Orals: Tue, 5 May, 10:45–12:30 | Room G2
Deformation within the upper crust is mainly accommodated through slip on fault systems. Slip can occur in various forms, ranging from aseismic creep (i.e., stable motion) to dynamic earthquake (i.e., unstable motion). Interestingly, a single fault is not restricted to a specific slip mode. Recent geodetic observations have shown that a fault can exhibit both stable and unstable motions. The different slip behaviours have been attributed to fault spatial heterogeneity of the frictional properties, rheological transitions, or geometric fault complexity.
To comprehensively characterise the effect of frictional heterogeneities, we deformed heterogeneous fault samples in a triaxial apparatus, at confining pressures ranging from 30 to 90 MPa. The fault planes, sawcut at a 30° angle from the sample axis, are composed of two materials: granite and marble. Experiments were conducted with both marble asperities embedded in granite and vice versa, alongside homogeneous fault samples of single lithology. The selection of granite and marble was based on their different frictional properties, with granite exhibiting seismic behaviour, while marble demonstrated aseismic behaviour under the tested conditions.
Our results show that the stress drops of seismic events are dependent on fault composition, with faults containing higher granite content exhibiting larger stress drops. In addition, local strain measurements close to the fault allow us to investigate the spatial and temporal distribution of fault slip. In the case of homogeneous faults, the seismic event nucleation is relatively straightforward, initiating in the highest stressed region and propagating uniformly. Conversely, heterogeneous faults display a shorter nucleation phase, followed by a dynamic strain drop restricted close to the granite areas. Away from the dynamic event, the fault remains locked and is subjected to an increase in strain. This strain deficit is then released by a long-lasting decay similar to post-seismic afterslip observed on natural fault systems after large earthquakes. For the heterogeneous samples exhibiting post-seismic deformation, elevated confining pressure favours longer and higher amplitude of afterslip. Further data analysis demonstrated that large afterslip observed at higher confining pressures must originate from the combination of 1) larger co-seismic stress/strain drop and 2) higher frictional stability around the area of dynamic stress/strain drop, both enhanced at larger confining pressures.
How to cite: Noël, C., Dublanchet, P., Twardzik, C., and Passelègue, F.: The effect of frictional heterogeneities on the seismic cycle: Insights from triaxial experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5279, https://doi.org/10.5194/egusphere-egu26-5279, 2026.
Changes in fault-zone properties over successive earthquake cycles drive variations in fault slip behavior and seismic hazard. Typically, fault zones are believed to evolve towards tabular damage zones surrounding a low cohesion fault core and are characterized by increased fracture density and reduced elastic stiffness. However, interseismic mineral alteration and fracture healing can either weaken or strengthen fault zones, influencing future earthquake ruptures. Here, we document post-earthquake fault-zone-strength enhancement through field and laboratory observations of fault core and damage zone rocks from subsidiary faults partially activated during the 2019 Ridgecrest earthquake sequence. Analysis of coseismic slip observed with InSAR shows that only some portions of faults in the Spangler Hills experienced slip during the Ridgecrest earthquake sequence even though Coulomb failure stress change analysis predicts the entire length of the faults would have been stressed towards failure. Field investigations have revealed the presence of pseudotachylyte along the faults which is evidence of ancient earthquakes. Mineralogical analysis of healed pulverized rock within these fault zones suggests that these early earthquakes occurred near the brittle-ductile transition prior to exhumation. We measured the tensile and uniaxial compressive strength, Young's modulus, Poisson's Ratio, cohesion, and angle of internal friction of exhumed fault zone rocks and nearby plutonic rocks using a Split Hopkinson Pressure Bar. We find that post-earthquake healing via propylitic albitic alteration within the fault zone increased damage zone tensile and compressive strength and stiffness, and fault core cohesion by ~150% in the location where no slip was observed. These observations are further supported by multispectral Landsat and ASTER analyses, which indicate that surface slip along subsidiary faults is preferentially localized within zones of pre-existing phyllic hydrothermal alteration and terminates at the boundaries of propylitic alteration zones. Together, these results demonstrate that fault-zone cohesive healing can exert long-lasting control on fault slip behavior and seismic hazards.
How to cite: Smith, Z., Bürgmann, R., Waligora, F., Griffith, A., Nevitt, J., Materna, K., Gleeson, M., Yan, R., and Idzakovich, M.: Post-earthquake Fault Zone Overstrengthening Influences Slip during Future Earthquakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13810, https://doi.org/10.5194/egusphere-egu26-13810, 2026.
Volcanic calderas, including Campi Flegrei (Italy), are characterized by intense shallow seismicity (<4 km depth, magnitude <4), commonly associated with hydrothermal fluid circulation. In these settings, seismogenic volumes constitute highly reactive systems where pressurized fluids, elevated temperatures, and mineral reactions interact to modulate fault strength, wall rock and fault zone stiffness, and slip behavior (i.e., from aseismic creep to seismic slip). Despite the dense monitoring network at Campi Flegrei—one of the most active and densely populated volcanic–geothermal systems worldwide—the mechanical and chemo-textural behavior of shallow faults under hydrothermal conditions and its implications for the seismic cycle remain poorly constrained.
Here, we investigate the coupled mechanical, mineralogical, geochemical, and microstructural evolution of experimental faults composed of Neapolitan Yellow Tuff, a highly reactive pyroclastic rock representative of the shallow (<1 km) intra-caldera faulted volume at Campi Flegrei. We performed fourteen hydrothermal rotary-shear experiments at constant slip velocity (10 µm s⁻¹), systematically varying temperature (T = 23–400 °C), effective normal stress (σₙeff = 5–30 MPa), and pore-fluid pressure (Pf = 5–30 MPa) to reproduce liquid, vapor, and supercritical water conditions expected within the upper ~2 km of the caldera. Slip stability was assessed from the occurrence of stick–slip events (laboratory earthquakes), and associated stress drops, while friction coefficients and apparent fault stiffness were retrieved from stick-slip cycles. Mechanical observations were complemented by post-mortem mineralogical, geochemical, and microstructural analyses using X-Ray Diffraction, X-Ray Fluorescence, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, and Energy Dispersive System microanalysis (XRPD, XRF, FTIR, SEM–EDS).
At room temperature, deformation is dominated by stable or slow slips associated with distributed grain-size reduction and limited induration. With increasing temperature, thermally activated mineral reactions alter fault rheology (and behavior). Between 300 and 400 °C, zeolite dehydration, clay dehydroxylation, volcanic glass dissolution, and rapid secondary mineral precipitation promote pervasive cementation and pore-space sealing, producing a dense, welded fault fabric. These processes strengthen grain-to-grain contacts, increase friction coefficients (from ~0.67 at room temperature to ~0.84 at 400 °C), and significantly enhance fault stiffness (from ~2.5 GPa/m at 23 °C up to ~10 GPa/m at 400 °C), leading to strongly unstable, earthquake-like slip with laboratory stress drops of up to ~25 MPa. Increasing effective normal stress further amplifies frictional instabilities through compaction, strain localization, and strengthening of grain contact junctions. In contrast, vapor-dominated conditions at ≥300 °C inhibit cementation, resulting in smaller stress drops while maintaining unstable fault slip behavior.
Our results demonstrate that hydrothermal fluid–rock interactions can rapidly shift shallow volcanic faults across slip modes by modifying fault fabric, stiffness, and strength. Temperature-driven mineral breakdown and pore-space sealing play a fundamental but often overlooked role in the shallow seismicity at Campi Flegrei, as well as in similar tuffaceous geothermal reservoirs, with important implications for fault mechanics, seismic hazard, and volcanic dynamics across small-to-long term time scales.
How to cite: D'Ippolito, G., Tesei, T., Mormone, A., Gomila, R., Piochi, M., and Di Toro, G.: Seismicity driven by rapid fault cementation in tuffs under hydrothermal conditions of the Campi Flegrei caldera (Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6931, https://doi.org/10.5194/egusphere-egu26-6931, 2026.
Deformation on planetary bodies is characterized by processes that act at strain rates of more than 15 orders of magnitude difference. With the advent of advanced geophysical techniques with ever increasing resolution in time and space, we are now able to detect some of these intriguing dynamics. However, to improve earthquake related hazard assessments, advancing from observations of apparent dynamics of geophysically detected deformation events to in-depth understanding of the underlying physical processes is urgently needed. One “type” example of a deformation phenomenon encompassing deformation at different rates are Slow Earthquakes (SEs). In SEs, slip occurs more slowly than in regular earthquakes, but significantly faster than can be attributed to long-term plate motion. Although SEs are abundant, their geophysically observed characteristics cannot be reconciled with current understanding of how rocks deform: new evidence of slip processes need to be discovered in the geological record.
Rock outcrops from an example of exhumed subducted crust in New Caledonia are interpreted to contain zones of former SEs. Microstructural characterization combining EBSD and EDS analyses deciphers controlling deformation processes, while phase petrology is used to evaluate stages of fluid ingress, production or egress. Based on our observations, we interpret that several deformation processes directly associated with the presence and movement of fluids governed rock behaviour. Relatively “slow” dissolution-precipitation creep is the main “background” deformation process responsible for the observed shape- and crystallographic-preferred orientations, in-grain compositional variations and grain boundary alignment. Geometric features akin to soft sediment deformation structures and water escape structures that developed at high grade conditions suggest that intermittently, local liquefaction is triggered by episodic high fluid pressures induced by mineral dehydration reactions. Based on these observations, we propose that wet granular flow at high fluid pressure may occur in subduction zone environments. This process is transient and relatively fast contrasting the slow, continuous viscous background flow. Catastrophic failure and flow by wet granular flow represents a viable candidate process for geophysically observed transient high slip rates in fluid rich subduction environments.
How to cite: Piazolo, S., Carpenter, M., Chapman, T., Clarke, G., Hansen, L., and Hawthorne, J.: Signatures of changing deformation rate dynamics in deforming rocks: Examples from the exhumed Slow Earthquake Zone of New Caledonia, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14249, https://doi.org/10.5194/egusphere-egu26-14249, 2026.
Serpentinites are “weak" rocks common in several geodynamics settings, including mid-ocean ridges, transform faults and subduction zones. Because of this, serpentinites play a pivotal role in the nucleation and propagation of slow and regular earthquakes.
We studied serpentinites pertaining to the exhumed Monte Fico shear zone (Elba Island, Italy) that reached greenschist facies conditions during deformation. The shear zone, tens to hundreds of m thick, is made of 10-100 cm lenses of metaperidotite, mainly composed by the lizardite and chrysotile, wrapped by foliated serpentinites. Bulk deformation is accommodated by anastomosing and pervasive S/C foliations. The lenses are bounded by 1-3 cm thick brittle faults decorated by slickenfibers composed of chrysotile and polygonal serpentine.
To determine the frictional and healing properties of the serpentinite-bearing shear zones and faults under realistic ambient shallow-subduction conditions, we performed 39 slide-hold-slide experiments at σ’n=20 MPa, Pf=6 MPa, Vshear= 10 µm/s and at temperatures from 25°C to 400°C. We sheared with a rotary shear apparatus equipped with a hydrothermal vessel (ROSA-HYDROS, Dept. of Geosciences, UniPD, Italy) the powders obtained from the metaperidotite and the slickenfibers.
Regarding the frictional properties of the metaperidotite, when the water is in a liquid state, the friction coefficient increases from 0.3 at 25°C to 0.5 at 300°C; when water is in vapour and supercritical states, the friction coefficient is strain hardening (0.6-0.89 at 300°C-400°C). Stick-slip behaviour (i.e., seismic slip) is observed only at 400°C. In the case of slickenfibers, when the water is in liquid state, the friction coefficient increases from 0.23 at 25°C to 0.34 at 300°C. When water is in vapour conditions the friction coefficient is 0.47 at 300°C and 0.57 at 400°C. In conclusion, the metaperidotite can deform by aseismic creep or seismic slip and the slickenfibers deform by aseismic creep.
Regarding the frictional healing properties, it differs between metaperidotite and slickenfibers. Frictional healing of the metaperidotite is positive and increases with temperature, independently of the physical state of water. Instead, frictional healing of the slickenfibers is negative for nearly all the conditions, with a maximum positive healing between 150°C and 250°C.
Our results show that the frictional response of low-grade serpentinites sheared in the laboratory at shallow-subduction hydrothermal conditions is controlled by the mineral assemblage and temperature. As a consequence, in nature, the combination of frictional and healing behaviour is highly heterogeneous and becomes the driver for enhanced instabilities on the weak but fast-healing slickenfibers in a narrow temperature window between 150°C and 250°C.
How to cite: Salvadori, L., Di Toro, G., and Tesei, T.: Temperature-dependent frictional and healing behaviour in serpentinite shear zones: implications for subduction zone seismicity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9588, https://doi.org/10.5194/egusphere-egu26-9588, 2026.
Faults in the shallow brittle crust are rarely frictionally homogeneous. Structural and mineralogical heterogeneities such as phyllosilicate-rich shear zones mixed with competent lenses generate, respectively, velocity-strengthening (VS) and velocity-weakening (VW) domains that strongly influence earthquake nucleation and rupture dynamics. Geological observations and laboratory experiments show that VW patches typically nucleate unstable stick-slip, whereas VS regions promote stable creep and can transfer stress on neighbouring VW patches. Although this heterogeneous patch framework supports models of shallow seismicity, induced seismicity, and subduction zones, direct experimental investigations with rock samples and realistic patch geometries remain limited.
Here we present a new experimental framework for testing heterogeneous fault slip in cm-scale rock samples. Using the newly developed “BeeAx” servo-controlled biaxial apparatus, we sheared 15 × 17 cm Pennant Sandstone blocks at slow displacement rates (~1 µm/s) and 2, 5 and 8 MPa of normal stress. We compare three frictional sliding configurations: (1) homogeneous sandstone–sandstone (VW-dominated), (2) homogeneous graphite-coated sandstone (VS-dominated), and (3) heterogeneous samples with four circular uncoated sandstone patches embedded within a graphite background, comprising 50% of the sliding surface. High-resolution and calibrated acoustic emission (AE) monitoring (16 sensors) allows hypocentre location and source parameter retrieval, enabling direct comparison of microseismicity and frictional stability across configurations.
The homogeneous graphite experiment produced stable sliding with very low friction (µ≈0.15), while the homogeneous sandstone samples exhibited unstable stick-slip and higher friction (µ≈0.5). The heterogeneous samples displayed hybrid behaviour: a low overall friction (µ≈0.20) comparable to graphite, yet persistent dynamic stick-slip events. AE hypocentres concentrated on the sandstone patches perimeters, revealing that aseismic creep in the weak graphite transfers shear stress onto the stronger patches, which subsequently fail seismically. Compared to homogeneous sandstone, heterogeneous samples showed larger stress drops, stronger localization of microseismicity, and reduced Gutenberg-Richter b-values. Temporal b-value evolution differed between configurations: constant and low for graphite, cyclic for sandstone (decreasing during interseismic loading and increasing post-mainshock), and intermediate but systematically lower in the heterogeneous case, consistent with enhanced stress transfer and patch interaction.
These results demonstrate that discrete weak VS regions can modulate and even enhance the seismicity of stronger VW patches by acting as creeping load reservoirs. This provides laboratory support for models invoking patchy asperities on shallow faults and in induced seismicity settings, where strong and weak rock patches coexist within a fault. More broadly, the experimental platform enables controlled studies of rupture nucleation, asperity geometry, interaction, and seismicity evolution in frictionally heterogeneous fault systems.
How to cite: Bigaroni, N., Mecklenburgh, J., and Rutter, E.: Seismic–Aseismic Slip Partitioning on a Frictionally Heterogeneous Fault: An Experimental Approach, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19671, https://doi.org/10.5194/egusphere-egu26-19671, 2026.
Unlike typical surface rupture zones that develop along the main fault of mainshocks, the January 23, 2024, Wushi earthquake in Xinjiang, despite its magnitude of Mw 7.0 and relatively shallow focal depth of 22 km, did not produce any co-seismic surface rupture along the seismogenic South Maidan Fault (SMDF). Instead, seven days later, an Mw 5.7 aftershock along a reactivated shallow back-thrust occurred approximately 3.7 km northwest of the SMDF, generating a co-seismic surface rupture about 4.7 km long. Together, these two faults form a pop-up structure with opposite vergence, triggered by the aftershock thus causing significant and localized surface uplift. This unusual case offers new insight into faulting dynamics, landscape evolution, and underscores the need to reassess the seismic hazard posed by shallow secondary faults.
On the rupture surface, a layer of red fault gouge approximately 0.5–1 cm thick has developed. Outward from this, there is a layer of light yellow fault breccia about 20–30 cm thick. A clear linear boundary exists between the two, and the entire assemblage is enclosed within a fragmented zone composed of Xiyu conglomerate. We combined rupture energy, δD, the Kübler index parameter, and multi-grain-size structural analyses with K-Ar dating of synkinematic illite separated from both the red fault gouge and surrounding fault breccia rocks.
The dating results show that the red fault gouge records a new round of strong earthquake clusters beginning at 0.34 ± 0.03 Ma. The detrital illite age was 2.04 ± 0.13 Ma, which is consistent with the sedimentary age of the Xiyu conglomerates. In contrast, the surrounding yellow-breccia rocks obtain an older clay mineral ages: the authigenic illite age is 204.0 ± 5.8 Ma and the detrital illite age is 419.4 ± 23.6 Ma. Considering that the Xiyu Conglomerate was deposited during the Late Cenozoic and exhibits relatively poor diagenetic consolidation, the terminal ages of its clay minerals likely represent two distinct periods: the initial collision and orogenic phase of the South Tianshan during the Late Silurian to Early Devonian, and a distal response to the closure of the Tethys Ocean during the Indosinian period.
Our findings confirm that the South Tianshan region has entered a new phase of tectonic activity since the Quaternary. Intense crustal shortening triggered extensive erosion, leading to widespread faulting activities and the deposition of the Xiyu conglomerates. By the Middle Pleistocene, back-thrust faults developed as stress accumulated, forming pop-up structures that controlled regional uplift and landscape evolution. These shallow, low-energy secondary faults are still capable of generating surface ruptures during seismic events, and carries important implications for seismic hazard assessment, particularly regarding surface deformation risk.
How to cite: Zheng, Y.: K-Ar dating of fault gouge from the surface rupture of the January 23, 2024 Ms7.1 Wushi Earthquake, Xinjiang, China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5332, https://doi.org/10.5194/egusphere-egu26-5332, 2026.
Holocene slip rates of active faults are critical to understand the kinematics of crustal deformation interior of the Tibetan Plateau. Here, we quantitatively studied the sinistral strike-slip rate the northern Yadong-Gulu rift (YGR), the unique one that has left-lateral component among the main seven N-S treading rifts in southern Tibet. By detailed fieldwork, UAV topographic data and 10Be cosmogenic dating, we document 2.5-3.5 mm/yr (3.0±0.5 mm/yr) Holocene left-lateral slip rate along the northern YGR at two moraine sites. This rate is consistent with GPS results (4 mm/yr) of the conjugate strike-slip faults in central Tibet. Both of this fault and the northern Beng Co dextral strike-slip fault (4.2-5.4 mm/yr) probably comprise a conjugate fault system, contributing to the extension rate of Gulu rift (~6±1.8 mm/yr) and accommodating the eastward extrusion of central Tibet.
How to cite: wang, S., Tapponnier, P., Chevalier, M.-L., Benedetti, L., and Xu, X.: Inhomogeneity of East-West Extension Interior of the Tibetan Plateau, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-394, https://doi.org/10.5194/egusphere-egu26-394, 2026.
This study presents an analysis of the tectonic evolution and structural features of west-central Taiwan, focusing on the fold-and-thrust belt developed from the Late Miocene–Holocene. Integrating surface geological mapping, borehole data, and seismic reflection profiles, we establish a refined tectonic model that emphasizes the significant influence of the preexisting normal fault on structural development. Our findings demonstrate that pronounced variations in stratigraphic thickness, notably within the early foreland basin sequence, indicate syndepositional normal faulting, creating substantial accommodation space during sedimentation. The normal fault acted as a mechanical barrier was overstepped by a thrust ramp during later compressional phases. These inherited structural features significantly influence seismicity and deformation patterns, exemplified by mechanical barriers linked to the 1999 Chi-Chi earthquake. Our structural cross sections reveal a characteristic ramp-flat-ramp geometry linked to the Changhua Thrust, the Chelungpu Thrust, and the Chusiang Fault. The resulting structural model illustrates a sequential tectonic evolution, transitioning from early extensional regimes to complex compressional environments.
How to cite: Chang, C.-W., Huang, W.-J., and Huang, T.-C.: Preexisting Normal Fault with Pliocene Syndepositional Controls on the Structural Style Transition: Implications for the Structural Evolution in Thrust Belts, West-Central Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15085, https://doi.org/10.5194/egusphere-egu26-15085, 2026.
Abstract: The collision and compression between the Indian and Eurasian plates have resulted in intense crustal shortening and deformation in the Tian Shan since the Cenozoic, leading to its renewed uplift and making the Tian Shan one of the most intensely deformed intracontinental orogenic belts and seismically active regions in the world. Cenozoic deformation of the Tian Shan is characterized by north–south crustal shortening, which is mainly controlled by approximately E-W striking thrust faults, N-W striking dextral strike-slip faults, and N-E striking sinistral strike-slip faults. Concurrently, pronounced tectonic deformation occurred within the Tien Shan, forming a series of E-W trending intermontane basins, including the Turpan, Kumishi, Yanqi, and Yili basins. The Yanqi Basin, located in the southeastern Tian Shan, has experienced significant tectonic deformation due to the continuous uplift of the Tien Shan. Since the Late Quaternary, tectonic deformation has been mainly concentrated along the northern and southern margins of the basin. Two major active tectonic systems are developed along the northern margin, including the Yanqi Basin north-edge thrust fault and the piedmont thrust-fold belt. Along the southern margin, the Yanqi Basin south-edge thrust fault is developed, striking E-W and dipping southward, forming a complex thrust-fold belt. Within this fold belt on the southern margin of the basin, Quaternary geomorphic surfaces are well preserved and display fault scarps of variable heights. Multiple generations of alluvial fans are dissected along the fault scarps. In this study, high-resolution topographic data of faulted alluvial fan landforms along the southern margin thrust-fold belt were acquired using airborne LiDAR. Detailed geomorphic interpretation and quantitative analysis were conducted to identify multiple generations of landforms developed perpendicular to the fault strike. Based on comprehensive geomorphic interpretation and field investigations, deposits from different generations of alluvial fans were sampled for surface age determination. Furthermore, based on measurements of exposed strata and the construction of characteristic topographic profiles across the alluvial fans, we established the cross-sectional geometry and deformation model of the thrust fault-fold belt at the south-edge of the Yanqi Basin. By integrating the tri-shear fault-propagation fold model from fault-related fold theory, we constrain the shortening deformation characteristics of the thrust fault-fold belt. This allows us to estimate the shortening amount and shortening rate for the belt at the southern margin of the Yanqi Basin. Combined with analysis of surrounding fold deformation and fault slip rates, this work not only reveals the deep geometry and activity mechanism of the thrust fault at the south-edge of the Yanqi Basin but also provides constraints for understanding intracontinental deformation within the Southern Tian Shan.
How to cite: Yang, J., Zheng, W., and Zhang, D.: Quantitative constraints on shortening deformation characteristics of the fold at the south-edge thrust fault of the Yanqi Basin, Southern Tien Shan , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15676, https://doi.org/10.5194/egusphere-egu26-15676, 2026.
The Gulang-Zhongwei Fault is a northeast splay of the Haiyuan fault (east of 102°E) accommodating mostly left-lateral slip and some thrusting. Its slip history and kinematics are critical for understanding the mechanisms of slip partitioning along the oblique convergence at the margin of the plateau. Up to now, a consensus on its slip rate remains elusive; previous investigations have yielded divergent estimates ranging from <1 to 6 mm/yr. The central segment of the Gulang–Zhongwei Fault has been targeted for a detailed geomorphological and paleoseismological study. We used drone-based lidar to collect aerial imagery and dense point cloud to generate a digital elevation model (DEM) with a spatial resolution of better than 6 cm. The DEM clearly reveals the fault trace with faulted geomorphic features such as offset terraces and alluvial fans along the southern piedmont of Jingtai Xiaohongshan and Guanguan Ling . We performed detailed geomorphological mapping and displacement measurements at five sites over a fault length of ~6 km. The initiation time of fault slip accumulation was constrained by sub-surface 14C and OSL dating of various terraces and fan surfaces. Systematic and repeated offsets of multiple alluvial fans and terraces, with an average displacement of 12 m, postdating their emplacement in the early Holocene (8–10 ka), imply a millennial slip rate of 0.9–1.5 mm/yr. Altogether, our results indicate that the long-term left-lateral strike-slip rate of the Gulang-Zhongwei Fault ranges from 0.9 to 1.5 mm/yr. Although it accommodates only 10 % to 15 %of the left-lateral shear between Gobi-Ala Shan to the north and northeast Tibet to the south, it has been responsible for some major earthquakes in the past (1709 and 1920). Determination of its slip rate at various time scales is paramount for understanding how northeast convergent strain is distributed along the various faults at the regional scale and is key to seismic hazard assessment.
How to cite: Li, B., Shao, Y., Yao, Y., Zhang, H., and Dong, Y.: New Slip Rate of the Central Gulang-Zhongwei Fault determined from high resolution topography and, OSL and 14C dating, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2143, https://doi.org/10.5194/egusphere-egu26-2143, 2026.
Silica layers composed of quartz grains typically a few micrometers or smaller are texturally distinct from typical quartz veins and occur as μm- to mm-thick layers along fault slip zones. The ultrafine quartz within these layers exhibits uniform interference colors in optical microscopy. Such features are commonly interpreted as indicating crystallographic preferred orientation (CPO), a fabric typically associated with ductile deformation but developed within brittle fault zones. Despite their widespread occurrence in faults developed in various rock types, the deformation mechanisms and ultrafine quartz CPO-forming mechanisms through the seismic cycle remain poorly understood.
In this study, we analyze the microstructures of silica layers observed in three upper crustal faults in Korea developed in sedimentary rocks, granite, and rhyolite within the Cretaceous Gyeongsang Basin, where average burial depths reach ~6 km. All observed natural silica layers are composed of fine-grained quartz (<2 μm. These layers display uniform interference colors in optical microscopy, while EBSD analyses reveal clustering of quartz c-axes. However, some faults are characterized by densely packed comminuted grains with nanopores, whereas others display polygonal quartz grains together with nanopores and illite aligned parallel to the fault plane, as well as shape-preferred orientation of quartz and adjacent calcite grains. These observations suggest that ultrafine quartz within silica layers may have experienced diffusion-related processes in the presence of fluids.
To investigate whether diffusional processes active after fault slip and cataclasis affect CPO development, we conducted hydrothermal rotary shear experiments on single-crystal quartz gouge (<63 μm) under identical P-T-fluid conditions (600°C, effective normal stress of 120 MPa, pore fluid pressure of 80 MPa) using three different velocity histories: (1) fast slip alone (V=300 μm/s), (2) fast slip followed by slow slip (V=0.1 μm/s), (3) fast slip followed by hydrothermal holding without further shear (22 h). The fast slip produces intense comminution within slip localized zones without the development of CPO-like features. In contrast, both the subsequent slow slip and hydrothermal holding result in the development of CPO-like features at the optical scale within the grain-size-reduced zones, accompanied by surface indentations on larger quartz grains and linear aggregates of euhedral ultrafine quartz. However, TEM observations reveal that ultrafine quartz grains within these zones display random crystallographic orientations, with no evidence for a preferred orientation.
Integrating natural and experimental observations, we interpret silica layers to form through a two-stage process: intense grain-size reduction by comminution during seismic slip, followed by fluid-assisted, time-dependent reorientation of ultrafine quartz during post-seismic or interseismic periods. Silica layers characterized by CPO-like features at the optical scale therefore record transitions in deformation mechanisms during the seismic cycle and provide key geological constraints for understanding slip behavior, mechanical properties, and the role of fluids in upper crustal faults. Further investigation is required to clarify the relationship between these optical features and crystallographic orientations at the nanoscale.
How to cite: Gu, D., Han, R., Niemeijer, A., Kim, D., Roddatis, V., and Schreiber, A.: Deformation mechanism transitions during the seismic cycle recorded by quartz CPO in fault-related silica layers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2652, https://doi.org/10.5194/egusphere-egu26-2652, 2026.
Fault healing is a fundamental process in the seismic cycle, allowing faults to relock and restrengthen during the interseismic period. Numerous geophysical studies have shown that the rate of fault healing plays a key role in controlling both earthquake magnitude and recurrence interval, in laboratory experiments as well as in natural fault systems. At the laboratory timescales (1–10⁵ s), fault healing is predominantly frictional and results from the time-dependent growth of contact area due to plastic deformation of the contact asperities. In contrast, seismic cycles in nature occur over much longer timescales, allowing additional healing mechanisms, often driven by chemically assisted processes, to become dominant.
Field observations reveal that chemically cemented fault rocks, such as cataclasites, are commonly present within the cores of several exhumed faults. Despite their widespread occurrence, the interplay between chemically-driven healing processes and fault stability remains poorly constrained by laboratory studies, largely due to the limited experimental timescales.
Here we present a suite of laboratory friction experiments specifically designed to overcome these limitations. We use analogue fault gouges composed of highly reactive materials, including hydraulic cement and anhydrite, tested under both nominally dry and fluid-saturated conditions. This approach allows us to investigate the combined and competing effects of frictional and chemically driven healing on fault slip behavior.
Microstructural and geochemical analyses reveal the formation of newly precipitated mineral phases under fluid-saturated conditions, consistent with the expected reaction for both gouge materials. Compared to purely frictional healing, chemically driven healing produces larger, non-log-linear fault restrengthening and a time-dependent increase in fault cohesive strength. Moreover, faults undergoing chemically driven healing exhibit unstable fault slip, characterized by recurrent stick–slip cycles.
These results indicate that chemically-driven healing processes play a fundamental role in interseismic fault restrengthening and may critically influence fault stability over geological timescales. Our results also suggest that these chemically-driven healing processes may favor the development of favorable conditions for unstable slip even at shallow depths, with relevant implications for natural and induced seismicity.
How to cite: Volpe, G., Affinito, R., Pozzi, G., and Collettini, C.: Cohesion-driven fault instability, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3578, https://doi.org/10.5194/egusphere-egu26-3578, 2026.
The Hambaeksan Fault is a major structural feature in the Taebaeksan Basin, South Korea, characterized by a significant right-lateral strike separation of approximately 3–5 km. Although it truncates the Baekunsan Syncline, the precise timing and kinematic history of the fault remain poorly understood. This study investigates the internal structures, fault rock characteristics, and kinematic indicators of the Hambaeksan Fault through field observations and microstructural analysis at the Sorotgol road outcrop in Taebaek.
At the study site, the fault juxtaposes the limestone of the Duwibong Formation against the shale of the Geumcheon-Jangseong Formation. The fault zone strikes N-S with a 65°E dip, comprising a ~30 cm thick fault core and a damage zone exceeding 10 m. The fault core consists of limestone breccia, shale breccia, and mixed breccia. Linear structures within the shale breccia (N5°E, 10°) confirm a dominant strike-slip movement.
Microstructural analysis reveals a complex deformation history characterized by several distinct features. A well-defined, linear Principal Slip Zone (PSZ), approximately 300 μm thick, is developed within the shale breccia and sharply truncates earlier clasts. Within the mixed breccia, the presence of clay-rich matrices exhibiting fluidized textures and injection structures suggests the occurrence of seismic slip, possibly involving rapid fluidization. Furthermore, kinematic overprinting is evident in the shear bands adjacent to the PSZ, where sinistral shear senses are superimposed on earlier dextral shear senses. These observations indicate that the Hambaeksan Fault has experienced a multi-stage evolution, beginning with a primary dextral strike-slip movement followed by localized subsequent deformation within the mechanically weaker shale units.
These findings suggest that the Hambaeksan Fault underwent at least one episode of seismic slip along the lithological boundaries (mixed breccia) prior to the formation of the current PSZ in the shale unit. The observed kinematic overprinting indicates that after the initial juxtaposition of the formations via dextral strike-slip movement, subsequent deformations were localized within the mechanically weaker Geumcheon-Jangseong shale. This study provides critical insights into the seismic behavior and structural evolution of major crustal faults in the Korean Peninsula.
How to cite: Kim, J. H., Ree, J.-H., Han, R., and Woo, J.: Internal Structure and Kinematic Evolution of the Hambaeksan Fault, Taebaeksan Basin, South Korea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4687, https://doi.org/10.5194/egusphere-egu26-4687, 2026.
Active strike-slip fault systems in addition to oblique slip producing vertical displacement are often linked to contemporaneous thrust faults, which together contribute to regional uplift. However, how slip is partitionned along the different faults of the strike-slip fault system and the underlying mechanisms of slip-paretionning remain poorly understood. To address this issue, we investigated the northwestern margin of the Tibetan Plateau, focusing on the Altyn Tagh fault—a complex fault system that has undergone major Cenozoic tectonic deformation due to the ongoing convergence between the Indian and Asian plates. Using drainage networks and geomorphic indices, we developed a composite index of Relative Uplift Rate (RUR) to map spatial variations in uplift rates. Our analysis reveals significant along-strike variations in tectonic uplift and identifies four major tectonic anomalies from south to north: Qiemo, Subei, Changma, and the northern Qilian Shan. By integrating these results with geological constraints on horizontal slip rates, we explore the mechanism of slip partitioning along the fault. We find that although horizontal slip rates generally decrease northeastward, the vertical relative uplift rates do not follow the same pattern. Instead, within the Qilian Shan, vertical uplift rates are the highest compared to the southwestern fault junction areas. A key finding is that areas with high uplift rates correspond spatially to zones of geometrical fault complexity, such as fault bends or branching. These anomalies are further supported by patterns of seismic activity.
How to cite: Gao, M., van der Woerd, J., and Cai, J.: Slip partitioning of the Altyn Tagh fault based on geomorphic indices reveals uplift pattern, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5101, https://doi.org/10.5194/egusphere-egu26-5101, 2026.
The NE-SW Carboneras Fault is one of the main active structures in the Alborán Sea and, together with the Al-Idrisi Fault, forms the Trans-Alborán Shear Zone, which connects the Betic and Rif Cordilleras. The accurate representation of the offshore 3D geometry and seismogenic characteristics (e.g., slip rate or maximum magnitude) of this large left-lateral strike-slip fault system is essential for assessing the seismic potential in a slow-deforming region, such as the Alborán Sea. Here, we combine the interpretation of multibeam bathymetric and high-resolution multi-channel seismic reflection (HR-MCS) data to reassess the offshore extent of the Carboneras Fault and the adjacent morphostructural elements. Topographic attributes were applied to the bathymetric data to enhance the visualization of the fault trace at the seafloor. Our findings suggest that the offshore Carboneras Fault extends for 110.3 km, exceeding previous estimates. We identified twenty fault segments along the Carboneras Fault trace, with individual lengths up to 46 km and azimuths varying from N40º to N238º. Geomorphic features typically associated with strike-slip systems such as deflected drainage systems, shutter and pressure ridges, and horsetail splays were also identified. Structural seismic attributes applied to the HR-MCS profiles highlight major subsurface discontinuities, allowing detailed mapping of the fault geometry at depth. Seismic interpretation also accounted with the identification of seven horizons offset by the Carboneras Fault, comprising the Paleozoic-Triassic basement, the Messinian unconformity and Early Pliocene to Late Quaternary seismic units. Based on this information, we constructed fault and horizon surfaces to develop the first 3D model of the Carboneras Fault. This model provides key constraints on crustal architecture and offers new insights into fault growth mechanisms, thereby reducing uncertainties and improving the assessment of the seismogenic potential of this major offshore fault system.
How to cite: Mattos, N., Perea, H., Martínez-Loriente, S., and Canari, A.: Reassessment of the Carboneras Fault 3D geometry based on new bathymetric and high-resolution multi-channel seismic reflection data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5564, https://doi.org/10.5194/egusphere-egu26-5564, 2026.
Understanding the spatial and temporal evolution of creep along continental faults is key to identifying where stress is released aseismically and where it may accumulate, potentially leading to future seismic events. Typically, creeping segments exhibit rate-strengthening behaviour, where frictional resistance increases with sliding velocity, resulting in stable sliding. However, the northern Aceh segment of the Sumatran Fault Zone (SFZ), a right-lateral strike-slip fault, presents a notable exception. Here, we observe creep signals using satellite radar interferometry, capturing the temporal evolution of creep, which decreased by ~60% from the 2007–2010 period to the 2017–2023 period. Numerical modelling constrained by these observations identifies two distinct creep events. The first was triggered by stress transfer from Mw 9.2 2004 earthquake more than 150 km away, and a second localized re-acceleration due to nearby continental earthquakes. These results reveal that the fault behaviour is more consistent with a brittle Coulomb surface and lacking the usual self-stabilizing influence of velocity- and slip history-dependent friction. This central creeping section, sandwiched between two large locked domains, instead exhibits nearly velocity-neutral behaviour at the fault segment scale, making it highly sensitive to both local and regional stress changes. These findings provide evidence of long-range fault interactions, where both the subducting megathrust and possibly the oceanic mantle drive creep on a continental strike-slip fault. Located within a densely populated region, the velocity-neutral Aceh fault may participate in future earthquakes nucleating in surrounding locked segments, highlighting elevated seismic hazard in northern Sumatra.
How to cite: Sathiakumar, S., Salman, R., Mallick, R., Feng, L., Qiu, Q., Susilo, S., Wibowo, S. T., Hill, E., and Yun, S.-H.: Coulomb-like creeping segment acts as a stress sensor in Northern Sumatra, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5619, https://doi.org/10.5194/egusphere-egu26-5619, 2026.
The left-lateral, NW-trending Katouna–Stamna Fault (KSF) in Western Greece marks the NE-boundary of the Ionian-Akarnania block. The Global Navigation Satellite System (GNSS) displacement field suggest the potential presence of aseismic slip, which is corroborated by the minimal seismicity in the area. To better understand the current kinematics of the KSF, we integrate full-coverage surface displacement rates derived from 7-yr-long Interferometric Synthetic-Aperture Radar (InSAR) time-series with the GNSS rates, field observations, and structural analyses.
While previous geodetic studies suggested a strike-slip rate of ∼10 mm/yr, our distributed-slip model indicates a strike-slip rate of up to 19 ± 1 mm/yr and a dip-slip rate of up to 11 ± 7 mm/yr. In particular, sinistral slip localises in the right-stepping south-central fault segment, while the highest dip-slip value is found in the northwestern part. In the model, aseismic slip reaches the surface, but the highest slip rates are found below 5 km depth. Furthermore, only the northernmost part of the fault appears locked and accumulating elastic strain, which also corresponds to the location of an earthquake occurred in 2014, supporting our model.
Field evidence indicates complex fault kinematics with multiple deformation phases. The younger generation of NNW-trending striae shows mostly oblique motion with dominating strike-slip component, in agreement with geodetic observations. Such agreement indicates that the geological kinematic regime under which they formed may be relatively recent and possibly still in place.
Overall, our geological observations highlight several possible drivers of aseismic slip. The fault bounds a so-called ‘salt wall’, represented by an elongated evaporite – mainly gypsum – diapir intruding the carbonate bedrock. Although dry gypsum does not display aseismic behaviour on its own, the interaction between evaporites and fluids could promote pressure-solution creep in wet gypsum. Pressure-solution creep in the fault gouge has been reported for other creeping faults, namely the San Andreas Fault, the North Anatolian Fault and the Longitudinal Valley Fault.
Structural evidence of ductile shear deformation within the evaporite-bearing rock along the KSF suggests that, at least in the shallow part, slip may occur with predominantly ductile creeping. This process is facilitated by the high solubility of evaporites and by the presence of fluids, and could explain both ductile deformation and abundant veining observed on the field. Furthermore, the fact that pressure-solution is slower between identical minerals, due to healed boundaries, but faster between different minerals, notably halite and calcite, would explain why the deformation is localised along the contact between evaporites and carbonates, possibly on both sides of the salt wall.
How to cite: Crosetto, S., Szrek, J., Metzger, S., Gomba, G., Faccenna, C., and Jolivet, R.: Aseismic slip along the evaporite-rich Katouna-Stamna fault in Greece, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7648, https://doi.org/10.5194/egusphere-egu26-7648, 2026.
Understanding why earthquakes nucleate unusually deep in the crust is essential for improving seismic hazard assessments. These events occur under pressures and temperatures where rocks are expected to deform ductilely, challenging standard models of rock strength and faulting. Constraining the conditions that allow frictional instabilities to persist at depth therefore has important scientific and societal implications. To investigate the transition from potentially seismic to aseismic slip in lower-crustal environments, we conducted hydrothermal friction experiments on simulated gouges derived from epidote-rich (65%) and amphibole-rich (58%) basement gneiss from the western branch of the East African Rift System. We characterized their frictional strength and stability across temperatures of 350-600 °C, 150 MPa effective normal stress, and 100 MPa pore fluid pressure. We tested three slip-velocity protocols spanning slow (0.01-1 μm/s), intermediate (0.1-10 μm/s), and fast (1-100 μm/s) rates. Both samples show frictional strengths that vary with temperature and slip velocity. Rate-and-state friction parameters (a-b) indicate that the epidote-rich gouge exhibits velocity-weakening behaviour between 350-500 °C and at 0.3-100 μm/s, whereas the amphibole-rich gouge remains velocity-weakening across the full temperature range and at 1-100 μm/s. Microstructural observations indicate that deformation is primarily accommodated within a broad slip zone, where frictional granular flow and cataclasis dominate under both high- and low-temperature conditions. At the highest temperatures (600 °C) and slow slip rates, however, additional evidence for dissolution-precipitation creep was found, indicating the operation of viscous deformation. Our results suggest that epidote and amphibole-rich gouges can host seismic slip under lower-crustal temperature conditions at elevated slip rates. Under natural lower-crustal conditions, these elevated slip rates, sufficient to trigger frictional instability, could be facilitated temporarily by stress transfer, strain localization, or transient fluid-pressure variations.
How to cite: Mensah, L., Niemeijer, A. R., Herwegh, M., and Berger, A.: From seismic to aseismic slip in the lower crust: Results from hydrothermal ring-shear experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8288, https://doi.org/10.5194/egusphere-egu26-8288, 2026.
In this study, we constructed a 3D viscoelastic finite element model of the lithosphere in the eastern Tibetan Plateau, incorporating fine-scale 3D fault geometries rigorously constrained by multi-source data. We quantitatively analyzed the seismogenic mechanisms and controlling factors of the Longmenshan Fault Zone (LMSFZ). The results indicate that: (1) Regional deformation is co-governed by the synergistic mechanism of "rigid blocking by the Sichuan Basin" and "kinematic decoupling along major strike-slip faults." (2) The 3D fault geometry serves as the primary factor controlling stress accumulation on the LMSFZ, following a physical control chain of "Geometry → Mechanical Response → Kinematic Characteristics." Vertically, the listric geometry results in a stratified feature of "deep-driving and shallow-locking"; along the strike, geometric variations dominate the mode transition from "thrust-strike-slip coupling" to "strike-slip dominance." (3) Seismic hazard assessment identifies a high-risk "unruptured asperity" near Dachuan in the southwestern segment, where the deep strain energy density is comparable to that of the Wenchuan earthquake nucleation zone. Conversely, the northeastern segment is characterized by a "low-resistance/slip-deficit" mode, indicating high long-term seismic hazard. Based on physically self-consistent heterogeneous continuum mechanical modeling, this study transcends the limitations of discrete surface observations. It achieves a transition from 2D surface projections to deep 3D continuous fields, providing a reliable physical basis for quantitatively unraveling the deep seismogenic mechanisms of faults.
How to cite: Yang, Y., Tao, W., Qiao, J., Sun, H., Yang, X., Lu, R., Wang, W., Sun, X., Xu, F., and Wang, X.: Dominant Control of 3D Fault Geometry on the Seismogenic Environment of the Longmenshan Fault: Insights from Multi-Source Data-Constrained 3D Numerical Modeling of the Eastern Tibetan Plateau, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9011, https://doi.org/10.5194/egusphere-egu26-9011, 2026.
In the inner Northern Apennines (Lerici and La Spezia inland) the exhumed Tellaro detachment fault system is exposed. It can be traced in an area larger than 20 Km2 and well observable in continuous kilometer-long coast exposures (Storti, 1995; Clemenzi et al., 2015). The major low-angle fault zone is marked by decameter-thick, carbonate-rich, cataclasites and gouges (“Calcare Cavernoso Fm.”) overlying a footwall of cataclastically deformed low-grade quarzites, phyllites and metaconglomerates (Ladinian-Carnian Verrucano Fm.) belonging to the Tuscan Metamorphic units (Molli et al., 2018). In the hanging wall, synthetic and antithetic splay faults affect the originally ~6 Km-thick Tuscan Nappe succession, thinned to less than 0.6 Km.
Detailed structural data collected at the meso- and microscale, combined with Raman spectroscopy, fluid-inclusion analysis, and mineralogical studies, allowed us to constraints deformation processes and fault activity in a temperature range of 260-120 °C at mid-shallow crustal depth (10-5 Km). Observable deformation structures in the footwall fault rocks provide evidence supporting mixed deformation mode and mechanisms, with intermitted cataclastic flow and unstable brittle slip (aseismic-to-seismic) during the fault activity.
How to cite: Molli, G., Berio, L., Pizzati, M., Lucca, A., Cecchini, P., Balsamo, F., and Storti, F.: Mixed mode of deformation and processes along the Tellaro Detachment (Northern Apennines, Italy), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9321, https://doi.org/10.5194/egusphere-egu26-9321, 2026.
The processes governing the nucleation and arrest of a rupture during slow slips remain speculative. The importance of understanding slip arrest mechanisms lies in the potential for slow slips to trigger destructive earthquakes and in the fact that not all slow slips lead to the nucleation of regular earthquakes.
At seismogenic depths (<30 Km, 100°-400°C), phyllosilicate-rich rocks (i.e., claystones, metasediments, serpentinites) are widespread lithologies that are also frictionally weak (µ≪0.6). The presence of these rock favors slip nucleation at weak fault patches which may or may not develop into fast unstable slip.
We performed hydrothermal friction experiments at the temperatures and pressures relevant to the seismogenic zone to understand the mechanism(s) behind slip nucleation and arrest. We tested experimental gouges of phyllite (Rio Marina Fm.) and meta-sandstone (Verrucano Fm.) from a natural shear zone exposed at the Elba Island (Italy). Experiments were performed at a shearing velocity of 10 µm/s over a wide range of effective normal stresses (20 to 150 MPa), high temperature (350 °C) conditions and to high strains (displacements up to 40 mm) using two hydrothermal Rotary shear machines hosted in Padova University (Italy) and Utrecht University (Netherlands).
Experimental results show that the phyllite sheared at low effective normal stresses (20-50 MPa), show a low friction coefficient of µ ~ 0.3 and a strain weakening behavior. With increasing normal stress (up to 150 MPa) we observe an initial low friction (0.35) that evolves with a strain hardening trend up to µ ~ 0.7-0.9. Conversely, experiments on the meta-sandstone show generally higher friction (0.6-0.7) even at small strains at all normal stress conditions.
Frictional weakness is due to the phyllosilicates’ ability to develop efficient foliations that accommodate the deformation. At low effective normal stresses (up to 60 MPa), we observe the development of a through-going phyllosilicate network within the phyllite gouge resulting in the observed low friction and strain weakening evolution. Conversely, at high normal stress, a through-going weak phyllosilicate network cannot develop because of the presence of frictionally strong high stress asperities, from which phyllosilicates have been extruded. The observed strain hardening and high friction trend results from comminution of intervening quartz and feldspar grains that we correlate with the occurrence of ultracataclasites in the microstructures. The experimental results on the meta-sandstone confirm this hypothesis, showing a friction and a microstructure similar to the “hardened” phyllite gouges. EDS maps of chemical elements in the phyllite gouge sheared under high normal stress confirm the absence of an interconnected network of phyllosilicates. In natural shear zones, at seismogenic depth, we may observe slip nucleation in weak phyllosilicate rich rocks (µ ~ 0.3). However, the fault patch may quickly strengthen if the propagating slip is fast enough to disrupt the foliation, which would decelerate slip. Our study provides a new mechanism by which slow slip events may nucleate and spontaneously arrest, potentially halting the growth of rupture into regular earthquakes.
How to cite: Tesei, T., Salvadori, L., Di Toro, G., and Niemeijer, A.: Strain hardening as a mechanism for slip nucleation and arrest in phyllosilicate-rich rocks, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10168, https://doi.org/10.5194/egusphere-egu26-10168, 2026.
The Yusuf Fault system (YF) is one of the largest active strike-slip structures in the Alborán Sea, where it acts as a lithospheric-scale boundary accommodating part of the NW-SE convergence between the Nubian and Eurasian plates. It trends WNW-ESE, extends for ~150 km, and is characterized by a complex fault array and a prominent pull-apart basin. Accurate representation of its 3D geometry and seismogenic behavior is essential to constrain its seismic potential and understand the tectonic evolution of the region. In this study, we integrate multibeam bathymetry, high-resolution multi-channel seismic reflection (HR-MCS) data, and scaled analogue modeling to characterize the structural architecture and kinematic evolution of the YF. Bathymetric analysis using topographic attributes enabled detailed mapping of the fault trace and associated geomorphic features, while seismic data interpretation revealed subsurface discontinuities and fault offsets affecting key seismostratigraphic units from the Upper Miocene to the Quaternary. This information allowed us to construct the first 3D structural model of the YF, showing that the system consists of multiple overlapping segments forming a complex strike-slip fault architecture. To explore the processes controlling pull-apart basin development, we conducted analogue experiments simulating strike-slip fault interaction. Results suggest that basin opening is controlled by overlapped fault geometry and lateral displacement rates, providing a physical framework for interpreting the observed morphostructural patterns. This integrated approach improves constraints on fault growth and segmentation, offering critical input for seismic hazard models in the western Mediterranean.
How to cite: Perea, H., Mattos, N., Ferrer, O., Gratacós, O., Carola, E., Canari, A., and Martíniez-Loriente, S.: 3D Structural Characterization and Analogue Modeling to Constrain the growth and evolution of the Yusuf Fault (Alborán Sea), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11504, https://doi.org/10.5194/egusphere-egu26-11504, 2026.
Vein stable isotope geochemistry and minerology provide a powerful record of fluid sources in subduction-zone fault systems, with implications for fault mechanics and seismic behaviour. We investigate fluid sources along out-of-sequence thrusts within an exceptionally well-preserved exhumed analogue of the shallow seismogenic zone: the Inuyama Sequence from the Jurassic Accretionary Complex in central Japan. The sequence comprises a coherent ocean-floor stratigraphy of siliceous claystone, ribbon chert, siliceous mudstone and clastic units, repeated by thrust imbrication.
Vein and host rock stable isotope data reveal the presence of two distinct vein sets, implying two distinct fluids, within the thrust sheets. One fluid is a cool pore water (𝛿18O = - 4 to 0 ‰), that precipitated quartz (21.2 to 25.7 ‰), calcite (20.6 to 21.1 ‰), and rhodochrosite (25.2 ‰) veins at ∼40 to100 ˚C throughout the exposed thrust sheets. This is consistent with a seawater-derived pore fluid in the shallow accretionary prism. In contrast, some quartz (2.1 ‰) and calcite (-0.4 to 6.2 ‰) vein clusters require a different and warmer fluid (𝛿18O = - 11 to - 8 ‰) possibly of meteoric origin. These isotopically lighter veins are restricted to discrete shear zones with well-developed scaly fabric and are generally focused along the margins of mechanically competent blocks. Notably, these discrete shear zones have far higher carbon contents than the host rocks, be that through pressure solution or direct carbon precipitation. The isotopically lighter calcite and quartz veins record significantly higher temperatures (∼170 to 220 ˚C), confirmed with chlorite geothermometry, and are in line with Raman and vitrinite reflectance temperature estimates for peak conditions for the area (Kameda et al., 2012; Ujiie et al., 2021).
The occurrence of isotopically light fluids at temperatures of 170-220˚C, corresponding to depths of ~ 8.5 to11 km given a relatively cool accretionary geotherm (20 ˚C/km), requires either (1) deep and lateral ingress of meteoric waters into the inner wedge as accreted sediments approached the coast, (2) late-stage vein precipitation during exhumation and fault reactivation, or (3) kinetic isotope effects associated with rapid precipitation during fault dilation that drives 𝛿18O lower than those predicted for equilibrium precipitation. Importantly, the hot and isotopically light fluids show a strong spatial relationship with highly concentrated black carbonaceous material that appears to control strain localisation at thrust sheet contacts. Consequently, these fluid-driven mechanical changes may have created carbon-rich asperities of very low frictional strength, encouraging local aseismic creep and stress build up at asperity boundaries.
How to cite: Robertson, R. V. M., Toffol, G., Fagereng, A., Ujiie, K., and Julve, J.: Tracing fluid sources, their influence, and mechanical consequences in the Inuyama accretionary complex, Japan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13844, https://doi.org/10.5194/egusphere-egu26-13844, 2026.
Earthquakes and ice sheet collapse are significant hazards that are both governed by friction. Fault interfaces and glacier beds share many frictional behaviours: slip stability and instability, seismicity, healing, and episodic slip. Rate-and-state friction (RSF), an empirically derived framework for describing frictional strength, has been successfully utilized for the last five decades to quantitatively characterize earthquake phenomena and has more recently been employed to describe stick-slip behaviour of glaciers. While RSF has been used to extract consistent parameters in both systems, frictional behaviours are rooted in the evolution of the asperities in contact at the interface. Although RSF is powerful (and practical!), it does not reveal the micromechanisms driving the behaviour it describes, nor does it account for other behaviours, such as rupture initiation and variation in stress drop. For this reason, we take advantage of the transparency of ice, its faster deformation timescales, as well as the frictional properties ice shares with rock, to directly observe the frictional interface in situ during shear. To do this, we employ a novel adaptation to our cryogenic biaxial device. As the interface cycles between periods of holds and shears, we use 1) an optical technique, total internal reflection, to light up the interface contacts and observe their evolution, and 2) acoustic emission sensors to listen to and locate slip events. This unique combination of data will allow us to more comprehensively understand the contact-level mechanisms that control friction on deforming interfaces, and help us to better interpret the seismological data we measure on faults and ice sheets. Here, we present recent results of this work.
How to cite: Bate, C., McCarthy, C., Steinhardt, W., and Saltiel, S.: Fault contact evolution seen via total internal reflection and heard via acoustic emissions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14779, https://doi.org/10.5194/egusphere-egu26-14779, 2026.
Understanding how connected fault segments may rupture sequentially or simultaneously to produce large earthquakes is a fundamental problem in earthquake physics and seismic hazard assessment. Addressing this issue requires integrated constraints on fault geometry, slip behavior, and rupture history across interconnected fault systems. The Subei triple junction along the eastern section of the Altyn Tagh fault, which connects thrust (Danghe Nan Shan thrust) and strike-slip fault segments (Altyn Tagh and Yema-Daxue Shan faults), is an ideal site for investigating such cascading rupture processes. In this study, we excavated three new paleoseismic trenches around the Subei junction, including two across the Danghe Nan Shan thrust and one across the Altyn Tagh fault. Detailed stratigraphic logging, identification of abundant paleoseismic indicators, and dense optical stimulated luminescence (OSL) dating constrain the timing of late Quaternary surface-rupturing earthquakes. Our results indicate that three to four surface-rupturing events occurred at these sites during the Holocene, with overlapping age ranges among the different fault segments. By integrating our new paleoseismic constraints with previously studies, we identify at least one Holocene earthquake that likely involved synchronous rupture of the Altyn Tagh fault, the Yema–Daxue Shan fault, and the Danghe Nan Shan thrust. Multicycle dynamic rupture modeling incorporating fault geometry and long-term slip rates further support such cascading ruptures across the strike-slip and thrust fault network. These results provide rare field-based evidence that large earthquakes on the eastern section of Altyn Tagh fault may involve multiple fault segment ruptures. Our findings highlight the importance of considering fault interactions and cascading rupture scenarios when assessing seismic hazard in complex continental fault systems.
How to cite: Shao, Y., van der Woerd, J., Liu-Zeng, J., Li, B., and Zhang, S.: Possible cascading ruptures on the eastern section of the Altyn Tagh fault, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15670, https://doi.org/10.5194/egusphere-egu26-15670, 2026.
Subduction zone megathrusts accommodate a wide range of slip modes, from earthquakes to slow slip events (SSEs) and aseismic creep. Understanding why different slip modes localize in specific regions of the shallow subduction interface remains a significant challenge. Exhumed accretionary complexes are an important natural laboratory for addressing this problem. In this study, we examine the mechanical behavior of representative lithologies and faults within the McHugh Complex in the Kenai Mountains of southern Alaska. The McHugh Complex is a Mesozoic accretionary wedge that exposes lithologies and fault zones representative of the shallow subduction interface. We integrate field observations with compositional and microstructural analyses and laboratory friction experiments to evaluate both fault failure conditions and potential slip modes. Direct shear experiments were conducted on powdered fault gouges and host rocks under dry and water-saturated conditions at normal stresses of 10–40 MPa, representative of shallow subduction zone conditions.
Our results demonstrate that mineralogical composition exerts a first-order control on fault strength and frictional stability. Increasing proportions of phyllosilicates reduce friction coefficients (μf) and promote velocity-strengthening behavior. Argillitic fault gouges rich in organic matter exhibit the lowest frictional strength (μf = 0.33), consistent with strain localization observed in these rocks in the field. Conversely, stronger lithologies, such as pillow basalts and cherts, display higher frictional strengths (μf = 0.53) and frictional stabilities that promote seismic slip initiation. However, fault zones within basaltic units that have undergone significant alteration and chlorite enrichment evolve toward velocity-neutral behavior, suggesting the potential to nucleate SSEs rather than earthquakes.
A cross-section through the exhumed accretionary wedge reveals that contrasts in mechanical strength often coincide with contrasts in permeability, suggesting that stress concentrations and transient fluid overpressure likely act together to trigger fault failure. Overall, our findings emphasize the role of lithologic heterogeneity in controlling both fault failure and slip mode along the shallow subduction interface. This provides a framework for linking rock composition to the spatial distribution of seismic and aseismic behavior. Future work will apply this integrated approach to additional cross-sections across the McHugh Complex.
How to cite: Rast, M., Behr, W., Madonna, C., and Guérin-Marthe, S.: Lithologic Controls on Frictional Behavior Along the Shallow Subduction Interface: Constraints From an Exhumed Accretionary Wedge (McHugh Complex, Alaska), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16525, https://doi.org/10.5194/egusphere-egu26-16525, 2026.
Slip-rate decrease along the eastern Altyn Tagh fault has long been interpreted as due to strain transfer from the strike-slip fault to the sequential thrust splays of the Qilian Shan. While the 2D kinematics of such strain transfer is now becoming well documented thanks to numerous field studies as well as an increase in geodetic and InSAR data, how the structures connect, interact during large rupturing events and evolve on the long-term is not well known. We focus on the Danghe Nan Shan thrust, a major splay of the Altyn Tagh fault (ATF), at one of these complex fault junctions. Near Subei, the western Danghe Nan Shan thrust comprises two left-stepping faults outlined by fault scarps in front of folded and uplifted alluvial fans and terraces. Age constraints of the accumulated slip of four terraces standing 7–60 m above the present stream bed yield shortening and vertical uplift rates of 0.5 ± 0.1 and 1.1 ± 0.3 mm/yr, respectively, over the last 130 ka on one of the thrust. Overall, about 1-1.4 mm/yr uplift and shortening rates are determined, in agreement with late Miocene long-term exhumation rate estimates. We emphasize the need for precise long-term slip rate determination and understanding the three-dimensional structures of fault connections to evaluate strain transfer between faults and related seismic hazard on these complex fault systems.
How to cite: van der Woerd, J., Shao, Y., Yuan, D., and Liu-Zeng, J.: The Subei triple junction: a complex strike-slip – thrust junction allowing strain transfer from the Altyn-Tagh fault to the Qilian Shan , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16986, https://doi.org/10.5194/egusphere-egu26-16986, 2026.
A relevant portion of seismic activity in subduction zones takes place along splay faults and other subsidiary structures of the subduction interface that cut across the upper plate accretionary complex. The heterogeneous lithology of accretionary complexes, reflecting the stratigraphy of the incoming ocean plate, exerts a first-order control on the seismic behaviour. Thus, investigating accretionary complexes exhumed from the seismogenic zone is relevant to understand upper plate seismicity.
The Inuyama Sequence, part of the Jurassic Accretionary Complex of central Japan, is the ideal natural laboratory to investigate the lithological control on faulting style and seismic/aseismic behaviour in the shallow, sediment-dominated portion of an accretionary prism. It consists of a coherent chert-clastic complex with ocean-floor stratigraphy (in ascending order: siliceous claystone unit, ribbon chert unit, siliceous mudstone unit, and a clastic unit composed of lower mudstone, sandstone and upper mudstone) repeated six times by out-of-sequence thrusts that delimit the thrusts sheets [1].
Here we focus on three of the out-of-sequence thrusts (T1, T2, T3 in ascending structural order) that are well exposed along the Kiso River: T1 separates siliceous mudstones of sheet 1 from black and grey cherts of sheet 2; T2 separates upper mudstones of sheet 2 from siliceous claystones and cherts of sheet 3; T3 separates upper mudstones of sheet 3 from siliceous mudstones and cherts of sheet 4 whose stratigraphic topping direction is overturned compared to the other sheets.
Fault zones are 10–50 metres in thickness and mostly accommodate strain in the weaker clay-rich lithologies (siliceous mudstones and siliceous claystones), typically localizing deformation along carbonaceous-material-rich layers. A pervasive foliation in the siliceous mudstones of T1 and 50–100 cm thick slip zones with scaly fabric in siliceous claystones and siliceous mudstone suggest predominant deformation by aseismic creep. The stiffer cherts are also involved in the fault zones. In T1, the hanging-wall derived brecciated cherts host a mm-thick pseudotachylyte fault vein recording earthquake slip [2]. In T3 a localized fault core in the hanging-wall cherts is rich in quartz clasts with pervasive 2-5 µm spaced deformation lamellae, recording high stress pulses.
Chlorite geothermometry applied on syn-kinematic chlorite and chlorite-quartz veins abundant in the fault rocks provide temperatures in the range 170–210 °C, in line with peak condition estimates for the area, confirming that the investigated structures were developed during accretion. Lower chlorite temperatures, down to 100°C, have also been measured in a scaly fabric fault zone, suggesting later reactivations of the fault at colder (shallower) conditions during exhumation.
These preliminary results highlight the importance of the heterogeneous stratigraphy of accretionary complexes in controlling faulting style: while weak mudstones accommodated most of slip by aseismic creep, the stiffer cherts hosted occasional high-stress pulses associated with seismic ruptures. Further questions to answer include how slip is partitioned and what factors promote seismic ruptures in the stiffer lithologies.
[1] Kimura, K., Hori, R. (1993) Journal of Structural Geology, 15(2), 145-161.
[2] Ujiie, K., et al. (2021) Earth and Planetary Science letters, 554, 11638
How to cite: Toffol, G., Robertson, R. V. M., Fagereng, Å., Ujiie, K., and Julve Lillo, J.: Accretionary complex heterogeneity controls the faulting style of upper plate thrusts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17287, https://doi.org/10.5194/egusphere-egu26-17287, 2026.
The northeastern Tibetan Plateau is bounded by two major left-lateral strike-slip faults: the Altyn Tagh and Haiyuan faults. Eastward crustal motion along these faults, driven by the ongoing Indo-Asian continental collision, diminishes progressively toward their eastern terminations. Here, the crustal deformation is dominated by crustal shortening, thrust faulting, and uplift, which collectively contribute to the lateral growth of the plateau. Understanding this tectonic transition is essential for interpreting the plateau’s expansion and the seismic hazard along its northeastern boundary. We integrated InSAR, GNSS, and precise-leveling data to reveal the present-day crustal deformation in the northeastern Tibetan Plateau. Our analysis indicates that eastward motion along the Altyn Tagh fault is absorbed by thrusting and uplift within the Qilian Shan. Similarly, the Haiyuan fault transitions into crustal shortening and uplift in the Liupan Shan orogen. These transitions are largely controlled by the geometry of the strike-slip faults and the presence of the rigid Alashan and Ordos blocks to the east, which impede eastward motion of the Tibetan crust. Our results of present-day crustal deformation align with late-Cenozoic geological structures in northeastern Tibet and the stress patterns inferred from earthquakes along its northeastern margins, supporting a consistent model of ongoing plateau growth through transitioning from the escaping tectonics to mountain building.
How to cite: Liu, M., Hong, S., and Li, Y.: Tectonic transition in the northeastern Tibetan Plateau: from tectonic escape to mountain building, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18134, https://doi.org/10.5194/egusphere-egu26-18134, 2026.
Posters virtual: Wed, 6 May, 14:00–18:00 | vPoster spot 1a
EGU26-5809 | ECS | Posters virtual | VPS30
Rotation of tectonic blocks controlled by strike-slip component along the Zahedan fault, IranWed, 06 May, 14:21–14:24 (CEST) vPoster spot 1a
The Zahedan fault zone in Iran constitutes an active tectonic zone characterised by a complex network of strike-slip faults that dominate the local deformation pattern. This area is located within a large-scale transpressional shear zone accommodating relative motion between the Central East Iranian block and the Afghan Helmand block. The region provides a natural laboratory for investigating the relationship between strike-slip faulting and tectonic blocks rotated around vertical axes.
We present herein, based on high-resolution 2025 Airbus satellite imagery and cartographic and geophysical data, a new strike-slip fault pattern that facilitated the development of the rotated tectonic blocks.
Our observations show that the major strike-slip fault zones are accompanied by dense networks of second-order faults, including single sets of antithetic and synthetic strike-slip faults, conjugate strike-slip fault sets, restraining and releasing stepovers, and thrust faults. In several sectors along the major faults occur the zones of deformation consisting of the rotated tectonic blocks. The scale, orientation, and spatial organisation of the mapped structures indicate that block rotation is controlled by the interaction between major strike-slip faults and subsidiary fault networks.
The individual second-order antithetic faults display that these faults commonly accommodate small displacements, but the faults play a critical role in allowing internal deformation within blocks and facilitate the progressive block rotation. The sense of movements along the major fault and the antithetic strike-slip faults bounding the tectonic blocks allows us to consider the structures as the blocks rotated around vertical axes in a domino-like orientation. Recognised examples of structures show that some rotating blocks are rigid, with no evidence of significant internal deformation, while other rotating blocks exhibit strong internal deformation.
Understanding these spectra of behaviours and the determination of the relationships between them will improve our knowledge of fault interaction processes in eastern Iran and related patterns of seismicity, and it also has implications for seismic hazard assessment in active transpressional settings.
How to cite: Paktarmani, Z., Konon, A., and Mikołajczak, M.: Rotation of tectonic blocks controlled by strike-slip component along the Zahedan fault, Iran, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5809, https://doi.org/10.5194/egusphere-egu26-5809, 2026.
