model provided a robust basis for high-precision relocation via the NonLinLoc code. In order to isolate significant spatio–temporal clusters from the 2008–2024 background seismicity (0.4 ≤ M_L ≤ 5.1), we utilized Kernel Density Estimation and β-statistics. The resulting dataset, together with the 2024 Langhirano sequence (comprising over 350 events), were relocated using the updated velocity model. In addition, to further enhance the kinematic framework, we improved the completeness of the existing dataset by computing new focal mechanism solutions for events with 2.5 ≤ M_L ≤ 3.9 using the FPFIT software. Relocated hypocentral depths are primarily concentrated between 15 and 30 km, and focal mechanisms indicate kinematics ranging from compressional to strike‑slip.

Our results reveal that current seismicity is predominantly accommodated by a system of antithetic structures to the basal thrusts, spanning depths between 15 and 25 km. While the basal thrust remains largely seismically silent at these depths, the high-resolution definition of these previously unrecognized antithetic faults provides a novel perspective on the structural partitioning of the arc. Stress inversion results support this framework, indicating a prevailing compressive regime with a sub-horizontal σ₁ reflecting ongoing crustal shortening. These findings suggest complex seismotectonic behavior where moderate-to-small magnitude events illuminate secondary structures, potentially acting as a release for internal deformation within the wedge. This complexity is further evidenced by the S_Hmax orientation, which rotates from a N–S trend to approximately NE–SW in the proximity of the intersection between the Emilia and Ferrara arcs.

This integrated approach allows for a refined 3D characterization of blind active faults while offering a critical perspective on deep crustal features. Such results contribute to a better definition of the seismotectonic potential of the region, providing fundamental insights for seismic risk assessment in this strategic industrial and residential area.

How to cite: Lelj, G., Talone, D., and Latorre, D.: Seismotectonic insights into the Emilia Arc: high-resolution earthquake relocation and 3D characterization of the 2024 Langhirano sequence, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10069, https://doi.org/10.5194/egusphere-egu26-10069, 2026.

EGU26-10069

Download Close

The Bayan Har block, located in the central part of the Tibet Plateau, is a key region characterized by intense tectonic activity and frequent major earthquakes within the plateau. It also serves as a crucial pathway for the eastward extrusion of the Tibet Plateau and continental shortening deformation. Systematically characterizing the current deformation and strain distribution of this block holds significant scientific value for understanding the plateau's tectonic deformation mechanisms and potential seismic hazards. Utilizing Sentinel-1 satellite data from 2015-2025 and integrating GNSS data, we obtained high-resolution three-dimensional deformation and strain rate fields for the Bayan Har block. The results reveal that the east-west component of the 3D velocity field exhibits significant cross-fault velocity discontinuities and gradients near the East Kunlun Fault, Xianshuihe Fault, and some secondary faults, reflecting the dominant deformation features of the overall eastward escape of the Bayan Har block and its boundary strike-slip faults. The north-south component is relatively smooth, primarily reflecting block-scale differential motion and GNSS interpolation constraints. The vertical component is dominated by slow subsidence, with localized patchy anomalies closely related to non-tectonic signals such as permafrost, hydrology, and surface processes. Current strain in the Bayan Har block is significantly concentrated along its boundaries and several major strike-slip fault zones. High shear strain rate belts spatially coincide with large faults like the East Kunlun and Xianshuihe faults, while areal strain rates reveal a mixed tectonic environment dominated by compression around strike-slip faults, with localized extension. Given that the InSAR observation period includes postseismic recovery processes from strong earthquakes such as the 2001 Kunlun Mountains and 2021 Maduo events, the high strain rates and pronounced cross-fault gradients along the faults reflect the combined effects of transient postseismic deformation and interseismic steady-state locking.

How to cite: Qu, C. and Chen, H.: Observation and Study on High Resolution Deformation and Strain Field Characteristics of the Bayan Har Block in the Central Tibetan Plateau, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5281, https://doi.org/10.5194/egusphere-egu26-5281, 2026.

EGU26-5281

Download Close

Fault systems are inherently heterogeneous, with barriers and asperities exerting a first-order control on rupture propagation and on the spatio-temporal distribution of seismicity. The persistence of long-lived seismicity raises questions about whether earthquake activity is primarily governed by local structural complexities or by large-scale tectonic loading acting on simplified, homogeneous fault surfaces.

In this study, we report persistent mid-crustal seismicity in Eastern-Central Italy along the Adriatic Basal Thrust (ABT), a major compressional structure deepening westward from the Adriatic offshore to the Apennine Foothills. Its 3D geometric and kinematic architecture was reconstructed combining geological information and a high-resolution seismological dataset of relocated earthquakes and focal mechanisms (de Nardis et al., 2022). Specifically, the seismic catalogue was refined using recordings from the ReSIICO seismic network and a 3D velocity model (Cattaneo et al., 2019). The ABT extends ~210 km along strike and dips at ~20°, with its main internal splay corresponding to the Near Coast Thrust (NCT). Seismicity is unevenly distributed; while the northern sector hosts instrumental earthquakes mainly at upper crustal levels, the southern sector appears locked at the surface but accounts for ~75% of the total deep seismicity, dominated by low-magnitude events (M_L mode ~0.8–0.9).

To extend the temporal perspective, we analyzed the Italian seismic catalogue over 40 years (1985–2024) (https://terremoti.ingv.it/). Fractal analysis and space–time clustering identify three persistent seismicity clusters: two shallow clusters likely related to anthropogenic processes (i.e., quarry blasts) and a third, dominant cluster consistently associated with the ABT. The spatio-temporal analysis reveals that within this tectonic cluster, ~76% of seismicity consists of non-triggered events representing background tectonic loading, with only a few moderate episodes of spatio-temporal clustering.

The remarkable long-term persistence of this activity prompted a deeper investigation into the underlying fault architecture through the high-resolution seismic catalogue. This analysis revealed that the seismicity highlights a complex structural duplex acting as a geometric asperity in the linkage zone between the ABT and the internal splay. This mid-crustal segment consists of two low-angle, west-dipping splays interconnected by high-angle ramps, forming a structural knot that hinders smooth slip. Overall, the spatial persistence, depth distribution, and geometric complexity of the microseismicity indicate that fault-scale heterogeneities and structural jams dominate over large-scale regional coupling. This implies that the continuous release of seismic energy within these complex structural nodes acts as a mechanical accommodation process, effectively controlling the segmentation and the maximum rupture potential of the entire fault system.

Cattaneo, M., Frapiccini, M., Ladina, C., Marzorati, S. & Monachesi, G. A mixed automatic-manual seismic catalog for Central-Eastern Italy: Analysis of homogeneity. Ann. Geophys. (2017).

de Nardis, R., Pandolfi, C., Cattaneo, M. et al. Lithospheric double shear zone unveiled by microseismicity in a region of slow deformation. Sci Rep 12, 21066 (2022).

How to cite: de Nardis, R. and Lavecchia, G.: Persistent Seismicity in Eastern-Central Italy: Evidence for a Complex Structural Asperity Dominating Mid-Crustal Deformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13568, https://doi.org/10.5194/egusphere-egu26-13568, 2026.

EGU26-13568

Download Close

The Africa–Eurasia plate boundary extends along the southern Tyrrhenian Sea in the Sicilian offshore, representing a tectonically complex region mainly characterized by compressional to transpressional regime. Deformation is unevenly distributed along the margin, and seismicity is predominantly characterized by low-to-moderate magnitude earthquakes. The large offshore extent of the area, combined with locally unfavorable seismic network geometry, often limits the resolution of traditional seismological analyses and hampers robust seismic source characterization. In this study, we present an integrated analysis of recent seismicity along the southern Italy segment of the Africa–Eurasia plate boundary, aimed at improving the characterization of active seismic sources and their kinematics through advanced, multi-method seismological approaches. Our investigation includes (i) a regional-scale clustering analysis of earthquakes recorded between 2010 and 2025, and (ii) a detailed characterization of a recent offshore seismic sequence in the southeastern Tyrrhenian Sea. At the regional scale, we apply a density-based spatial clustering algorithm using a space–time distance metric to a high-resolution relocated earthquake catalog. Seismic clusters are subsequently classified as swarm-type or mainshock–aftershock sequences using statistical descriptors of the seismic moment distribution over time. This analysis allows us to identify spatial variations in seismic release patterns and to infer differences in fault segmentation, loading conditions, and stress transfer along the plate boundary. At the local scale, we focus on a Mw 4.7 offshore earthquake sequence and propose an integrated workflow specifically designed to enhance seismic source characterization in offshore environments. The methodology combines Bayesian absolute hypocenter location, machine-learning-based phase picking and event detection, distance geometry solvers for relative relocation, and probabilistic moment tensor inversion. This approach resolves source geometry, fault orientation, and slip kinematics despite non-optimal network conditions, providing robust constraints on active fault planes. Overall, our results demonstrate that advanced, integrated seismological methods significantly improve the characterization of active seismic sources along the Africa–Eurasia plate boundary, offering new insights into fault behavior and deformation processes in offshore and structurally complex regions.

How to cite: Totaro, C., Mancuso, T., Cesca, S., Grigoli, F., Presti, D., and Orecchio, B.: Characterizing Low-to-Moderate Magnitude Earthquake Sequences and Seismic Sources Along the Africa–Eurasia Plate Boundary in Southern Italy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18739, https://doi.org/10.5194/egusphere-egu26-18739, 2026.

EGU26-18739

Download Close

Obtaining reliable moment tensor (MT) solutions for earthquakes is particularly challenging due to their strong dependence on station geometry, accurate hypocentral locations, and a well-constrained seismic velocity model. The estimation of seismic moment and magnitude, as well as the decomposition of the source mechanism into double-couple (DC), isotropic (ISO), and compensated linear vector dipole (CLVD) components, strongly depend on the assumed velocity model, which also controls the minimum resolvable magnitude. The limited availability of detailed three-dimensional crustal models often restricts MT inversions to low-frequency data, reducing resolution and negatively affecting both source parameter accuracy and inversion stability.

Recent improvements in seismic network coverage and instrument sensitivity have increased the resolving power, leading to a growing demand for MT solutions of progressively lower-magnitude earthquakes. This evolution imposes stricter requirements on the accuracy of 3D velocity models, which must properly represent small-scale heterogeneities, attenuation, and seismic anisotropy. In this context, Empirical Green’s Functions (EGFs) provide a practical approach to reduce the impact of simplified velocity models, empirically incorporating path and site effects, and improving high-frequency waveform fits.

In this study, we propose a methodological approach for earthquake MT inversion that includes EGFs into time-domain waveform inversion using the ISOLA code (Zahradník and Sokos, 2018). The methodology is based on the concept introduced by Plicka and Zahradník (1998), which enables the estimation of spatial derivatives of the EGF tensor directly from seismic observations, without requiring an a priori similarity among the focal mechanisms of weak earthquakes. Within this conceptual framework, a selected set of well-recorded small earthquakes within the same focal volume is first inverted for MTs using a standard waveform inversion procedure. These independently obtained MT solutions are combined with the corresponding observed waveforms to retrieve empirical Green’s tensor spatial derivatives, which are subsequently used to invert other earthquakes occurring in the same source region.

Within this framework, the use of EGFs significantly reduces modeling errors associated with simplified velocity structures and unresolved small-scale heterogeneities, while preserving sufficient resolution capability to extend MT analysis toward lower-magnitude earthquakes. The ISOLA code further enables systematic exploration of source parameters, quantitative assessment of solution quality through variance reduction and stability analysis, and consistent comparison among different inversion setups, providing an additional criterion for evaluating the reliability of the obtained solutions.

The proposed methodology is applied to the 2024–2025 seismic crisis at Campi Flegrei, a volcanically active area in Southern Italy, characterized by strong lateral heterogeneity and complex wave propagation effects. This dataset provides a representative test case to evaluate and validate the robustness of this approach under challenging geological and observational conditions, where complex rupture processes may be influenced by crustal fluids.

References

Plicka, V., and J. Zahradník (1998). Inverting seismograms of weak events for empirical Green’s tensor derivatives, Geophys. J. Int. 132, 471–478.

Zahradník, J., & Sokos, E. (2018). ISOLA code for multiple-point source modeling. In Moment tensor solutions: A useful tool for seismotectonics (pp. 1-28). Cham: Springer International Publishing.

How to cite: Susini, A., Adinolfi, G. M., Guinez-Rivas, F., Talone, D., and Vinciguerra, S. C.: Moment Tensor Inversion Using Empirical Green’s Functions: a Methodological Approach in Complex Media for Seismotectonic and Volcanic Studies, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14680, https://doi.org/10.5194/egusphere-egu26-14680, 2026.

EGU26-14680

Download Close

We present the finite element neotectonic dynamic modelling of crustal deformation for the Caucasus region based on the GPS observations, seismicity and main fault configurations. The data obtained from crustal deformation monitoring made using GPS systems in Azerbaijan, Georgia, Turkiye, Iran and Armenia aggregated and used to determine the dynamics of the main tectonic structures. Over 215 continuous and survey mode GPS site velocities were collected from several published papers, analyzed and after a careful filtration process were involved in modelling. The traces and parameters of main active faults in the region were obtained from different open access data bases in order to constrain more accurate and solid model for the analysis. The World Stress Map database released in 2025 used to take into account the regional seismicity and to calculate the fault slip rates, strain and stress directions associated with main seismic events. Our model shows that the high accumulation of strain is predominantly concentrated along the southeastern part of the Greater Caucasus Trust Belt, eastern part of Kur depression and Absheron peninsula. Relatively low strain accumulation is observed in Lesser Caucasus. Although, there are some indications of significant strain along other main subparallel faults in the region, the large majority of the Arabia-Eurasia convergence is accommodated by the lateral movement of the crust. Since earthquakes are known usually to occur in areas of very low strain rates, it is difficult to quantify hazards in such cases. However, with auxiliary information from paleoseismology and geomorphology will possibly help to constrain better models.

How to cite: Safarov, R., Gadirov (Kadirov), F., Carafa, M., and Mammadov, S.: Kinematic modeling of crustal deformation in the Caucasus territory, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16931, https://doi.org/10.5194/egusphere-egu26-16931, 2026.

EGU26-16931

Download Close

In northern Umbria (central Italy), the region between the Tiber Valley, Gubbio, and the main Apennine ridge is affected by persistent microseismicity (M_L < 3.0), occurring at an average rate of ~3 events per day. A significant portion of this activity is associated with the Alto Tiberina Fault (ATF), a ~60 km-long, low-angle normal fault that has been active since the Late Pliocene–Early Pleistocene. Within this tectonic framework, we analyse seven low-magnitude seismic sequences (M_L < 4.5) that occurred between 2010 and 2023 within the ATF hanging wall. These sequences are not linked to surface-exposed faults, raising questions about the nature and distribution of the seismogenic sources.

The main objectives of this study are to: (1) determine whether the observed seismicity is concentrated along discrete fault planes or instead distributed within fractured rock volumes; and (2) define the geometry and kinematics of the causative faults and assess their correspondence with structures imaged in available 2D seismic reflection profiles. Earthquakes were relocated using a high-resolution 3D velocity model and projected onto depth-converted seismic reflection sections.  Consequently, this work presents a methodological framework for analyzing low-magnitude seismic sequences by integrating active and passive seismic data.

Our results indicate that most ruptures occurred on high-angle normal faults that branch upward from the ATF detachment. The geometry of these faults is consistently constrained by both the depth distribution of relocated seismicity and the corresponding reflectors imaged in the seismic profiles, while their kinematic behaviour is compatible with that inferred for the mainshocks. The aftershock areas range from ~1 to 15 km², suggesting that the mainshocks ruptured only limited portions of larger fault segments. Additionally, the behaviour of these minor sequences, particularly in terms of rupture localization and aftershock spatial patterns, closely mirrors that observed for higher-magnitude sequences in the same region, indicating that similar seismotectonic processes operate across different magnitude scales.

How to cite: Riva, F., Marzorati, S., Latorre, D., and Barchi, M. R.: Assessing fault-earthquake relationships for low-grade seismic sequences (ML<4.5): examples from the extensional belt of central Italy, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9814, https://doi.org/10.5194/egusphere-egu26-9814, 2026.

EGU26-9814

Download Close

The junction between the central and southern Apennines represents a high-seismic-hazard region in the Mediterranean. Its seismotectonic setting is characterized by a complex, poorly understood interplay between SW-NE regional extension along the range axis and E-W mid-to-lower crustal shear zones in the Adria plate to the east. Although the range axis hosted several M6-7 historical earthquakes, their causative faults remain mostly debated. Monitoring by the Italian National Seismic Network (Rete Sismica Nazionale, RSN), with a station spacing of 10-30 km and a detection threshold of about M_L1.2 in the region, has proved insufficient to pinpoint and fully characterize source faults for recent low-to-moderate magnitude (M < 4) sequences.

To address these limitations, we conducted the first comprehensive study of background seismicity as part of the MOSAICMO project, an inter-disciplinary initiative investigating tectonic evolution and seismogenesis of this region. This study integrates a 2-year passive seismic experiment (2023-2025) with a re-analysis of the 2016-2022 RSN seismicity. Our objectives were to improve knowledge of the active faults and relationship between seismogenesis and physical properties of the crustal rocks. The seismic experiment integrated 13 temporary stations with 20 permanent stations of the RSN over an area of 60x60 km², reducing station spacing to 4-12 km. Initial analysis of the first nine months of the new dataset using a standard STA/LTA algorithm identified 470 events (0.2 < M_L < 2.8), representing a 220% increase over the RSN catalog. For these earthquakes, P-and S-phases were manually picked. For the 2016-2022 seismicity, we revised and augmented the phase picks for 1,400 selected events and applied cross-correlation template matching to a prolonged swarm-like sequence (2016-2017; Mw 4.3) to produce a high-resolution catalog.

We utilized these phase picks to construct catalogs, through: i) absolute locations using the probabilistic location software NonLinLoc and a new optimized 1D velocity model, based on a non-linear approach ii) high-precision relative locations using the double-difference technique HypoDD; iii) absolute 3D re-locations alongside with Vp and Vp/Vs crustal models derived from Local Earthquake Tomography on a 3 × 3 × 2 km grid.

Our results show that seismicity deepens eastward, from 3–12 km beneath the inner range to 15–22 km under the outer range. While the upper crust exhibits mixed extensional and strike-slip focal mechanisms, deeper eastern events are almost exclusively strike-slip. Most seismicity occurs in small, short-lived clusters. Along the inner range, seismicity concentrates at 5-10 km depth within high-Vp (6.0-6.7 km/s), low-Vp/Vs (1.70-1.85) zones. Here, high-precision relocations reveal NW-striking, NE-dipping alignments consistent with known Quaternary normal faults. Integrating these results with a subsurface geological model based on seismic commercial profiles and exploration wells, and a 2D magnetotelluric tomography, we find that: (i) axial seismicity is mainly hosted within the high-velocity, high-resistivity Mesozoic carbonates of the Apulia Platform, (ii) the 2016–2017 swarm-like seismicity also clusters within the Apulian Platform but correlates with a low-resistivity anomaly, suggesting a fluid-driven seismogenic mechanism.

How to cite: Improta, L., Bagh, S., Latorre, D., Marchetti, A., De Gori, P., Valoroso, L., Lucente, F. P., Riccio, G., Pucillo, S., Cogliano, R., Criscuoli, F., Buttinelli, M., Maesano, F., Maffucci, R., Vico, G., Romano, G., Siniscalchi, A., Khasi, R., and De Martini, P. M.: Seismotectonics of the Central-Southern Apennines Junction (Italy): New Insights from High-Quality Background Seismicity Data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20203, https://doi.org/10.5194/egusphere-egu26-20203, 2026.

EGU26-20203

Download Close

The Swabian Jura near the town Albstadt is one of the seismically most active regions of Germany. Concerning tectonics, the region is characterized by a NW-SE striking shallow (<2-3 km) aseismic graben structure, the Hohenzollerngraben (HZG), and the seismically active Albstadt Shear Zone (ASZ), a NNE-SSW striking sinistral strike-slip fault zone at about 1-18 km depth. The ASZ has an extension of at least 50 km, but there is no evidence for surface rupture. Beside the continuous low-magnitude seismic activity, in the 20^th century eight earthquakes with ML>5.0 occurred causing significant damage in the region of the Swabian Jura.

Here, we search for and then analyze very low-magnitude earthquake sequences during 2018 to 2020 in the area of the ASZ to image the seismically active faults. We apply a template matching detection routine, determine relative event locations for the identified earthquake sequences, calculate fault plane solutions based on first motion polarities and finally moment tensor solutions of earthquakes with ML greater than 3.5.

We identified six earthquake sequences and image three types of seismically active faults in the area of the town Albstadt. First, the known ASZ, with NNE-SSW striking sinistral strike-slip faulting at 5-10 km depth. Second, a so far not observed NW-SE striking dextral strike-slip fault at 11-15 km depth, beneath the HZG. A continuation with depth of the HZG surface faults is unlikely, but the co-location of the HZG and the NW-SE striking fault may indicate an inherited zone of weakness below the HZG. And finally, complex faulting in form of NNW-SSE striking sinistral strike-slip and normal faulting in 9-12 km depth indicating a heterogeneous deformation zone at the intersection of the ASZ and the newly discovered NW-SE striking fault zone.

Our results go into a revised seismotectonic model for the area of the ASZ, including two new types of seismically active faults in the area.

How to cite: Moser, S., Joachim, R., and Andrea, B.: A revised seismotectonic model for the Albstadt Shear Zone, Southwest Germany, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19561, https://doi.org/10.5194/egusphere-egu26-19561, 2026.

EGU26-19561

Download Close

The interpretation of deep 2D seismic profiles from the central Adriatic foreland of the External Dinarides in the area of the islands of Vis and Jabuka (Croatia), reveals a complex Mesozoic platform-to-basin architecture, and Cenozoic structural and sedimentary system developed on top of the central part of the Adriatic microplate (Adria). Tectonic subsidence and thick Paleogene to Neogene sedimentary loading in the latest Dinaric foredeep probably initially mobilized buried Middle Triassic evaporites from the proximal to distal foreland. Miocene tectonic is characterized by basement-rooted positive flower structures, pop-up blocks, and upward-diverging fault splays, diagnostic of a transpressional tectonic regime.

The crustal-scale Quaternary subvertical faults without apparent vertical throw are associated with positive and negative structures along the strike. In the overlying sedimentary cover, localized normal faulting and extensional arrays overprint transpressional structures, interpreted as gravitational collapse above pop-up blocks, roof collapse above ascending diapirs, and lateral collapse within a mechanically decoupled cover. The positive structures are associated with the Quaternary salt diapirs, some of which are still active. However, it is not clear which faults are inducing regional seismicity.

Instrumental seismicity is moderate to strong (up to M>5), shallow (≈5–15 km) and spatially clustered around the diapiric structures. Focal‑mechanism solutions predominantly indicate reverse to reverse–oblique faulting, yet the nodal planes do not clearly coincide with any single reverse fault imaged on 2D profiles, and many hypocenters project within or immediately above active salt diapirs. These observations suggest that salt diapirs act as mechanical and geometric controllers that focus stress and localize brittle failure on surrounding basement‑rooted faults, rather than being the primary source of seismic energy, which is difficult to reconcile with the seismic moment of M>5 events if salt flow alone were responsible.

Active salt structures are characterized by long stems and relatively small surface expressions that are aligned along Quaternary faults. Their geometry, disconnection with original depth of the Triassic evaporites, and limited lateral extent, indicate tectonic extrusion of deep evaporites. Variations and segmentation along strike, suggest localized strain and strong structural control on diapir rise. Overall, these observations indicate that diapir growth and surface expression are controlled by the interaction between deep shear zones, active faulting, and a mechanically decoupled overburden. Within this framework, seismicity reflects the interaction between deep shear zones, evaporite mobilization and upper‑crustal faulting, highlighting the need to re‑evaluate focal mechanisms with improved 3D velocity models and to explicitly incorporate salt‑controlled structures into seismic‑hazard assessments for the region.

“This work was supported by Croatian Science Foundation project SALTECTA (HRZZ-IP-2024-05-2957).”

Keywords: Central Adriatic Sea, 2D seismic profiles, Transpressional deformation, Salt diapirs, Active tectonics, Seismicity.

How to cite: Neji, C., Korbar, T., Rukavina, D., Markusic, S., Kamenski, A., and Alves, T.: Structural and kinematic controls on basement-influenced salt diapir geometries in the central Adriatic: Insights from 2D seismic profiles., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18590, https://doi.org/10.5194/egusphere-egu26-18590, 2026.

EGU26-18590

Download Close

This work aims to perform ground motion simulations using a simplified approach that allows fast yet accurate estimation of intensity measures (PGA, PGV, SA). The approach presented can be applied a few minutes after a strong earthquake, when knowledge of source parameters is still limited, or during the pre-emergency phase, contributing to more effective territorial planning. A similar goal can be achieved using physics-based models that account for source uncertainty. However, due to the limited time and data available immediately after an earthquake, physics-based models are not suitable for urgent computing (Stallone et al., 2025). The proposed method is based on two Python codes: HypoSmoothFaultSimulation (Di Naccio et al., 2025), a soon-to-be-released open-access software, which generates an ensemble of rupture scenarios starting from geometric and kinematic properties of the fault (length, strike, dip, depth, rake), and seismotectonic potential (magnitude). The second software, ProbShakemap (Stallone et al., 2025) computes ground shaking at different points of interest (POIs) by implementing one or more Ground Motion Models (GMMs), starting from the plausible hypocenters generated by HypoSmoothFaultSimulation. The latter code accounts for source parameter uncertainty by defining smoothed boxcar probability density functions (PDFs), which are subsequently sampled to generate the rupture scenarios. ProbShakemap accounts for both source-related and GMM-related uncertainties, producing multiple ground-shaking estimates for each POI. As a case study, the method was applied to the central Apennines, focusing on a representative sample of faults, by computing PGA maps on a regular grid, or at the location of RSN and RAN seismic stations. For the same sample of faults, the stochastic code EXSIM (Motazedian and Atkinson, 2005), which requires more detailed knowledge of source parameters and wave propagation effects, was also applied. These comparisons aim to highlight the differences between the proposed method and more complex physics-based models. It should be noted that the proposed method cannot provide reliable ground motion estimates in the near field, due to source-related effects such as velocity pulses, large peak accelerations and the effect of the vertical component, which strongly influence ground shaking close to the fault. However, the method is applicable in the intermediate field, which is still characterized by significant ground shaking during large earthquakes. Overall, this approach allows ground motion estimates to be obtained from a limited number of initial parameters while accounting for their associated uncertainty, enabling fast and simplified computation suitable for application before or immediately after a strong earthquake.

Bibliography

• DI NACCIO, Deborah; STALLONE, Angela; MC CARAFA, Michele. The Mt. Morrone seismotectonic source: analysis of fault model uncertainty for Ground Motion Prediction. In: EGU General Assembly Conference Abstracts. 2025. p. EGU25-12632.
• Motazedian, D., Atkinson, 2005. Stochastic Finite-Fault Modeling Based on a Dynamic Corner Frequency. Bull. Seismol. Soc. Am. 95, 995–1010. https://doi.org/10.1785/0120030207
• STALLONE, Angela, et al. ProbShakemap: A Python toolbox propagating source uncertainty to ground motion prediction for urgent computing applications. Computers & Geosciences, 2025, 195: 105748.

How to cite: Garofalo, A., Talone, D., Di Naccio, D., Stallone, A., and Carafa, M. M. C.: Influence of Source Representation on Damage Scenarios: Comparison Between Point and Finite Sources in the Intermediate Field, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13980, https://doi.org/10.5194/egusphere-egu26-13980, 2026.

EGU26-13980

Download Close

The Adria microplate represents the main geodynamic driver in the central Mediterranean, and its interaction with the surrounding plates controls the distribution of stress, strain and seismicity across the adjacent domains. In this context, the geometry and thermal structure of the lithosphere play a key role in partitioning the deformation across the Tyrrhenian–Apennines–Adriatic system. However, these properties remain poorly constrained by direct observations. Here, we address this problem through thermo-petrological forward modelling constrained by geophysical data aimed at quantifying lateral variations in lower-crustal seismic velocities.

The modelling was performed along a profile across the central Apennines, constructed using a structural and density model of the crust and upper mantle. The profile was sampled at multiple points to derive geothermal and lithostatic gradients from heat-flow and density data, thereby constraining pressure-temperature conditions along the section. Moho depth and its associated uncertainties were incorporated into the pressure-temperature estimates.

We adopted pyroxenite, peridotite, and metagabbro samples from well-exposed natural analogues as proxies for the lower crust and upper mantle of the Adria lithosphere. For each lithology, stable mineral assemblages, phase proportions, elastic properties and seismic velocities were computed as a function of pressure and temperature using the thermodynamic and elastic modelling code Perple_X (Connolly, 2005).

Calculations were performed using a mantle-oriented thermodynamic database and complemented by a sensitivity test based on an alternative parametrization optimized for crustal petrology, to quantify how differences in thermodynamic databases affect phase assemblages and the resulting seismic velocities.

Modelled P- and S- wave velocities were compared with independent laboratory measurements on representative rocks and with regional seismic tomography to assess the consistency between mineral assemblages, seismic velocities and independent constraints, indicating that the adopted thermo-petrological structure provides a realistic representation of the Adria lower crust and upper mantle.

References

Connolly JAD (2005). Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters 236:524-541.

How to cite: Amoroso, F., Kastelic, V., Carafa, M. M. C., Lopez Suarez, M. C., Vinciguerra, S. C., Santarelli, B., and Zanetti, A.: Thermo-petrological constraints on seismic velocities of the Adria lower crust, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19789, https://doi.org/10.5194/egusphere-egu26-19789, 2026.

EGU26-19789

Download Close