The island of Taiwan is situated in a complex tectonic setting at the top of a triple junction, where the Eurasian plate is subducting under the Philippine Sea plate and vice versa. These opposing subductions generate intense deformation, culminating in frequent megathrust earthquakes, and, at greater depths, produce seismic tremors. In addition to seismic extreme events, Taiwan experiences strong monsoon seasons during which typhoons deliver up to 1 m of precipitation locally. Here, we observe a transient reduction in megathrust slip rates following major typhoons, as evidenced by decreased geodetic velocities and reduced tremor and earthquake rates, lasting for approximately 15 days. We interpret the apparent reduction in subduction rates as a result of water-load-induced increases in normal stress on the plate interface, which, in turn, increases interplate coupling. While correlations between individual crustal fault activity and hydrological cycles have been previously reported, our study demonstrates that such effects operate at much larger scales, temporarily slowing an entire subducting slab. Our observations highlight the importance of studying the coupling between climate and tectonic dynamics.

How to cite: Makus, P., Münchmeyer, J., Turowski, J. M., Männel, B., and Chang, J.-M.: Evidence for hydrologically-induced, short-term variations in subduction interplate coupling in Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12123, https://doi.org/10.5194/egusphere-egu26-12123, 2026.

Deep‐focus earthquakes and their association with metastable olivine wedges (MOWs) remain enigmatic. Here, we perform a seismic-geodynamic analysis of the Pacific slab, which is stagnant at the 660 km deep bottom of the mantle transition zone. We investigate deep earthquakes with moment magnitudes (Mw) ranging from 5.3 to 6.9 from 2009 to 2017. They exhibit only minor (mostly implosive) isotropic components, yet they display strongly varying CLVD components. For the largest studied earthquake (Mw 6.9, 2010-02-18), we demonstrate significant stress-drop heterogeneity on a subhorizontal fault and a spatial change in radiation efficiency. We interpret the earthquakes with an evolutionary numerical subduction model with realistic mineralogy and rheology, including non‐uniform plate aging and subduction disruption due to the Izanagi–Pacific ridge sinking in the early Cenozoic. This process resulted in a present-day slab with a bent tip that agrees with tomography. The slab maintains low temperatures (900-1000 K), allowing the presence of a metastable olivine and thus potentially forming MOW with a correspondingly bent geometry. The accompanying internal deformation controls the deep seismicity in the slab tip with apparent changes in seismic radiation efficiency and rupture speed across the modeled temperature gradients. From a broader perspective, the MOW contortion may contribute to deformational anisotropy in the shallow lower mantle. Our results underscore the importance of joint interpretations of the evolutionary subduction models and seismic source inversions.

Reference:

Liu, J., Zahradník, J., Plicka, V., Gallovič, F., Bina, C. R., Čížková, H. (2025). Deep-Focus Earthquakes Under Northeast China—An Imprint of the Complex Tectonic History of Pacific Plate Subduction, J. Geophys. Res. Solid Earth 130, e2024JB030215. https://doi.org/10.1029/2024JB030215.

How to cite: Gallovič, F., Čížková, H., Zahradník, J., Plicka, V., Liu, J., and Bina, C. R.: Could a deep earthquake cluster under Northeast China be associated with transformational faulting in an old Pacific slab?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5275, https://doi.org/10.5194/egusphere-egu26-5275, 2026.

The transport and release of fluids play a fundamental role in subduction zone settings, including for the genesis of seismicity and arc volcanism. Understanding how fluids escape from the slab and move into and through the overlying mantle wedge is key for understanding these processes. The presence and distribution of fluids within these regions has been primarily investigated through estimates of the Vp/Vs ratio, which serves as a key indicator of fluid and magma content.

Local earthquake travel-time tomography (LET) has been extensively used to image Vp/Vs structure in subduction zones, providing critical insights into the morphology of the subducted slab and the properties of the overlying mantle wedge. LET typically involves the inversion of seismic data to simultaneously determine P- and S-wave velocity models alongside hypocentre locations. Differences in data coverage and quality lead both to the need for different regularisation in the Vp and Vs inversions and to mismatched resolution between the two velocity models. Together, these effects make a simple division to compute Vp/Vs inappropriate and result in unreliable and uninterpretable estimates of the Vp/Vs ratio. Therefore, several widely used algorithms incorporate a direct inversion for the Vp/Vs ratio, but these typically assume identical ray paths for P and S waves. The requirement to have both S- and P- wave arrivals also means that valuable data are discarded. Moreover, many LET schemes provide limited information on model uncertainty and resolution, complicating the assessment of model reliability.

In this work, we address these issues by utilising the Backus-Gilbert based SOLA method (Zaroli, 2016) to obtain robust and consistent Vp/Vs models in local earthquake tomography. The SOLA method provides direct control over model resolution and has recently been applied to obtain multiple physical parameters with the same local resolution. Here, we present the methodology and preliminary work towards implementing SOLA with LET, with the goal of improving constraints on fluid distribution in subduction zones. As an initial step towards this, we use data from the Lesser Antilles subduction zone (Bie et al, 2022) to obtain Vp and Vs models within a linearised framework. We will present preliminary findings for the Vp/Vs ratio including uncertainty and resolution information.

How to cite: Mojica Boada, M., Koelemeijer, P., Hicks, S., Zaroli, C., and Serra, E.: Towards imaging fluids in subduction zones through Backus-Gilbert inference of Vp/Vs structure: initial application to the Lesser Antilles, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14860, https://doi.org/10.5194/egusphere-egu26-14860, 2026.

The south-central Chilean seismogenic zone has produced some of the largest megathrust earthquakes ever recorded, including the 2010 Mw 8.8 Maule event. To understand the rupture behaviour of this earthquake in the region of maximum coseismic slip at 34.5°S, we analyse the tectonic and elastic structure of the margin using 2D wide-angle seismic (WAS) data, and spatially coincident 2D multichannel (MCS) data acquired with a 15-km-long streamer.

To improve the seismic velocity model relative to previous results along the same WAS line, we jointly invert travel times from WAS and MCS data using a combined refraction–reflection tomographic approach and statistical uncertainty analysis. In addition, we apply downward continuation to the MCS shot gathers to increase the number of usable MCS travel times and to improve ray coverage with refracted arrivals from the shallow part of the velocity model. This approach enhances coverage and reduces tomographic velocity uncertainties, and improves constraints on the position of the interplate reflector from the megathrust.

The resulting 2D P-wave velocity (Vp) model includes the velocity structure of a 50-60 km-wide accretionary prism, and a sharp velocity transition into crystalline basement landward. We convert the velocity structure of the upper plate into density and S-wave seismic velocity to then calculate rigidity (Shear modulus), and infer dynamic rupture parameters such as slip and rupture velocity. Comparison of the expected slip distribution from our results with existing kinematic slip models shows significant discrepancies, particularly beneath the accretionary prism, where the time-migrated 127-km-long seismic profile reveals intense internal deformation and increasing thrust faulting and folding towards the trench. We discuss potential upper-plate coseismic deformation processes to explain such discrepancy.

How to cite: Boekholt, A., Prada, M., Gómez de la Peña, L., E. Jiménez-Tejero, C., Bangs, N., and R. Ranero, C.: The Structure of the Chilean Subduction Zone from Seismic Imaging and Tomography at 34.5°S, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18475, https://doi.org/10.5194/egusphere-egu26-18475, 2026.

Understanding the coupled multi-scale geodynamic and tectonic processes related to the subduction of the oceanic Nazca Plate beneath the South American Plate is one pre-requisite to better assess seismic hazards in the region. It is essential to identify all relevant forces and related stress-strain relationships within the subduction system, such as the negative buoyancy of the subducting slab or the degree of mechanical coupling between the Andean domain and the Pampean foreland. We approach this by investigating the present-day physical, in particular rheological state of the lithosphere-asthenosphere system, including first-order variations in pressure, temperature, and rock composition as constrained by multi-disciplinary observations.

This study integrates gravity, active and passive seismic data to construct a high-resolution lithospheric scale density model for the Central Andes, enabling detailed analysis of crustal structural differentiation. For this, a combined workflow of forwardgravity field calculation using IGMAS+ and gravity inversion for the crustal layer using SimPEG is applied. By inverting for crustal densities constrained by both gravity field observations and seismic shear wave velocity conversions for the crust and upper mantle, this work reveals lateral and vertical variations in crustal density that challenge traditional models of upper/lower crustal dichotomy.

With this contribution, we will discuss how the combined information on seismic velocity and gravity-constrained density helps inferring lithological variations within the crust. This is a pre-requisite for investigating variations in the thermal field and mechanical strength of this complex lithosphere-asthenosphere system. In addition, our results provide new insights into the distribution of seismic events in relation to crustal heterogeneity.

How to cite: May, T., Bott, J., and Scheck-Wenderoth, M.: Unveiling Crustal Heterogeneities in the Central Andes: A High-Resolution Density Model Derived from Integrated Gravity and Seismic Modeling, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7613, https://doi.org/10.5194/egusphere-egu26-7613, 2026.

In subduction zones, seismic imaging reveals increasing permanent active deformation toward the trench, particularly within accretionary systems. Yet, how these rock bodies deform coseismically and influence megathrust rupture behavior is elusive. Here we combine geophysical observations from seismic imaging with visco-plastic dynamic rupture simulations to investigate how realistic upper-plate rock bodies influence megathrust earthquake dynamics and off-fault deformation. Our models reproduce the elastic structure of three subduction systems that differ primarily in the width of the accretionary prism, a key parameter for comparison. These configurations, referred to as Models I, II, and III, include a narrow compliant prism of approximately 20 km, an intermediate prism of about 60 km, and a wide prism exceeding 100 km, respectively. Elastic rock properties for each upper-plate model are derived from 2D P-wave velocity models obtained from controlled-source seismic data. Upper-plate bulk cohesion and bulk friction define visco-plastic strength and are set to depend on rigidity distribution and empirical observations. Based on laboratory measurements from JFAST drilling samples, we use rate-and-state friction law with strong velocity weakening in the shallow portion of the fault.

Results show that coseismic upper-plate plastic deformation in Model I is confined to the ~20-km-wide wedge, whereas in Models II and III it extends 40–60 km landward from the trench. This is consistent with seismic reflection profiles that reveal increasing active internal deformation of the prism at similar distances from the trench in regions such as the Japan Trench, Chile, and Sumatra. Such contrasting upper-plate deformation patterns lead to distinct uplift scenarios, particularly in their high-frequency response. In particular, Model I produces shorter-wavelength uplift near the trench, likely generating a tsunami with higher-frequency content than Model II and III, where uplift exhibits a longer wavelength. Although we do not explicitly simulate independent faults within the prism, the bulk plastic strain can be considered a proxy for the amount of deformation that is accommodated by these structures. Our results suggest that permanent deformation within accretionary prisms is active during trench-breaching megathrust earthquakes, indicating that substantial prism deformation occurs coseismically. Plastic deformation leads to a reduction in slip toward the trench, implying that coseismic energy is absorbed by the overlying rock body. This effect explains the low-radiated-energy of near-trench earthquakes, including tsunami earthquakes. Depending on the plastic strength of upper-plate material and the available energy along the fault, this effect may even prevent the rupture from reaching the trench, while still producing substantial coseismic uplift and horizontal seafloor displacement. Overall, this study indicates that identifying permanently deformed, low-rigidity regions near the trench can serve as a proxy for locating areas where coseismic deformation is strongly accommodated and tsunamigenic uplift is likely amplified.

How to cite: Prada, M., R. Raner, C., Sallarès, V., and Ulrich, T.: The role of permanent upper-plate deformation in coseismic deformation and megathrust earthquakes dynamics, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11604, https://doi.org/10.5194/egusphere-egu26-11604, 2026.

The capture of microplates by the Pacific Plate drives the transition from subduction to intracontinental, strike-slip motion along the San Andreas Fault (SAF). However, the underlying mechanics behind microplate capture and formation of intracontinental strike-slip faults remain unclear. Through 3D thermo-mechanical models with fluid migration, we find that northwestward Pacific Plate motion transitions from being accommodated at the Pacific-Farallon ridge to the megathrust between the Farallon slab and North America, and finally to an emergent, fluid-weakened intracontinental strike-slip fault. This transition occurs during slab detachment, triggered by decaying subduction convergence, strengthening of the megathrust with slowing water release, and eventual subduction termination. With the detached Monterey slab paleogeographically restored, forward large-scale convection models show that the paleo slab corresponds to the prominent, high-seismic velocity anomaly in the mantle transition zone below Nevada and Utah. The extension of the overriding American Plate facilitates the formation of the strike-slip fault. The computations suggest the connection between the low viscosity and high permeability subducted plate interface and the North American lower crust may lead to shearing, fluid transfer, and serpentinization and eventual SAF formation, offering insights into the spatial variations of volcanism, fault creeping, and seismicity along the SAF.

How to cite: Mao, W. and Michael, G.: Dynamics of San Andreas Fault Formation: Capture of a Microplate during the Demise of Subduction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15591, https://doi.org/10.5194/egusphere-egu26-15591, 2026.

The eastern margin of the Sea of Japan is a zone of great seismic and tsunami hazard due to multiple offshore and nearshore reverse faults. The 2024 Mw 7.5 Noto Peninsula Earthquake highlights this hazard. It resulted from the combined rupture of multiple adjacent faults. The specific hazard caused by each fault in the back-arc is however difficult to assess owing to long earthquake recurrence intervals. Diagnostic fingerprints in the landscape, onshore and offshore, can reveal clues and augment our understanding of the local earthquake cycle.

Here, we compare coseismic deformation of the 2024 Noto Peninsula Earthquake with 4,767 individual marine terraces attributed to 16 successive sea-level stages over the last Myr. This reveals that thereverse faults responsible for the quake were reactivated and started slipping between 326 and 238 ka. The emerged landscape is still adjusting to it while nearshore underwater scarps mark the active faults. Applied to nearby Sado Island, these observations reveal the likely location of an active fault that drives its fast deformation. Active faults defining the edge of uplifting land are likely found in the near shore domain, drowned by the current sea-level high stand.

New luminescence dating constraints on uplifted marine terraces further quantify the rate of deformation on Noto. These ages are in the final phase of analysis at the time of writing. Preliminary results appear to largely confirm the existing morphostratigraphic assumptions for the 120 ka terrace of Noto and a recurrence interval for 2024 Mw 7.5-type earthquake on the order of 2 kyr.

How to cite: Malatesta, L. C., Sueoka, S., Weiss, N.-M., Tsukamoto, S., Gailleton, B., Bonerath, V., Keum, D., Takahashi, N., Ishimura, D., Nishimura, T., Komatsu, T., Kataoka, K., Iwasa, Y., and Norton, K.: Earthquakes, “young” faults, and landscapes in Japan’s back-arc, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18752, https://doi.org/10.5194/egusphere-egu26-18752, 2026.

The Andaman–Nicobar segment of the Sunda subduction zone hosts some of the world’s highest tsunamigenic hazards, exemplified by the 2004 Mw 9.1-9.3 Sumatra–Andaman event. Highly oblique convergence promotes slip partitioning between the megathrust and upper-plate strike-slip faults; however, the detailed fault architecture of the Andaman arc remains uncertain. To better constrain the neotectonic framework, we perform regionally comprehensive kinematic block modelling using the most up-to-date geodetic velocities and earthquake slip vector azimuths, allowing us to quantify slip rates on major crustal faults from southern Myanmar to Java and assess along-strike variations in deformation style. Our results reveal several new features of the tectonic system. 1) A distinct transition in trench behaviour occurs near the Nicobar Islands, separating independently moving Andaman (Burma plate) and Sumatran segments of the rigid forearc. This boundary coincides with pronounced changes in slip magnitude, rake and rupture velocity during the 2004 great Andaman–Sumatra earthquake, implying that the rupture spanned two kinematically distinct plate boundaries that are interseismically loaded at different rates and in different directions. This boundary is expressed by sharp changes in gravity, bathymetry, and trench obliquity gradient, analogous to those observed at the Sunda Strait, particularly marked by a negative residual Bouguer anomaly indicative of a mechanically weak zone capable of accommodating differential block motion. 2) The strike-slip Andaman-Nicobar Fault, the offshore continuation of the Sumatran Fault, has a slip rate of >30 mm/yr, about twice that of the Sumatran Fault. The lack of recorded large earthquakes along this system and abundant swarm seismicity, imply that deformation may be accommodated by a combination of fault creep and localized locked patches, and/or by distributed slip across multiple structures. Our results have important implications for seismic hazard assessment and for future tectonic and tsunami-generation models, which must account for the structural barrier and complex strain accommodation in this part of the subduction system. 

How to cite: Agrawal, H., Lythgoe, K., Bradley, K., and Feng, L.: Kinematic boundary between Burma and Sumatra at the Nicobar Trench, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19995, https://doi.org/10.5194/egusphere-egu26-19995, 2026.

The safe management of subsurface-related economic activities, such as fluid extraction or storage (groundwater, hydrocarbon, H2, CO2, etc.), requires a reliable assessment of local seismicity. In intraplate regions, such assessments are difficult because earthquakes are often scattered and difficult to associate with active structures. Thus, studies of the local seismicity in intraplate settings often require detailed long-term seismic surveys. 

In this work, we present the results of more than six years of seismic monitoring in the Basque-Cantabrian Zone (BCZ), a region of great economic and geological interest in the eastern continuation of the Pyrenees along northern Iberia. Although the BCZ has long been an area of intensive subsurface use and resource exploitation, knowledge of its background seismicity and active structures remains limited. During our six-year long survey, we recorded more than 1200 earthquakes and computed 42 new focal mechanisms. 

The observed seismicity is generally dispersed and concentrates primarily to the east of the studied area, in the transition between the BCZ and the Western Pyrenees (WP). Within the BCZ, seismicity is associated with salt diapirs and blind faults that likely affect the Paleozoic basement, as well as with a major south-dipping Mesozoic normal fault. In the WP, seismicity primarily clusters along a steeply dipping fault that we interpret as the Ollín fault, reaching ~40 km depth. In the Southern Pyrenean Zone, we observed two seismic crises that appear to be related to blind faults. In the northern Iberian Range, seismicity is scattered over a wide range of depths, both all of them occurring above and below the frontal thrust (Cameros thrust). 

Finally, we analyzed the regional stress regime by inversting the newly-derived focal mechanisms. Our results indicate a predominantly extensional stress regime in the BCZ, with localized strike-slip components in several areas, including the South Pyrenean Zone. 

How to cite: Olivar Castaño, A., Díaz-González, A., Álvarez Pulgar, F. J., Pedreira, D., González-Cortina, J. M., Gallastegui, J., Diaz, J., and Gallart, J.:   Seismicity and seismotectonics of the Basque-Cantabrian Zone (northern Iberia) from six years of observations using a dense temporary network of broadband seismic stations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22644, https://doi.org/10.5194/egusphere-egu26-22644, 2026.