This session invites interdisciplinary contributions that address the mechanics controlling seismicity and fault deformation in subduction and collisional settings. We welcome studies that integrate seismological, geodetic, and modelling approaches to address key questions including: (i) what physical processes control seismicity patterns and fault behaviour across different depths and tectonic settings?; (ii) How do stress interactions, rheology, fluids, climate and surface processes drive the spatial and temporal evolution of seismicity?; (iii) How can multi-scale observations, from high-resolution geophysics to paleoseismology, improve our understanding of active fault systems and short- to long-term seismic hazard assessments?
By bridging insights from different convergent margins, this session aims to advance our understanding of earthquake generation and the factors shaping seismic hazard worldwide.
Orals: Wed, 6 May, 14:00–15:45 | Room D1
Three models have been proposed to explain the downdip limit of the subduction seismogenic zone. The first is a temperature-controlled transition in rate-and-state frictional properties between 350-510°C, which inhibits earthquake nucleation. The second places the limit at the frictional and viscous failure envelope intersection. The third combines thermal and lithological controls, where ‘warm’ subduction zones are controlled by a 350°C frictional transition and ‘cold’ subduction zones are limited by the overriding plate Moho. To evaluate these hypotheses, we integrate thermal models with seismicity catalogs from 17 subduction zones. Observed depth limits remain remarkably consistent (~50 km) across a temperature range exceeding 250°C, indicating that the temperature-controlled rate-and-state friction model cannot fully explain observed depths. While warm subduction zones can be reasonably explained as a rate-and-state stability transition, the overriding plate Moho in cold subduction zones is too shallow, challenging the combined thermal-lithological model. To test the frictional-viscous model, we analyze power law creep and low-temperature plasticity for quartz, feldspar, olivine, antigorite, and talc. We find that power law creep in any tested mineral is overly temperature sensitive. In contrast, wet olivine, antigorite, and talc low-temperature plasticity fits observed depth limits to a ~6 km misfit. However, only talc is consistent with the weak megathrust paradigm of effective friction coefficients <0.1 and shear strengths of tens of MPa. We conclude that a frictional-viscous transition with a weak and temperature-insensitive viscous mechanism, such as talc low-temperature plasticity, is most consistent with the downdip seismicity limit and constraints on megathrust strength.
How to cite: Moser, L., Cattania, C., and Peč, M.: Temperature insensitive viscous deformation limits megathrust seismogenesis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-789, https://doi.org/10.5194/egusphere-egu26-789, 2026.
Subduction zones host the majority of global earthquakes, spanning shallow megathrust events, outer-rise earthquakes, and deep intraplate seismicity within subducting slabs. Although earthquakes form narrow, coherent belts in map view, their three-dimensional spatial distributions exhibit complex, case-dependent patterns when depth is considered. The physical processes governing these patterns, particularly for deep earthquakes, remain incompletely understood.
In this study, we develop realistic three-dimensional spherical geodynamic models constrained by multiple geophysical datasets to investigate long-term slab dynamics across multiple subduction zones worldwide. By comparing modeled slab deformation with global earthquake distributions, we identify a coherent spatial correlation between the deformation rate predicted by the models and the observed distribution of seismicity within subducting slabs. Regions of strong long-term deformation systematically coincide with zones of concentrated deep seismicity, whereas areas of weak deformation are characterized by sparse earthquake occurrence.
These results indicate that large-scale slab dynamics exert a first-order control on the spatial distribution of deep intraplate seismicity, providing a dynamics-based framework for interpreting global earthquake patterns.
How to cite: Li, Y. and Ribe, N.: From Slab Dynamics to Seismicity: A Global Perspective, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11619, https://doi.org/10.5194/egusphere-egu26-11619, 2026.
Subduction zones host the largest seismogenic zones on earth and hence the largest earthquakes. However, although some subduction margins generate some of the most destructive earthquakes (e.g., Japan, Sumatra), others appear to slip less dramatically in slow slip events, by aseismic creep or in small-moderate earthquakes (e.g., north Hikurangi). Subduction interface topography (‘roughness’) has emerged as a leading parameter in controlling the seismicity at subduction zones, although there is strong debate as to whether rough patches are asperities or barriers to large rupture. This issue persists because observational studies are limited to individual margins, seafloor bathymetry used as proxy for plate interface topography is not direct measurement, and historical earthquake record is short, which together make the precise assessment of earthquake potential in a subduction margin challenging. Here we test whether geodetic interplate coupling is an indicator of earthquake potential in lieu of a longer historical records. We then use direct seismic reflection observations from 35 plate boundary faults to quantify three types of roughness at 1–10 km length scales. We find a strong and positive relationship between maximum magnitude and interplate coupling. Strikingly, no relationship is observed between any of the roughness parameters and maximum earthquake magnitudes/interplate coupling. This result challenges the long-standing paradigm that the plate interface roughness is a pivotal factor in governing seismogenic behaviour. We suggest that short-wavelength (£10 km) roughness has different effects on earthquake nucleation depending on the prevailing stage of earthquake cycle. We conclude that plate roughness alone is not a good proxy to assess a margin’s seismic potential. Instead, interplate coupling provides a better indicator of seismic potential, highlighting the need for enhanced marine geodetic observations.
How to cite: Yang, X., Bell, R., Whittaker, A., Xu, H., Han, X., Knowlson, A., and Locher, V.: Rough or smooth plate interface? It doesn’t matter when it comes to great earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8635, https://doi.org/10.5194/egusphere-egu26-8635, 2026.
Standard models of lithospheric strength indicate an increase in frictional fault strength with depth. The dependence suggests that also earthquake stress-drop (Δσ) values may increase with depth, if the stress release scales with the stress on the fault. However, the range of uncertainty in Δσ values and the lack of constraints on stress in the lithosphere make it difficult to establish how stress drop, fault strength and depth are related. Here we present the main outcomes of a recent study (Bocchini et al., 2025), in which we investigated the Δσ dependence on depth and fault strength based on 11 years of seismicity in the Northeastern Japan Arc following the 2011 M9 Tohoku-Oki megathrust earthquake. We show that Δσ values increase with depth within the seismically active upper 60 km of the lithosphere by about 0.08 MPa/km. Furthermore, a comparison of the Δσ values with quantitative fault-strength estimates from finite-element models reveals that the Δσ values systematically increase with fault strength and that earthquakes within the study region release, on average, 10-30 % of the shear stress on the fault. Our results support the hypothesis that stress drop increases with fault strength, but also show that fault strength increases significantly less with depth than in standard models. Our findings further imply that temporal variations in average Δσ values may reflect changes in fault strength. In northeastern Japan, Δσ values remained roughly constant in the decade following the Tohoku-Oki earthquake, suggesting only small changes in fault strength since the mainshock.
Bocchini, G.M., Dielforder, A., Kemna, K.B., Harrington, R.M., Cochran, E.S. (2025). Earthquake stress-drop values delineate spatial variations in maximum shear stress in the Japanese forearc lithosphere. Communications earth & environment 6. https://doi.org/10.1038/s43247-025-02877-y
How to cite: Dielforder, A., Bocchini, G. M., Harrington, R. M., and Cochran, E. S.: Average earthquake stress-drop values delineate variations in fault strength in the Northeastern Japan Arc, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2142, https://doi.org/10.5194/egusphere-egu26-2142, 2026.
The central Apennines have experienced several destructive normal-faulting earthquakes in the last decade, but fundamental questions about the tectonic mechanisms driving extension persist. Multiple mechanisms have been proposed, including differences in gravitational potential energy (GPE), independent motion of the Adriatic plate, and large-scale uplift following slab detachment. In terms of structure, debates continue about whether the slab has detached and whether the continental Mohos overlap. However, none of these hypotheses have been tested through self-consistent geodynamic modeling. We employ 2D instantaneous seismo-thermo-mechanical models with a visco-elasto-plastic rheology and a strongly slip-rate dependent friction. We systematically explore different lithospheric structures, rheologies and forcings to test these hypotheses and identify the key driving mechanisms of surface deformation and seismicity in the central Apennines.
Our results confirm that the slab beneath the central Apennines is detached: only a detached slab reproduces normal-faulting earthquakes in the orogen and a gradual increase of horizontal surface velocities up to 3 mm/yr. An attached slab instead produces strong compression and vertical motions inconsistent with observations. The primary driver of extension is Adriatic plate motion, which accounts for approximately two-third of the horizontal surface velocities. The secondary driver is eduction of subducted upper crust, which contributes to approximately one-third of the horizontal surface velocities and facilitates decoupling between the Adriatic and Tyrrhenian plates. On the contrary, differences in GPE arising from topography only have a minor contribution to extension and seismicity. Density differences up to the lithosphere-asthenosphere boundary do play a significant role as it controls upper crust eduction. Lower- and upper crust rheology also control the occurrence and intensity of eduction, thereby affecting plate coupling and seismicity. Additionally, lower crust viscosity of the plate contact area strongly modulates the transfer of deep velocities to the surface, and thereby controls the location of highest surface velocity gradient and seismicity. Hence, our results show that deep structures, rheologies, temperatures and processes have a large control over the location and intensity of crustal seismicity. By refining the geodynamic structure and deciphering the tectonic drivers of seismicity, this study advances the understanding of Apennine geodynamics and seismicity.
How to cite: Fonteijn, M., Pathier, E., Socquet, A., and van Dinther, Y.: What drives extension and seismicity in the central Apennines (Italy)? Insights from 2D seismo-thermo-mechanical modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20670, https://doi.org/10.5194/egusphere-egu26-20670, 2026.
Upper-plate aftershocks following megathrust earthquakes can pose severe time-dependent hazard because they often occur close to densely populated regions, increasing the risk to structures already weakened by the mainshock. Although aftershock rates commonly follow Omori–Utsu temporal decay, the physical mechanisms controlling their non-linear time dependency and the diversity of faulting styles in the upper plate remain unclear. Because coseismic static stress transfer cannot explain this time dependency, transient postseismic processes — afterslip, viscoelastic relaxation, and fluid-driven pore-pressure diffusion — are potential candidates.
Here, we combine comprehensive seismological and geodetic observations with a 4D (space–time) hydro-mechanical numerical model to identify the dominant postseismic stress-change process controlling upper-plate aftershocks of the 2014 Mw 8.2 Iquique megathrust earthquake in northern Chile. We reproduce GNSS-observed postseismic deformation during the first nine months and separate the contributions from afterslip, viscoelastic relaxation, and poroelastic deformation in both horizontal and vertical components. In particular, poroelastic deformation contributes substantially to the near-field vertical signal. We then compute spatiotemporal Coulomb Failure Stress (CFS) changes for each process and compare them to the distribution of upper-plate aftershocks.
Our results show that CFS changes driven by coseismically induced pore-pressure changes best explain the observed aftershock pattern in both space and time. Furthermore, increasing pore pressure reduces effective normal stress largely independent of fault orientation, promoting failure across a broad range of faulting styles, consistent with observed focal-mechanism diversity. This implies that time-independent elastic ΔCFS calculations on optimally oriented faults may be insufficient to assess the response of upper-plate faults to megathrust earthquakes, and that transient, pore-pressure stress changes must be considered. Overall, our results link postseismic deformation, stress transfer, and pore-fluids in the upper plate, and provide a basis for physics-based, time-dependent aftershock forecasting constrained by forearc hydraulic properties.
How to cite: Peña, C., Heidbach, O., Metzger, S., Schurr, B., Moreno, M., Bedford, J., Oncken, O., and Faccenna, C.: Pore-pressure diffusion controls upper-plate aftershocks following the 2014 Mw 8.2 Iquique earthquake (northern Chile), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6139, https://doi.org/10.5194/egusphere-egu26-6139, 2026.
The South-Central Andes topography results from a three-plate framework, where the Andean block is compressed between the Nazca plate to the west and the South American craton to the east. Interseismic GNSS observations consistently show that basal décollements beneath the eastern fold-and-thrust belts accommodate permanent shortening through aseismic thrust creep. However, their behavior during great megathrust earthquakes has remained poorly understood.
We combine constraints from the 2010 Mw 8.8 Maule earthquake with previous evidence from the 2015 Mw 8.3 Illapel and the 1995 Mw 8.0 Antofagasta earthquakes and demonstrate that basal décollements systematically creep in a normal sense during the coseismic phase. This backsliding occurs as a mechanical response to abrupt stress changes from megathrust rupture: the direction of décollement slip during earthquakes is opposite to their interseismic motion.
By integrating these coseismic observations with independent three-plate interseismic models, we present a unified framework for Andean orogenic-wedge dynamics that reconciles forearc-to-backarc deformation. This framework provides the first comprehensive explanation for the long-observed obliqueness deficiency in Andean megathrust slip distributions. Our results demonstrate that three-plate models are essential for accurately capturing both long-term orogenesis and the complete earthquake cycle, representing a paradigm shift from conventional two-plate approaches with broad implications for other subduction boundary zones and seismic hazard assessment worldwide.
How to cite: Figueroa, M. A., Gómez, D. D., Bevis, M. G., Smalley, Jr., R., Folguera, A., Spagnotto, S., Griffith, W. A., Kellmayer, B., Caccamise II, D., Kendrick, E., and Smith, P.: Three-plate Dynamics of the Andean Earthquake Cycle, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6016, https://doi.org/10.5194/egusphere-egu26-6016, 2026.
The subduction megathrust and its frictional properties are central to controlling the short- and long-term dynamics of subduction zones. While the frictional properties are largely controlled by the 3D structure of the megathrust interface, our understanding of this structure is limited. Exhumed outcrops provide evidence for a complex mélange of ductile and brittle materials, which is segmented in a fractal manner. However, for active subduction zones, we lack direct evidence for such fine-scale structural segmentation as well as quantitative constraints on the segmentation structure.
Here, we use two high-resolution earthquake catalogs from the South American subduction margin to characterize the fine-scale segmentation in situ. We show that two overlying processes govern the fractal distribution of earthquake hypocenters. At short time scales, aftershock clustering dominates the earthquake distribution. At long time scales, averaging over many mainshock-aftershock sequences, the underlying structure shows. However, with typical catalog durations of only a few years, it is crucial to infer structure from short-term catalogs as well. We show, both theoretically and in our observational data, that even in short-term catalogs, structural constraints can be derived by looking at near-field interactions (< 100 m).
Based on our analysis, we find a fractal segmentation of the subduction interface in Northern Chile and Southern Peru with a fractal dimension D=1.6-1.7. This is consistent with the fractal distribution of brittle inclusions in exhumed outcrops. Notably, the fractal distribution is stable down to the hypocenter uncertainty (< 10 m), suggesting self-similarlity over several orders of length. We find an increase in fractal dimension with depth, suggesting a more uniform interface in the downdip region. This work provides a method to gain direct insights into the structure of the subduction interface and systematically quantify it. This way, we aim to connect structural observations to frictional properties and large scale dynamics.
How to cite: Münchmeyer, J., Frank, W., Marsan, D., Schurr, B., and Socquet, A.: Measuring the fine-scale segmentation of the subduction megathrust in situ, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7591, https://doi.org/10.5194/egusphere-egu26-7591, 2026.
Lateral changes of lithospheric density structure and associated topography create spatial variations of Gravitational Potential Energy (GPE) that exert a primary control on the direction and magnitude of crustal stresses, the style of active faulting and, therefore, the location, spatial density and magnitude of crustal earthquakes. Zones of positive/negative GPE with respect to a stable region, should be characterized by an extensional/compressional stress regime, driving crustal deformation toward an ideal situation of spatially homogeneous GPE with no lateral gradients. Along active continental margins, these relationships can be altered by forces associated to subduction, namely the far-field tectonic forces due to plate convergence, elevated shear stresses along the interplate megathrust and basal drags driven by mantle wedge flow. Testing the role of GPE on crustal stresses and seismicity requires an adequate representation of the 3D density structure and a large dataset of stress field indicators and focal mechanism to allow a significant statistical comparison between model predictions and observations, both of which are commonly scarce.
In this contribution we will show results of a study performing this test along the Central and Southern Andean margin (5º-45ºS) that use a refined geophysically-constrained 3D density model, complemented by an analysis of Geoid anomalies, and a recently compiled dataset of several hundred stress tensors derived from Pliocene-to-Recent fault slip data and shallow earthquake focal mechanisms. These results show a strong first-order correlation between GPE anomalies and the large-scale stress field with positive/negative GPE correlating with normal/reverse faulting and near neutral GPE associated to strike-slip faulting. However, local misorientation of existing crustal faults with respect to this field causes stress rotations. First- and second-order partial derivatives of GPE are associated to the 2D stress tensor and compares well with the maximum horizontal stresses SHmax derived from the available data, confirming the main role of GPE on driving crustal deformation. This is further analyzed verifying a correlation between the spatial density of crustal seismic events and the magnitude of GPE gradients, which shed light about the level of stresses at crustal faults and the mechanism of their seismic activation. These results have important implications for understanding the forces driving crustal deformation and the controls on crustal seismicity in active orogenic systems.
How to cite: Tassara, A., Giambiagi, L., Spagnotto, S., Cabello, C., and Araya, R.: Control of Gravitational Potential Energy and associated stress field on crustal seismicity along the Andean margin, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22671, https://doi.org/10.5194/egusphere-egu26-22671, 2026.
South Peru subduction is marked by a transition between flat slab, where the Nazca ridge enters into subduction, to dipping slab further South. Using a dense seismo geodetic network installed in the area together with Sentinel InSAR time series, we monitor in great details the seismic structure, the seismicity and the deformation in the area.
The subduction of the Nazca ridge is associated with low interseismic locking as well as seismic swarms and repeaters on the interface likely indicative of the occurrence of shallow slow slip events. There, the overriding plate is characterized by a wide zone of deformation as shown by InSAR data and by crustal seismicity. The flat slab also exibits an intense intraslab seismicity that shows an intriguing correlation with the vertical surface deformation.
Further south, the slab is dipping steeply and exhibits much less seismicity, maybe due to long lasting post-seismic relaxation following the 2001 Mw8.4 Arequipa earthquake and to high interseismic locking on the interface. Crustal seismicity is more localised: along the volcanic arc and associated tectonic structures, and along faults systems in the forearc.
At the transition between flat to dipping slab, regular Mw~7+ earthquakes occur every ~5 years. The last one occurred in June 2024 and has been captured by our seismo-geodetic deployment. This Mw7.2 earthquake was preceded by a series of foreshocks, and followed by numerous aftershocks, both of which exhibit an intriguing extent down to 80km depth within the slab, likely guided by subducted oceanic structures along the edge of the Nazca ridge that mark the transition from flat to dipping slab.
Our observations image the transition from flat to dipping slab in South Peru and its impact on the seismicity features, and on the upper plate deformation. We notably show that the subduction of the Nazca ridge cannot sustain the flat slab alone. Full waveform tomographic images instead show that the oceanic lithosphere is anomalously thin with asthenosphere upwelling, suggesting that it is thermally eroded by Easter hot spot, that contributes to the buoyancy of the flat segment. We also see that the flat slab has likely contributed to the delamination of the continental lithosphere that is almost absent, implying a significant viscous coupling between the slab and the overriding plate. The enhanced landward motion above the flat slab, seen by InSAR and GNSS, could be due to a viscous drag of the continental plate by the flat slab. Finally, the surface uplift imaged by InSAR can only partly be explained by a viscoelatic subduction model, including interseismic coupling on the interface and an elastic cold nose. Far inland, at about 250km from the trench, a secondary uplift zone correlates remarkably well with intraslab seismicity. We suggest that these intriguing features could be explained by the bending – unbending of the slab.
How to cite: Socquet, A., chalumeau, C., lovery, B., chevrot, S., chlieh, M., radiguet, M., villegas, J. C., munchmeyer, J., doin, M. P., sanchez-reyes, H., norabuena, E., tavera, H., monteilller, V., and kan, L.: The transition from flat to steep subduction in south Peru and its impact on seismicity and deformation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11813, https://doi.org/10.5194/egusphere-egu26-11813, 2026.
The island of Taiwan is situated in a complex tectonic setting at the top of a triple junction, where the Eurasian plate is subducting under the Philippine Sea plate and vice versa. These opposing subductions generate intense deformation, culminating in frequent megathrust earthquakes, and, at greater depths, produce seismic tremors. In addition to seismic extreme events, Taiwan experiences strong monsoon seasons during which typhoons deliver up to 1 m of precipitation locally. Here, we observe a transient reduction in megathrust slip rates following major typhoons, as evidenced by decreased geodetic velocities and reduced tremor and earthquake rates, lasting for approximately 15 days. We interpret the apparent reduction in subduction rates as a result of water-load-induced increases in normal stress on the plate interface, which, in turn, increases interplate coupling. While correlations between individual crustal fault activity and hydrological cycles have been previously reported, our study demonstrates that such effects operate at much larger scales, temporarily slowing an entire subducting slab. Our observations highlight the importance of studying the coupling between climate and tectonic dynamics.
How to cite: Makus, P., Münchmeyer, J., Turowski, J. M., Männel, B., and Chang, J.-M.: Evidence for hydrologically-induced, short-term variations in subduction interplate coupling in Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12123, https://doi.org/10.5194/egusphere-egu26-12123, 2026.
Deep‐focus earthquakes and their association with metastable olivine wedges (MOWs) remain enigmatic. Here, we perform a seismic-geodynamic analysis of the Pacific slab, which is stagnant at the 660 km deep bottom of the mantle transition zone. We investigate deep earthquakes with moment magnitudes (Mw) ranging from 5.3 to 6.9 from 2009 to 2017. They exhibit only minor (mostly implosive) isotropic components, yet they display strongly varying CLVD components. For the largest studied earthquake (Mw 6.9, 2010-02-18), we demonstrate significant stress-drop heterogeneity on a subhorizontal fault and a spatial change in radiation efficiency. We interpret the earthquakes with an evolutionary numerical subduction model with realistic mineralogy and rheology, including non‐uniform plate aging and subduction disruption due to the Izanagi–Pacific ridge sinking in the early Cenozoic. This process resulted in a present-day slab with a bent tip that agrees with tomography. The slab maintains low temperatures (900-1000 K), allowing the presence of a metastable olivine and thus potentially forming MOW with a correspondingly bent geometry. The accompanying internal deformation controls the deep seismicity in the slab tip with apparent changes in seismic radiation efficiency and rupture speed across the modeled temperature gradients. From a broader perspective, the MOW contortion may contribute to deformational anisotropy in the shallow lower mantle. Our results underscore the importance of joint interpretations of the evolutionary subduction models and seismic source inversions.
Reference:
Liu, J., Zahradník, J., Plicka, V., Gallovič, F., Bina, C. R., Čížková, H. (2025). Deep-Focus Earthquakes Under Northeast China—An Imprint of the Complex Tectonic History of Pacific Plate Subduction, J. Geophys. Res. Solid Earth 130, e2024JB030215. https://doi.org/10.1029/2024JB030215.
How to cite: Gallovič, F., Čížková, H., Zahradník, J., Plicka, V., Liu, J., and Bina, C. R.: Could a deep earthquake cluster under Northeast China be associated with transformational faulting in an old Pacific slab?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5275, https://doi.org/10.5194/egusphere-egu26-5275, 2026.
The transport and release of fluids play a fundamental role in subduction zone settings, including for the genesis of seismicity and arc volcanism. Understanding how fluids escape from the slab and move into and through the overlying mantle wedge is key for understanding these processes. The presence and distribution of fluids within these regions has been primarily investigated through estimates of the Vp/Vs ratio, which serves as a key indicator of fluid and magma content.
Local earthquake travel-time tomography (LET) has been extensively used to image Vp/Vs structure in subduction zones, providing critical insights into the morphology of the subducted slab and the properties of the overlying mantle wedge. LET typically involves the inversion of seismic data to simultaneously determine P- and S-wave velocity models alongside hypocentre locations. Differences in data coverage and quality lead both to the need for different regularisation in the Vp and Vs inversions and to mismatched resolution between the two velocity models. Together, these effects make a simple division to compute Vp/Vs inappropriate and result in unreliable and uninterpretable estimates of the Vp/Vs ratio. Therefore, several widely used algorithms incorporate a direct inversion for the Vp/Vs ratio, but these typically assume identical ray paths for P and S waves. The requirement to have both S- and P- wave arrivals also means that valuable data are discarded. Moreover, many LET schemes provide limited information on model uncertainty and resolution, complicating the assessment of model reliability.
In this work, we address these issues by utilising the Backus-Gilbert based SOLA method (Zaroli, 2016) to obtain robust and consistent Vp/Vs models in local earthquake tomography. The SOLA method provides direct control over model resolution and has recently been applied to obtain multiple physical parameters with the same local resolution. Here, we present the methodology and preliminary work towards implementing SOLA with LET, with the goal of improving constraints on fluid distribution in subduction zones. As an initial step towards this, we use data from the Lesser Antilles subduction zone (Bie et al, 2022) to obtain Vp and Vs models within a linearised framework. We will present preliminary findings for the Vp/Vs ratio including uncertainty and resolution information.
How to cite: Mojica Boada, M., Koelemeijer, P., Hicks, S., Zaroli, C., and Serra, E.: Towards imaging fluids in subduction zones through Backus-Gilbert inference of Vp/Vs structure: initial application to the Lesser Antilles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14860, https://doi.org/10.5194/egusphere-egu26-14860, 2026.
The south-central Chilean seismogenic zone has produced some of the largest megathrust earthquakes ever recorded, including the 2010 Mw 8.8 Maule event. To understand the rupture behaviour of this earthquake in the region of maximum coseismic slip at 34.5°S, we analyse the tectonic and elastic structure of the margin using 2D wide-angle seismic (WAS) data, and spatially coincident 2D multichannel (MCS) data acquired with a 15-km-long streamer.
To improve the seismic velocity model relative to previous results along the same WAS line, we jointly invert travel times from WAS and MCS data using a combined refraction–reflection tomographic approach and statistical uncertainty analysis. In addition, we apply downward continuation to the MCS shot gathers to increase the number of usable MCS travel times and to improve ray coverage with refracted arrivals from the shallow part of the velocity model. This approach enhances coverage and reduces tomographic velocity uncertainties, and improves constraints on the position of the interplate reflector from the megathrust.
The resulting 2D P-wave velocity (Vp) model includes the velocity structure of a 50-60 km-wide accretionary prism, and a sharp velocity transition into crystalline basement landward. We convert the velocity structure of the upper plate into density and S-wave seismic velocity to then calculate rigidity (Shear modulus), and infer dynamic rupture parameters such as slip and rupture velocity. Comparison of the expected slip distribution from our results with existing kinematic slip models shows significant discrepancies, particularly beneath the accretionary prism, where the time-migrated 127-km-long seismic profile reveals intense internal deformation and increasing thrust faulting and folding towards the trench. We discuss potential upper-plate coseismic deformation processes to explain such discrepancy.
How to cite: Boekholt, A., Prada, M., Gómez de la Peña, L., E. Jiménez-Tejero, C., Bangs, N., and R. Ranero, C.: The Structure of the Chilean Subduction Zone from Seismic Imaging and Tomography at 34.5°S, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18475, https://doi.org/10.5194/egusphere-egu26-18475, 2026.
Understanding the coupled multi-scale geodynamic and tectonic processes related to the subduction of the oceanic Nazca Plate beneath the South American Plate is one pre-requisite to better assess seismic hazards in the region. It is essential to identify all relevant forces and related stress-strain relationships within the subduction system, such as the negative buoyancy of the subducting slab or the degree of mechanical coupling between the Andean domain and the Pampean foreland. We approach this by investigating the present-day physical, in particular rheological state of the lithosphere-asthenosphere system, including first-order variations in pressure, temperature, and rock composition as constrained by multi-disciplinary observations.
This study integrates gravity, active and passive seismic data to construct a high-resolution lithospheric scale density model for the Central Andes, enabling detailed analysis of crustal structural differentiation. For this, a combined workflow of forwardgravity field calculation using IGMAS+ and gravity inversion for the crustal layer using SimPEG is applied. By inverting for crustal densities constrained by both gravity field observations and seismic shear wave velocity conversions for the crust and upper mantle, this work reveals lateral and vertical variations in crustal density that challenge traditional models of upper/lower crustal dichotomy.
With this contribution, we will discuss how the combined information on seismic velocity and gravity-constrained density helps inferring lithological variations within the crust. This is a pre-requisite for investigating variations in the thermal field and mechanical strength of this complex lithosphere-asthenosphere system. In addition, our results provide new insights into the distribution of seismic events in relation to crustal heterogeneity.
How to cite: May, T., Bott, J., and Scheck-Wenderoth, M.: Unveiling Crustal Heterogeneities in the Central Andes: A High-Resolution Density Model Derived from Integrated Gravity and Seismic Modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7613, https://doi.org/10.5194/egusphere-egu26-7613, 2026.
In subduction zones, seismic imaging reveals increasing permanent active deformation toward the trench, particularly within accretionary systems. Yet, how these rock bodies deform coseismically and influence megathrust rupture behavior is elusive. Here we combine geophysical observations from seismic imaging with visco-plastic dynamic rupture simulations to investigate how realistic upper-plate rock bodies influence megathrust earthquake dynamics and off-fault deformation. Our models reproduce the elastic structure of three subduction systems that differ primarily in the width of the accretionary prism, a key parameter for comparison. These configurations, referred to as Models I, II, and III, include a narrow compliant prism of approximately 20 km, an intermediate prism of about 60 km, and a wide prism exceeding 100 km, respectively. Elastic rock properties for each upper-plate model are derived from 2D P-wave velocity models obtained from controlled-source seismic data. Upper-plate bulk cohesion and bulk friction define visco-plastic strength and are set to depend on rigidity distribution and empirical observations. Based on laboratory measurements from JFAST drilling samples, we use rate-and-state friction law with strong velocity weakening in the shallow portion of the fault.
Results show that coseismic upper-plate plastic deformation in Model I is confined to the ~20-km-wide wedge, whereas in Models II and III it extends 40–60 km landward from the trench. This is consistent with seismic reflection profiles that reveal increasing active internal deformation of the prism at similar distances from the trench in regions such as the Japan Trench, Chile, and Sumatra. Such contrasting upper-plate deformation patterns lead to distinct uplift scenarios, particularly in their high-frequency response. In particular, Model I produces shorter-wavelength uplift near the trench, likely generating a tsunami with higher-frequency content than Model II and III, where uplift exhibits a longer wavelength. Although we do not explicitly simulate independent faults within the prism, the bulk plastic strain can be considered a proxy for the amount of deformation that is accommodated by these structures. Our results suggest that permanent deformation within accretionary prisms is active during trench-breaching megathrust earthquakes, indicating that substantial prism deformation occurs coseismically. Plastic deformation leads to a reduction in slip toward the trench, implying that coseismic energy is absorbed by the overlying rock body. This effect explains the low-radiated-energy of near-trench earthquakes, including tsunami earthquakes. Depending on the plastic strength of upper-plate material and the available energy along the fault, this effect may even prevent the rupture from reaching the trench, while still producing substantial coseismic uplift and horizontal seafloor displacement. Overall, this study indicates that identifying permanently deformed, low-rigidity regions near the trench can serve as a proxy for locating areas where coseismic deformation is strongly accommodated and tsunamigenic uplift is likely amplified.
How to cite: Prada, M., R. Raner, C., Sallarès, V., and Ulrich, T.: The role of permanent upper-plate deformation in coseismic deformation and megathrust earthquakes dynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11604, https://doi.org/10.5194/egusphere-egu26-11604, 2026.
The M9.1 2011 Tohoku earthquake occurred in a region with complex heterogeneous Earth structure, including non-uniform slab geometry and strong velocity contrasts. Prior studies of the deformation caused by this earthquake have generally utilized simplified Earth structures based on homogeneous or layered models. Here, we present an analysis of the Tohoku earthquake deformation field based on a model that incorporates realistic Earth structure, including three-dimensional (3D) velocity structure, slab geometry, and topography and bathymetry. We find that the presence of 3D material structure significantly alters predicted surface displacement by producing greater uplift far from the trench and smaller near-trench uplift, and by reducing near-trench horizontal displacement. These findings demonstrate the potential for 3D structural variations in the Tohoku region to bias slip estimates for the 2011 Tohoku earthquake. Our results suggest that it may be appropriate to re-visit conclusions drawn from prior analysis of geodetic data for the Tohoku earthquake.
How to cite: Langer, L. and Materna, K.: Heterogeneous Earth structure controls on surface deformation caused by the 2011 Tohoku earthquake, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17702, https://doi.org/10.5194/egusphere-egu26-17702, 2026.
The capture of microplates by the Pacific Plate drives the transition from subduction to intracontinental, strike-slip motion along the San Andreas Fault (SAF). However, the underlying mechanics behind microplate capture and formation of intracontinental strike-slip faults remain unclear. Through 3D thermo-mechanical models with fluid migration, we find that northwestward Pacific Plate motion transitions from being accommodated at the Pacific-Farallon ridge to the megathrust between the Farallon slab and North America, and finally to an emergent, fluid-weakened intracontinental strike-slip fault. This transition occurs during slab detachment, triggered by decaying subduction convergence, strengthening of the megathrust with slowing water release, and eventual subduction termination. With the detached Monterey slab paleogeographically restored, forward large-scale convection models show that the paleo slab corresponds to the prominent, high-seismic velocity anomaly in the mantle transition zone below Nevada and Utah. The extension of the overriding American Plate facilitates the formation of the strike-slip fault. The computations suggest the connection between the low viscosity and high permeability subducted plate interface and the North American lower crust may lead to shearing, fluid transfer, and serpentinization and eventual SAF formation, offering insights into the spatial variations of volcanism, fault creeping, and seismicity along the SAF.
How to cite: Mao, W. and Michael, G.: Dynamics of San Andreas Fault Formation: Capture of a Microplate during the Demise of Subduction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15591, https://doi.org/10.5194/egusphere-egu26-15591, 2026.
The eastern margin of the Sea of Japan is a zone of great seismic and tsunami hazard due to multiple offshore and nearshore reverse faults. The 2024 Mw 7.5 Noto Peninsula Earthquake highlights this hazard. It resulted from the combined rupture of multiple adjacent faults. The specific hazard caused by each fault in the back-arc is however difficult to assess owing to long earthquake recurrence intervals. Diagnostic fingerprints in the landscape, onshore and offshore, can reveal clues and augment our understanding of the local earthquake cycle.
Here, we compare coseismic deformation of the 2024 Noto Peninsula Earthquake with 4,767 individual marine terraces attributed to 16 successive sea-level stages over the last Myr. This reveals that thereverse faults responsible for the quake were reactivated and started slipping between 326 and 238 ka. The emerged landscape is still adjusting to it while nearshore underwater scarps mark the active faults. Applied to nearby Sado Island, these observations reveal the likely location of an active fault that drives its fast deformation. Active faults defining the edge of uplifting land are likely found in the near shore domain, drowned by the current sea-level high stand.
New luminescence dating constraints on uplifted marine terraces further quantify the rate of deformation on Noto. These ages are in the final phase of analysis at the time of writing. Preliminary results appear to largely confirm the existing morphostratigraphic assumptions for the 120 ka terrace of Noto and a recurrence interval for 2024 Mw 7.5-type earthquake on the order of 2 kyr.
How to cite: Malatesta, L. C., Sueoka, S., Weiss, N.-M., Tsukamoto, S., Gailleton, B., Bonerath, V., Keum, D., Takahashi, N., Ishimura, D., Nishimura, T., Komatsu, T., Kataoka, K., Iwasa, Y., and Norton, K.: Earthquakes, “young” faults, and landscapes in Japan’s back-arc, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18752, https://doi.org/10.5194/egusphere-egu26-18752, 2026.
The Andaman–Nicobar segment of the Sunda subduction zone hosts some of the world’s highest tsunamigenic hazards, exemplified by the 2004 Mw 9.1-9.3 Sumatra–Andaman event. Highly oblique convergence promotes slip partitioning between the megathrust and upper-plate strike-slip faults; however, the detailed fault architecture of the Andaman arc remains uncertain. To better constrain the neotectonic framework, we perform regionally comprehensive kinematic block modelling using the most up-to-date geodetic velocities and earthquake slip vector azimuths, allowing us to quantify slip rates on major crustal faults from southern Myanmar to Java and assess along-strike variations in deformation style. Our results reveal several new features of the tectonic system. 1) A distinct transition in trench behaviour occurs near the Nicobar Islands, separating independently moving Andaman (Burma plate) and Sumatran segments of the rigid forearc. This boundary coincides with pronounced changes in slip magnitude, rake and rupture velocity during the 2004 great Andaman–Sumatra earthquake, implying that the rupture spanned two kinematically distinct plate boundaries that are interseismically loaded at different rates and in different directions. This boundary is expressed by sharp changes in gravity, bathymetry, and trench obliquity gradient, analogous to those observed at the Sunda Strait, particularly marked by a negative residual Bouguer anomaly indicative of a mechanically weak zone capable of accommodating differential block motion. 2) The strike-slip Andaman-Nicobar Fault, the offshore continuation of the Sumatran Fault, has a slip rate of >30 mm/yr, about twice that of the Sumatran Fault. The lack of recorded large earthquakes along this system and abundant swarm seismicity, imply that deformation may be accommodated by a combination of fault creep and localized locked patches, and/or by distributed slip across multiple structures. Our results have important implications for seismic hazard assessment and for future tectonic and tsunami-generation models, which must account for the structural barrier and complex strain accommodation in this part of the subduction system.
How to cite: Agrawal, H., Lythgoe, K., Bradley, K., and Feng, L.: Kinematic boundary between Burma and Sumatra at the Nicobar Trench, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19995, https://doi.org/10.5194/egusphere-egu26-19995, 2026.
The safe management of subsurface-related economic activities, such as fluid extraction or storage (groundwater, hydrocarbon, H2, CO2, etc.), requires a reliable assessment of local seismicity. In intraplate regions, such assessments are difficult because earthquakes are often scattered and difficult to associate with active structures. Thus, studies of the local seismicity in intraplate settings often require detailed long-term seismic surveys.
In this work, we present the results of more than six years of seismic monitoring in the Basque-Cantabrian Zone (BCZ), a region of great economic and geological interest in the eastern continuation of the Pyrenees along northern Iberia. Although the BCZ has long been an area of intensive subsurface use and resource exploitation, knowledge of its background seismicity and active structures remains limited. During our six-year long survey, we recorded more than 1200 earthquakes and computed 42 new focal mechanisms.
The observed seismicity is generally dispersed and concentrates primarily to the east of the studied area, in the transition between the BCZ and the Western Pyrenees (WP). Within the BCZ, seismicity is associated with salt diapirs and blind faults that likely affect the Paleozoic basement, as well as with a major south-dipping Mesozoic normal fault. In the WP, seismicity primarily clusters along a steeply dipping fault that we interpret as the Ollín fault, reaching ~40 km depth. In the Southern Pyrenean Zone, we observed two seismic crises that appear to be related to blind faults. In the northern Iberian Range, seismicity is scattered over a wide range of depths, both all of them occurring above and below the frontal thrust (Cameros thrust).
Finally, we analyzed the regional stress regime by inversting the newly-derived focal mechanisms. Our results indicate a predominantly extensional stress regime in the BCZ, with localized strike-slip components in several areas, including the South Pyrenean Zone.
How to cite: Olivar Castaño, A., Díaz-González, A., Álvarez Pulgar, F. J., Pedreira, D., González-Cortina, J. M., Gallastegui, J., Diaz, J., and Gallart, J.: Seismicity and seismotectonics of the Basque-Cantabrian Zone (northern Iberia) from six years of observations using a dense temporary network of broadband seismic stations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22644, https://doi.org/10.5194/egusphere-egu26-22644, 2026.
