The Aceh Fault, part of Indonesia's Great Sumatran Fault System, exhibits recent faulting through prominent scarps along its 250-kilometer length. Running northwest-southeast, it spans northwestern Sumatra from Tripa to Banda Aceh, a city of over 268,000 residents. Understanding the complete faulting history is essential for assessing seismic risk, as instrumental records are too recent to capture long-term patterns. We study the fault by combining remote sensing using 8-m resolution DEM (DEMNAS) for the entire area and 15-cm resolution (LiDAR drone survey) for selected areas, field methods, and paleoseismology. We excavated two paleoseismic trenches across the fault and documented evidence of at least three well-dated ground-rupturing earthquakes from the upper 2 meters of strata spanning the last ~1000 years. The event chronology is constrained by 15 radiocarbon dates on detrital charchoal. This new paleoseismic data confirms that the Aceh Fault is active. Our study delineates the active trace of the fault zone and provides the first detailed information about significant prehistoric earthquakes along this fault. These findings improve seismic hazard maps and enhance understanding of the region's seismic risks.

How to cite: Marliyani, G. I., Klinger, Y., Yao, W., Setianto, A., Helmi, H., Kurniawan, T., Triyono, R., Rusdin, A. A., Rohadi, S., and Karnawati, D.: Preliminary Results of the Paleoseismology of Aceh Fault in northern Sumatra, Indonesia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19994, https://doi.org/10.5194/egusphere-egu25-19994, 2025.

Stress interactions between neighbouring faults plays a key role in controlling earthquake recurrence and size, and therefore in the seismic hazard posed by individual faults within a fault network. In this study, we investigate how differences in the predominant arrangement of faults, specifically, whether it is along-strike or across-strike, affect earthquake recurrence rates and magnitude of earthquakes. To address this topic, we use the boundary-element code QDYN to simulate earthquake cycles of two fault systems within the actively extending region of the Italian Apennines: one to the south where faults are predominantly arranged along-strike, and another in the central Apennines where faults are predominantly arranged across-strike.  The different styles of fault network between the Central and Southern Apennines, and high seismic hazard of the region, make this the ideal area to investigate the role of fault geometry on earthquake behaviour across multiple seismic cycles in this region.

The models account for variable fault slip rates between faults and network geometry to determine their impact on seismic cycles and earthquake statistics. These simulations produce spontaneous ruptures, with slip modes encompassing full and partial ruptures as well as slow-slip events. We found a good fit between the modelled magnitudes and the ones derived from historical ruptures and empirical relationships. Fault networks with multiple across-strike faults produce more complex seismic sequences, including greater variability in recurrence times and higher proportion of partial ruptures, compared to fault networks with faults arranged predominantly along-strike. Lastly, we assessed the seismic hazard in the studied regions based on the modelled earthquake rates and magnitudes. Our findings show that the spatial distribution of peak ground acceleration corresponding to a 50-year exceedance probability has a greater heterogeneity compared to classical seismic hazard assessment approaches. Hazard levels are elevated in areas where multiple faults overlap, highlighting the influence of fault interactions on regional hazard patterns. These findings show the influence of fault system geometry on how stresses redistribute across multiple earthquake cycles and associated seismic hazard.

How to cite: Rodriguez Piceda, C., Mildon, Z., Andrews, B., Yin, Y., Ampuero, J.-P., van den Ende, M., and Sgambato, C.: Seismic sequences in the Italian Apennines influenced by fault network geometry , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-876, https://doi.org/10.5194/egusphere-egu25-876, 2025.

The characteristics and factors that control the Off-Fault Deformation（OFD）remain poorly understood. The existing studies shows the 2021 Mw 7.4 Maduo earthquake produce the largest OFD than other earthquake cases. We try to use the China Gaofen-serie-satellite images to re-constrain the OFD deformation. By correlating pairs of images before and after this earthquake, we obtain the coseismic deformation parttern of  the 2021 Mw 7.4 Maduo earthquake. By measuring the coseismic deformation, we constrain the near-field and far-field surface displacement distribution. The result shows that this earthquake accommodated 69% of total surface deformation as OFD deformation over a mean deformation-zone width of 237 m.  Our result show the OFD proportation of the Maduo earthquake is large, but our result is lower than the result by using the SPOT and Sentienl-2 images. By analying the fault geometry  and geological deposit, we think the magnitude and width of off-fault deformation along the rupture is primarily controlled by the fault maturity and structural complexity of the fault. 

How to cite: Kang, W., Li, Z., and Xu, X.: Large Off-Fault Deformation of 2021 Mw 7.4 Maduo Earthquake along an Immature Strike-Slip Fault, Tibetan Plateau, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2435, https://doi.org/10.5194/egusphere-egu25-2435, 2025.

Subduction megathrusts are among the largest fault systems on Earth and are responsible for generating megaearthquakes-the most powerful earthquakes and one of the most destructive natural phenomena. However, obtaining natural data on the Subduction Earthquake Cycle (SEC) in these areas is challenging due to the long recurrence intervals of such events. To overcome this limitation, we used analogue models to reproduce in the laboratory hundreds of seismic cycles under different conditions in just a few minutes. The models feature a single velocity weakening asperity (i.e., rice) surrounded by a velocity-neutral material (i.e., sand). Using a parametric approach, we systematically varied two key parameters of our single asperity model: (1) the rheology of the upper plate, which affects its stiffness and (2) the normal load (σ_n) applied on the asperity. We performed four distinct models, each with a different upper plate stiffness. For each upper plate stiffness we implemented four σ_n (i.e., 16 models in total). High-resolution monitoring of our models, combined with Particle Image Velocimetry, allowed for a detailed analysis of the analog earthquakes. The variation in upper plate rheology enabled the models to simulate the transition from stick-slip behavior to stable sliding, governed by the ratio k/kc, the stability parameter within the rate-and-state framework. Moreover, the models demonstrate that this variation is a controlling factor of magnitude and recurrence time of the analogue events. Comparing the results with natural data, we found that all the models exhibit moment magnitudes (Mw) comparable to those of natural megaearthquakes. The possibility of crossing the k/kc=1 threshold allows us to explore the stick-slip behavior in a regime that includes period doubling linked to the coexistence of faster and slower slip rates. The findings in our experimental models demonstrate the influence of the upper plate rheology in the spectrum of megathrust slip behaviors, providing constraints that could potentially be applied to natural subduction zones. 

How to cite: Guastamacchia, S., Corbi, F., Mastella, G., Brizzi, S., and Funiciello, F.: Subduction Earthquake Cycle through the lens of analogue modelling: the role of the upper plate rheology, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4056, https://doi.org/10.5194/egusphere-egu25-4056, 2025.

Normal faults in a rate-and-state friction model release seismic energy in distinct, instantaneous seismic events (i.e. earthquakes) between steady state periods. However, recent geodetic work in the Gulf of Corinth, Greece suggests that some seismogenic normal faults can also undergo transient aseismic slip events above steady state deformation rates in interseismic periods. Integrating the full range of fault slip behaviours into fault evolution frameworks is required to better constrain how normal faults accommodate and release strain with implications for rift development and seismic hazard. Therefore, further detailed observation of both coseismic and aseismic slip behaviours across normal faults at all time scales are needed.

In this analysis, we exploit open-source, vertical ground motion data from the European Ground Motion Service (EGMS), derived from five-years of Interferometric Synthetic Aperture Radar (InSAR) measurements, to evaluate uplift and subsidence in areas of active tectonics. While vertical ground motion data likely reflects a range of geological, hydrological and anthropogenic processes, isolating tectonic signals allows quantification of fault motion on annual to decadal time scales using the Europe-wide dataset. Therefore, this data bridges an important time-scale gap between event-specific InSAR studies and geological assessments and provides regional context to ground motion. Here, we use time series spanning 2019 to 2023 to assess vertical ground motion across normal faults in Central Greece that have, and have not, hosted large earthquakes in this period.

Spatio-temporal ground motion analysis is conducted for the March 2021, Mw > 6 earthquakes in the Larissa Basin (Thessaly). The cascading rupture style of the earthquakes and aftershocks is resolved in EGMS time series, and geometries of uplift and subsidence are plotted to define rupture parameters and fault plane projections. High coseismic uplift to subsidence ratios of 1:6 – 1:9 reflect the tight structural controls on this earthquake sequence. In contrast to Larissa, EGMS time series across the Coastal Fault System of the North Gulf of Evia imply aseismic normal fault slip. Differential vertical ground motion is recorded across both the Kamena Vourla and Arkitsa fault segments with little to no associated seismicity. Time-averaged throw rates of 2 - 3 mm/yr are measured at an uplift to subsidence ratio of 1:2. These throw rates exceed the long-term, geodetic extension rates across the North Gulf of Evia suggesting that the faults are moving in a transient period of elevated aseismic slip between 2019 and 2023. The nearby Atalanti Fault, which hosted two Mw > 6.4 earthquakes in 1894, shows no differential ground motion across its plane reflecting that the fault is in a locked state. The observed variable shallow crustal behaviour of normal faults implies long-term, geologically derived throw rates on normal faults likely combine transient periods of elevated aseismic slip, coseismic slip, and steady state strain accommodation.

How to cite: Wood, J., Whittaker, A., Bell, R., Kranis, H., Ganas, A., and Peron-Pinvidic, G.: Coseismic and aseismic normal fault slip in Central Greece from InSAR time series, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4323, https://doi.org/10.5194/egusphere-egu25-4323, 2025.

How to cite: Martinelli, C., Hollingsworth, J., Jolivet, R., and Thomas, M.: How well can displacement be resolved close to earthquake surface ruptures using optical image correlation?  , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5877, https://doi.org/10.5194/egusphere-egu25-5877, 2025.

On September 5, 2022, a magnitude Ms6.8 earthquake occurred in Luding County, Sichuan Province. This earthquake occurred at the key part of the southeast-clockwise extrusion of material on the eastern margin of the Tibetan Plateau, the Y-shaped confluence of the Xianshuihe, Longmenshan and Anninghe fault zones. In this study, the three-dimensional dynamic crustal density changes in the earthquake area are obtained by the typical gravity change data from 2019 to 2022 before the earthquake and gravity inversion by growing bodies. The results indicate that gravity changes presented an obvious four-quadrant and gradient belt distribution in the Luding area before the earthquake. The three-dimensional density horizontal slices show that small density changes occurred at the epicenter in the mid-to-upper crust between 2019.9 - 2020.9 and 2019.9 - 2021.9. At the same time, the surrounding areas exhibited a positive and negative quadrant distribution. These observations indicate that the source region was likely in a stable locked state, with locking in shear forces oriented in the NW and NE directions. From 2021.9 to 2022.8, the epicentral region showed negative density changes, indicating that the source region was in the expansion stage, approaching a near-seismic state. The three-dimensional density vertical slices reveal a southeastward migration of positive and negative densities near the epicenter and on the western of the Xianshuihe Fault Zone, indicating that the material is flowing out to the southeast. The observed local negative density changes at the epicenter along the Longmenshan Fault Zone are likely associated with the NE-oriented extensional stress shown by the seismic source mechanism. The above results can provide a basis for interpreting pre-earthquake gravity and density changes, thereby contributing to the advancement of earthquake precursor theory.

How to cite: Wang, J., Tan, Q., and Shen, C.: Dynamic changes of gravity field before the Luding Ms6.8 earthquake and its crustal material migration characteristics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6505, https://doi.org/10.5194/egusphere-egu25-6505, 2025.

Mongolian tectonics is shaped by the far-reaching effects of the Indo-Eurasian collision, which drives deformation and stress over 2000 km behind the Himalayan front. During the 20th century, Mongolia experienced four earthquakes with magnitudes greater than 8, making it an exceptional location for studying intraplate seismicity, predominantly with strike-slip components. Among these events, the Tsetserleg-Bulnay fault system recorded the largest intraplate earthquake doublet, with two magnitude 8 earthquakes occurring 14 days apart in 1905, rupturing more than 500 km of fault. The surface rupture, remarkably well-preserved due to the region's low erosion rate, has enabled extensive paleoseismic investigations. Despite this, the junction between the two faults remains unclear at the surface, and the fault structures at depth are still poorly constrained, leaving the interactions between fault segments not well understood.

In the present day, the significant microseismic activity affecting the Bulnay and Tsetserleg faults is anomalous given the low regional deformation rate and overall Mongolian seismicity. This persistent microseismicity could be interpreted as aftershocks that illuminate the faults’ structures more than a century after their mainshocks. By tracking this microseismicity with precision, we aim to map the faults’ 3D geometry at depth and address several questions including: how do these faults interact, why did the Bulnay earthquake occur only 14 days after the Tsetserleg earthquake, and why is its epicenter located 150km west of the junction zone?

In 2024, the French Atomic Energy Commission (CEA) and the Mongolian Institute of Astronomy and Geophysics (IAG) collaborated to strategically deploy a temporary seismic network, TDBnet, at the Bulnay-Tsetserleg junction. This network, comprising 10 geophones in addition to 5 broadband stations, operated altogether for five months, complementing the national network, and recorded local seismicity with unprecedented resolution. The collected data are being processed to automatically detect seismic phases using state-of-the-art methods, including the EQTransformer artificial neural network implemented in Seisbench. The detected events are then precisely located using an absolute location method, followed by an absolute relocation corrected with a Source Specific Station Time approach as proposed in the NonLinLoc-SSST framework. We present the experiment along with preliminary results, including a precisely determined earthquake epicenter map.

Acknowledgement  : We sincerely acknowledge the IAG-DASE technical team for their collaboration in the deployment of the temporary seismic network (TDBnet): Narmandakh Adyasuren⁴, Dorjdavaa Myagmar⁴, Youndonjunai Sodvoobavuu⁴, Nyamdorj Badarch⁴, Munkhbat Dagva⁴, Enkhtuvshin Begzsuren⁴, Purevsuren Dosmaa⁴, Leo Chazellet^1,4, Serge Olivier^1,4, Vincent Lisette^1,4, Denis Lubin^1,4.

How to cite: Bollinger, L., Manceau, L., Klinger, Y., Letort, J., Munkhuu, U., Bakthuu, B., Ganbold, T., and Technical_team, I.: Investigating the Tectonic Complexity of the Bulnay-Tsetserleg Fault Junction in Mongolia Using a Temporary Seismic Network, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9148, https://doi.org/10.5194/egusphere-egu25-9148, 2025.

How to cite: Allemand, A., Klinger, Y., and Scholtès, L.: A 3-D numerical model to bridge long- and short-term approaches of deformation on a strike-slip fault, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11845, https://doi.org/10.5194/egusphere-egu25-11845, 2025.

Following the 2004 Mw 9.2 Sumatra-Andaman event, a series of earthquakes occurred along the Sunda megathrust of the Sumatran subduction zone, extending from the southern terminus of the 2004 rupture to Bengkulu. A notable exception is the Mentawai seismic gap, spanning from just south of the Batu Islands to Sipora for ~200 km in length. Historical records of regional seismicity from paleogeodetic measurements (i.e. coral microatolls) indicate that the last major event that ruptured the current seismic gap occurred in 1797. An adjacent patch ruptured in 1833, broadly coincident with the 2007 Bengkulu rupture. More recent M≥7 events surrounding the Mentawai seismic gap have occurred in 2007, 2008, 2010, and 2023. However, slip distributions of these events show limited slip propagation into the gap and a significant slip deficit remains. Thus, a potential earthquake in the region poses a threat to local communities from both ground shaking, as well as a potential tsunami. Previous geodetic estimates of coupling in the region indicate low coupling at shallow depths on the megathrust. However, these estimates lack near-trench observations and ignore the influence of stress shadows originating from frictionally locked asperities downdip, and thus may underestimate the tsunami hazard, especially in light of the 2010 Mentawai tsunami earthquake that ruptured to the trench at depths of <6 km. Additionally, new estimates of long-term slip rates on the Sumatran Fault indicate the forearc sliver is deforming as rigid block and substantial oblique convergence is taken up within the oceanic plate. By correcting published geodetic velocities to remove the motion of the forearc sliver, we place updated constraints on subduction obliquity. Combining these observations with paleogeodetic uplift and subsidence rates, we invert for a coupling distribution on the Sunda megathrust, accounting for the effect of stress shadows, and constraining the coupling direction based on earthquake slip vectors. We find, in contrast to previous estimates, that the megathrust appears coupled to the trench. This coupled region extends from just north of Siberut south to the Pagai Islands and includes the region of the 2007 Bengkulu rupture. While the risk for large earthquakes in this region is relatively well known, our results indicate that the Mentawai seismic gap contains a strongly coupled patch that extends to the trench, suggesting that the tsunami hazard is significantly higher than inferred from previous coupling estimates. Additionally, this updated coupling model allows us to place new constraints on the influence of tectonics on regional sea level projections.

How to cite: Perry, M., Feng, L., Hill, E., and Sarkawi, G.: Updating megathrust coupling models for the Mentawai Seismic Gap and surrounding regions, Sumatra, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14410, https://doi.org/10.5194/egusphere-egu25-14410, 2025.

Earthquakes represent a significant hazard to human life, having claimed nearly a quarter of a million lives worldwide and strongly affecting an additional 125 million people between 1998 and 2017 (WHO). The Himalayan Front is an especially active continental collision zone spanning over 2500 kilometers across five countries, with its Himalayan Frontal Thrust (HFT) producing surface ruptures at the southern leading edge of the front (Kumar et al, 2001). Although recent earthquakes have produced surface ruptures along eastern and western sections of the HFT, paleoseismic and historical investigations have not revealed any surface rupture-forming earthquakes in the central Himalayas since at least the 17th century (Arora and Malik, 2017). This gap raises the potential for a mega-earthquake (> Mw 8) in coming years (Wesnousky, 2020). Here, we share preliminary results from a paleoseismic investigation of an exposed river section on the central HFT adjacent to Shahjahanpur village, 20 km southwest of Dehradun, Uttarakhand, India (30° 12 '04.6"N, 77° 49' 39.6"E). Optically stimulated luminescence (OSL) bulk sediment dates in combination with river section interpretations will aid in evaluating the presence of surface ruptures related to a major 1505 earthquake event in this area, for which numerous historical accounts exist (Jackson, 2002). Implications of these results include an improved estimation of this event’s western lateral extent in conjunction with previous studies. This will allow for the calculation of a more accurate paleo magnitude for the 1505 earthquake, ultimately informing the region’s seismic hazard potential.

How to cite: Soederberg, M., Arora, S., Cochran, D., and Singh, G.: Determining the Western Extent of the 1505 Central Himalayan Earthquake through a Paleoseismic Investigation of Surface Ruptures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14536, https://doi.org/10.5194/egusphere-egu25-14536, 2025.

Under the assumption that the plate convergence rate is distributed across faults along the plate boundary, in the Chungliao Tunnel area of southwest Taiwan, the total surface velocity change between the Chegualin fault (CGLF) to the west and the Chishan fault (CSNF) to the east exceeds 90 mm/yr, which is larger than the palte convergence rate of approximately 82 mm/yr in Taiwan. However, the physical processes driving these high-rate deformation is still debated. As the deformation is mainly aseismic, and to increase the spatial resolution of the large-scale surface deformation field, we used GNSS and ALOS-2 InSAR to understand tectonic processes. To examine the spatial continuity of the ultra-rapid deformation beyond the Chungliao Tunnel, InSAR processing was conducted using ALOS-2 ascending and descending datasets to improve the spatial extension and resolution of surface deformation. We introduced a priori phase discontinuity at mapped fault trace by setting the temporal coherence to correct the unwrapping errors. Then several Line-Of-Sight (LOS) velocity discontinuities are consistent with fault traces, indicating shallow creep along those faults. Furthermore, we demonstrated the continuity of few-hundred meters of high deformation between the CGLF and the CSNF with LOS velocity of 30-40 mm/yr, a LOS velocity gradient of 20-30 mm/yr across two faults. A 3D velocity reconstruction inverted by combining GNSS and ALOS-2 InSAR result reveals a local counter-clockwise rotation from NW to SW align north to south and the significant uplift (~80 mm/yr) in the narrow band between the Chishan fault and Chegualin fault near the Chungliao Tunnel. The local deformation implies the opposite lateral components of CSNF and CGLF in different segments of two faults as well, providing precise constraints to enhance the tectonic interpretation of this area. This rapid deformation identified in the narrow zone may be resulting from the interaction between the thrust faults and the surrounding mobile shale, in agreement with the hypothesis of a mud diapir of large mud diatreme that developed in the thick two thrusts.

How to cite: Wang, I.-T., Ching, K.-E., and Pathier, E.: Revisiting Rapid Surface Deformation in Southwestern Taiwan Using GNSS and ALOS-2 InSAR Data: Case study in Chungliao Tunnel, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16771, https://doi.org/10.5194/egusphere-egu25-16771, 2025.

Lacustrine paleoseismology, which focuses on sedimentary traces of past earthquakes in lakes, has gained increasing attention over the past two decades, even though on-fault trenching remains the most common technique in paleoseismology. This field primarily investigates Mass Wasting Deposits (MWD) and Soft Sediment Deformation Structures (SSDS) in lake sediments. Additionally, catchment response (CR), characterized by a temporary increase in erosion rates within catchments due to strong ground motions, is another significant trace of past earthquakes in lake sediments. In this study, past earthquake traces were analyzed in 19 gravity cores (98.880-138.70 cm in length) retrieved from the varved sediments of Köyceğiz Lake. High-resolution elemental profiles and optical images were obtained using ITRAX micro-XRF core scanner. ITRAX optical and XRF data along one core was used to generate varve chronology, and Ca/Ti profiles of the other cores were used to chronostratigraphically correlate 19 cores. Although the region experienced several notable earthquakes over the past 600 years, no MWDs were identified in Köyceğiz sediments; instead, SSDS and CR were observed. Distinct anomalies in Cr/Ti profiles related to the 1959 earthquake were evident in all cores. Conversely, CR associated with a mid-19^th-century earthquake was detected only in the northern basin, which has significantly larger catchment than the southern basin. SSDS, including faults, intraclast breccias and laminae disturbances were identified in Köyceğiz sediments. While some of these SSDS correlate temporally with historical earthquakes, most do not correlate either with seismic events or with each other. This implies that, contrary to what has been thought so far, SSDS formation may not be limited to the water-sediment interface but could also occur in deeper parts of the sequence. Moreover, the study indicates that the formation of SSDS may be controlled not only by peak ground acceleration (PGA) but also by peak ground displacement (PGD) due to earthquakes.

How to cite: Doğan, A. and Avşar, U.: Sedimentary records of past earthquakes in varved lake sediments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17648, https://doi.org/10.5194/egusphere-egu25-17648, 2025.

In southwestern Taiwan, about 45-50 mm/yr of westward shortening occurs across the 40-45 km wide fold-and-thrust belt, accompanied with tectonic extrusion towards the southwest. Within this broad framework, measurements from a local ground-based geodetic network revealed rapid ground deformation surrounding two sub-parallel geological thrust faults, located only 500 m apart. 50 mm/yr of shortening occurs on the western fault and 32 mm/yr of extension across the eastern one. In-between the faults, uplift relative to the east block increases eastward from 20 to 80 mm/yr. Sharp deformation gradients indicate aseismic slip on both structures. This remarkable deformation raises the question of the deep structure and mechanism at play: Is it driven by tectonic forces, possibly released as transient slip events? Or does it involve shale tectonics related to fluid overpressure within the mudstone formation that dominates the geology?

To investigate this phenomenon, we monitored ground deformation using image correlation for horizontal displacements and DSM time series for vertical displacements, aiming at high-resolution observations covering a wider area than the ground-based network. Eight sets of aerial images acquired from 2008 to 2015 were processed using the MicMac photogrammetric software. The resulting horizontal velocities are in good agreement with ground-based observations. The compressional gradient across the western fault (the Chegualin Fault) vanishes northward, but remains clearly visible towards the south, with an increasing right-lateral component. While we detect extension across the eastern fault (the Chishan Fault), precise location and quantification of the deformation gradient remains challenging due to poor correlation caused by dense vegetation. Elevation differences based on the DSMs derived from aerial images have a similar spatial pattern as ground-based observations, but the amplitudes are overestimated. On-going refinement in the processing and time series based on LiDAR datasets are expected to improve the results.

This work was complemented by the field survey of the numerous bedrock shear zones in the area to build a structural map of active structures. We confirm the Chegualin Fault as an active thrust fault, with an oblique component along its southern part. Extension across the Chishan Thrust is accommodated by SE-dipping en-echelon normal faults, found up to 1.4 km north of the ground-based network. The change in rake of the slickenlines indicates an increasing right-lateral component northward. While the pattern of horizontal velocities may fit with the regional tectonics, the hypothesis of a shale piercement so far best explains the ratio between uplift and shortening. Achieving a better imaging of the vertical deformation would help further discussing this assumption and eventually propose a structural model consistent with local and regional observations, which will also allow further assessing the associated natural hazards.

How to cite: Chen, K.-F., Le Béon, M., Delorme, A., Klinger, Y., Rupnik, E., Zhang, L., Pathier, E., Ching, K.-E., and Pierrot-Deseilligny, M.: Measuring rapid aseismic ground deformation within the foothills of southwestern Taiwan using aerial image correlation and DSM time series, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17661, https://doi.org/10.5194/egusphere-egu25-17661, 2025.

On July 1905, two M~8 earthquakes occurred 14 days apart along the Bulnay Fault system, in northwestern Mongolia. These seismic events are among the largest recorded earthquakes in intracontinental regions. However, our current understanding of the earthquake behavior of the Bulnay Fault is quite limited due to the scarcity of paleoseismic data. Additionally, the geographic and climatic conditions of the region play a major key in permafrost development, posing challenges in the excavation of paleoseismological trenches and causing cryoturbation. Lacustrine environments, conversely, are isolated depositional systems that minimize the influence of external factors and provide high temporal resolution with continuous sedimentation. Here, we present our findings on earthquake-triggered turbidites of eight sedimentary cores collected from three lakes around the Bulnay Fault. These cores were analyzed using X-ray tomography, X-ray fluorescence, and hyperspectral imaging. We found that prior to the 1905 event, three large earthquakes ruptured the Bulnay Fault, with recurrence intervals of 1.5 to 3 kyr. By integrating our observations with previous paleoseismic trench investigations, we proposed that strain is primarily accommodated through large earthquakes along the Bulnay fault, and major events involving both the Bulnay and Tsetserleg faults, potentially analogous to the 1905 doublet.

How to cite: Klinger, Y., Pinzon Matapi, N., Sabatier, P., Duarte, E., Choi, J.-H., Kim, T., and Ga, B.: Potential record of large earthquakes from lacustrine sedimentary archives along the Bulnay fault system (Mongolia), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19066, https://doi.org/10.5194/egusphere-egu25-19066, 2025.

In Southwest Japan, interseismic deformation exhibits distinct patterns, particularly across Kyushu Island, where its magnitude decreases significantly from north to south. Various mechanisms, including plate motions, fault slip on onshore fault systems, dilatational sources, and variable interplate coupling along the Nankai and Ryukyu subduction zones, have been proposed to explain these features. While previous studies have effectively modeled horizontal deformation and attributed the rotational pattern in southern Kyushu primarily to plate motion, they often neglect or inconsistently predict vertical deformation, underscoring the need for further investigation.

In this study, we employ a three-dimensional (3D) viscoelastic finite element model (FEM) to analyze interseismic deformation in Southwest Japan, spanning the transition from the Nankai to the Ryukyu subduction zone. To focus on megathrust coupling, we exclude block motion and consider other factors as secondary influences. Our goal is to reconcile horizontal and vertical geodetic observations and provide a first-order estimate of megathrust coupling in this margin through a viscoelastic model, offering a direct comparison with previously published elastic models.

How to cite: Liu, Y., Hu, Y., and Cui, X.: Megathrust Coupling in Southwest Japan Inferred from Viscoelastic Modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11133, https://doi.org/10.5194/egusphere-egu25-11133, 2025.

We investigate the complexity of two types of scaled seismotectonic models mimicking subduction megathrust seismic cycles. Our research encompasses a variety of model sizes, materials, deformation rates, and frictional configurations. Using nonlinear time-series analysis tools and displacement as an input variable, we characterize the dynamics of laboratory earthquakes in different phases of the labquake cycle. The number of active degrees of freedom that we are able to retrieve is low (<5) during most of the cycle, akin to slow slip episodes observed in natural settings and friction experiments performed with quartz powder. Results seem insensitive to the along-strike frictional segmentation of the megathrust. Nonetheless, the instantaneous dimension d can reach large values (>10), revealing the complexity of the system. High values of d correlate with slip phases, while significant drops in the extremal index anticipate slip episodes. Our results suggest that prediction horizons are in the order of a fraction of slip duration similarly to prediction horizons inferred for slow slip events in nature. This research not only enhances our understanding of earthquake dynamics, but also validates scaled seismotectonic models as effective tools for studying frictional physics across diverse spatio-temporal scales.

How to cite: Corbi, F., Gualandi, A., Mastella, G., and Funiciello, F.: Complexity of scaled seismotectonic models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9782, https://doi.org/10.5194/egusphere-egu25-9782, 2025.