Faults grow through fault lengthening and slip accumulation, which are episodic processes related to the repetition of earthquakes. It is most often recorded in geomorphology. Meanwhile, the activity and seismic hazard of the ‘slow-moving’ faults are often overlooked due to their weak imprints in landforms, especially at their initial formation stage. The 2021 Mw 7.4 Maduo earthquake triggered a ~158-km long surface rupture along the poorly-known and geomorphically subtle Jiangcuo fault, which is one of the distributed faults in the Bayan Har block and splays that merge with the Kunlun Pass fault. The slip rate of the Jiangcuo fault is thus crucial for comprehending how the strain is distributed between the major and subsidy faults in the complete fault system of the Bayan Har block, as well as the broader deformation process at a large scale. In this study, we present three sites where the Jiangcuo fault left-laterally displaces Holocene geomorphic features (e.g., terraces, fans, and channels). Through the detailed interpretations of high-resolution Digital Elevation Models (DEMs), field investigations, and credible Optically Stimulated Luminescence (OSL) dating of displaced geomorphic features, we document an average left-lateral slip rate of 2.1 ± 0.2 mm/yr since ~12 ka of the Jiangcuo fault. Furthermore, we conservatively updated existing slip rates of the large strike-slip faults (East Kunlun fault, Ganzi-Yushu-Xianshuihe fault) bounding the Bayan Har block. Synthesizing the slip rate of the Jiangcuo fault with the updated rates of the bounding faults, our findings suggest that the Jiangcuo fault accommodates ∼10% of the total deformation in the Bayan Har block. This study provides valuable insights into the impact of younger faults on regional deformation processes.

How to cite: Yao, W., Liu_Zeng, J., Klinger, Y., Hu, G., Shao, Y., Liu, X., Qin, K., Liu, Z., Wang, Z., Gao, Y., and Han, L.: Unveiling the Activity of a Young Fault: Insights from the 2021 Maduo Earthquake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6640, https://doi.org/10.5194/egusphere-egu24-6640, 2024.

Earthquakes release strain energy that has accumulated between seismic events. Measuring strain accumulation rates is critical for understanding earthquake cycle and assessing earthquake potential, with fault slip rates serving as essential inputs for seismic hazard models. However, the Tibetan Plateau has been lacking comprehensive estimates of geologic slip rates on numerous faults. To address this gap, geodetic data have been invoked to derive fault slip (or slip deficit) rates using various methodologies. These include the commonly adopted classic and deformable block modelling approaches (Meade & Loveless, 2009) and the newly developed direct inversion of geodetic strain rates (Johnson et al., 2022), which has the advantage of not requiring blocks to be defined.  A comprehensive comparison of slip rates obtained from these different geodetic methods has been notably absent.

In this study, we focus on the southeastern Tibetan Plateau, utilising Sentinel-1 satellite data from 35 ascending and 32 descending frames spanning the period between 2014 and 2023, along with published GNSS velocities. We constructed high-resolution (1 km) maps of velocity and strain rate fields covering 1.3 million km². Using these maps, we derived slip rates on newly mapped faults (Styron, 2022) using classic block modelling, “deformable block” modelling, and by the direct inversion of strain rates. Our strain rate fields reveal a partition through focused shear on the Kunlun fault, the Xianshuihe-Xiaojiang fault system, the Longriba fault, the Longmenshan fault possibly influenced by the ongoing postseismic deformation of the 2008 M_w 7.9 Wenchuan earthquake, and the Lijiang-Xiaojinhe fault. On the deforming plateau there is diffuse deformation away from the major faults, with average shear strain and dilatation rates of 14.3 and 13.1 nanostrain/year, compared to 9.4 and 11.1 nanostrain/year in the Sichuan basin (which likely reflects the noise floor in the data). The geodetically-determined slip rates from the three methods generally align with available geologic rates, particularly along-strike variations on the Kunlun fault and the Xianshuihe-Xiaojiang fault system. Our block model consists of 103 blocks bounded by 326 fault sections in the southeastern Tibetan Plateau. The model is constrained by the combined geodetic horizontal velocities from 6617 observation points. Classic block modelling without considering internal strain tends to overestimate slip rates on faults that slip faster than 5 mm/yr, compared to deformable block model that accounts for homogeneous intrablock strain, constituting 5% of the total. The two block models explain approximately 45-50% of the geodetic strain, predicting focused strain on block boundaries even in the absence of observed strain concentrations. By directly inverting strain rates, we suggest that 40-50% of the geodetic strain is attributable to elastic coupling (back slip) on faults, while the remaining can be explained by off-fault distributed moment sources (body forces) in a thin elastic plate. We discuss limitations of different geodetic approaches in modelling deformation (velocities or strain rates) and implications for seismic hazard by comparing the seismic moment release rate from earthquakes and the geodetic moment accumulation rate from our geodetic models.

How to cite: Fang, J., Wright, T., Johnson, K., Ou, Q., Styron, R., Craig, T., Elliott, J., and Hooper, A.: Kinematics of the Southeastern Tibetan Plateau from Sentinel-1 InSAR and GNSS: Implications for Seismic Hazard Analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8280, https://doi.org/10.5194/egusphere-egu24-8280, 2024.

Previous studies have constrained the fault slip rates and block geometries of the SoutheasternTibetan Plateau (SETP) with contradictory results due to complex deformation patterns, limited datasets, and subjective choices of block boundaries. In this work, we address the issue of uncertain block geometries by employing an unsupervised machine learning (Euler pole clustering) algorithm that automatically resolves regions that behave as rigid blocks (clusters) using ~1000 GNSS velocity vectors. The optimal clustering results, determined by F-test and Euler-vector overlap analyses, indicate 4 elongated blocks exist in the SETP that are approximately parallel and delineated by a set of arcuate sinistral-slip faults. Our clustering results redefine the kinematicsof the SETP region with new block definitions which elucidate the dominance of sinistral-slipfaults.

How to cite: Xu, R. and Liu, X.: Clustering of GNSS Velocities Using Unsupervised Machine Learning in the Southeastern Tibetan Plateau: Block Identification and the Dominance of Sinistral-slip Faults, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13534, https://doi.org/10.5194/egusphere-egu24-13534, 2024.

Strike-slip faults of considerable scale play a pivotal role in accommodating crustal deformation resulting from the Cenozoic India-Eurasia collision. The manner in which strike-slip motion is transferred along faults remains a topic of ongoing debate. In this study, we have meticulously compiled millennial strike-slip rates and GPS-derived strike-slip data along the extensive ~1800 km East Kunlun Fault (EKF). Our objective is to discern the slip distribution pattern and evaluate the mode of strike-slip transfer. The findings reveal a segmented pattern of strike-slip activity, characterized by a consistently high strike-slip rate exceeding 10 mm/yr along the central segments. In contrast, the eastern segment exhibits a reduced slip rate, measuring less than 5 mm/yr, and further diminishes to approximately 1 mm/yr along its eastern fault tip zone. Notably, strike-slip drop events occur within the fault bending zone, or in areas where the fault bifurcates, forming a horsetail structure. To complement our observational insights, numerical modeling has been employed to validate that the fault geometry may serve as a crucial controlling factor in the observed variation of strike-slip rates, Additionally, it influences the local stress situation along the fault, further contributing to the earthquake risk along the fault and the associated hazards impacting the local area.

How to cite: Zhang, Y. and Jiao, L.: Mechanism of Strike-slip Transfer along the East Kunlun Fault in Northern Tibet, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7062, https://doi.org/10.5194/egusphere-egu24-7062, 2024.

The geometry of faults regulates the spatial patterns of interseismic, coseismic, and post-seismic surface deformation. Geodetic techniques can measure these deformation patterns during a seismic cycle and are expected to constrain the geometry of  seismogenic faults. However, the conventional linear inversion of geodetic data is unable to simultaneously estimate the fault slip distribution and the fault geometry. In this study, we propose a Bayesian framework that treats fault geometry as a time-invariant parameter. It can individually use coseismic deformation data or simultaneously utilize interseismic, coseismic, and post-seismic deformation data to invert for both fault slip distribution and non-planar fault geometry. Within this framework, geometry evidence informed by geophysical imaging, geological surveys, and microseismicity forms the basis for establishing the prior probability density function, while geodetic observations constitute the likelihood function. Our methodology provides an ensemble of plausible geometry parameters by sampling the posterior probability distributions of the parameters using Markov Chain Monte Carlo simulation. The performance of the developed method is tested and demonstrated through inversions for synthetic oblique-slip faulting models. Results demonstrate that assuming constant rake can significantly bias fault geometry estimates and data weighting. Additionally, considering the variability of slip orientations allows for plausible estimates of non-planar fault geometry with objective data weighting.We applied the method to the 2013 Mw 6.5 Lushan earthquake in Sichuan province, China. The results reveal dominant thrust slips with left-lateral components and a curved fault geometry, with the confidence interval of the dip angles ranging between 20° and 25° and 56° and 58°. Furthermore, the application of this method to the 2015 Gorkha earthquake in Nepal sheds light on the Main Himalayan Thrust, which serves as the interface between the Indian Plate and Eurasia. This may provide new insights into future seismic potential and topographic growth in the Nepal Himalaya.

How to cite: Wei, G., Chen, K., and Dal Zilio, L.: Development of a Bayesian non-planar fault geometry inversion using geodetic seismic cycle deformation data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7990, https://doi.org/10.5194/egusphere-egu24-7990, 2024.

Geometric complexity plays an important role for a fault’s seismotectonic behavior as it affects the initiation, propagation and termination of an earthquake and influences stress-slip relationships, fault-segment size, and the probability of multi-segment rupture. Consequently, geometric fault complexity is studied intensively and increasingly incorporated into computational earthquake rupture simulations. These efforts reveal a problem: While a natural fault’s geometry may be well quantifiable at the surface (i.e., the fault trace), its down-dip buried portion cannot be well constrained. At this point, it is not clear how this epistemic uncertainty affects the propagation of individual ruptures and a fault’s seismotectonic behavior (e.g., large-earthquake recurrence).

We address this issue computationally with a physics-based multi-cycle earthquake rupture simulator (MCQsim), enabling us to investigate various aspects of rupture propagation and earthquake cycle in a controlled environment (e.g., with well constrained fault geometry). We approximate fault geometric complexity as a 2-D random field using the “random midpoint displacement” method which allows us to represent fault roughness (i.e., incorporate its epistemic uncertainty) while keeping the fault surface trace unchanged. 

Using MCQsim, we create 20kyr-long synthetic earthquake catalogs for strike-slip faults that share the same complex fault surface trace but have different sub-surface fault geometries. We analyze the resulting variations in single-event rupture propagation (i.e., the kinematic source model) and long-term seismotectonic behavior. We find that kinematic source models of individual events differ substantially between different realizations of sub-surface geometry. However, the long-term seismotectonic behavior (e.g., large-earthquake recurrence) does not differ as much and is less sensitive to the epistemic uncertainties of sub-surface fault geometry.

How to cite: Zielke, O. and Mai, P. M.: Exploring the effects of sub-surface fault geometry on rupture propagation and long-term fault behavior, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7556, https://doi.org/10.5194/egusphere-egu24-7556, 2024.

At depth just below the seismogenic zone of the continental crust, i.e. at greenschist facies conditions, stresses increase during seismic rupturing within minutes from differential stresses on the order of a few tens of MPa to several hundreds of MPa. These fast stress-loading rates are manifested in characteristic microfabrics in fault rocks (cataclasites and pseudotachylytes) exhumed from these depths. The microfabrics indicate quasi-instantaneous cataclasis of almost all rock-forming minerals including garnet and quartz, as well as mechanical twinning of pyroxenes, amphiboles and titanite. In combination with experiments, the microfabrics can be used as paleo-stress gauges, i.e., paleopiezometers. The characteristic microstructures can occur distributed over the whole width of large-scale thrust faults, as the Silvretta basal thrust in the Central European Alps. There, twinned amphiboles record transient differential stresses of more than 400 MPa in a rock volume to about 300 m above the basal thrust exposed at the contact to the Penninic units of the Engadine window over several tens of km. Fast stress-unloading is indicated by growth of new undeformed quartz grains along cleavage cracks in host quartz generated coeval with seismic rupturing and missing evidence of quartz dislocation creep after pseudotachylyte formation. This fast stress-loading and unloading is recorded in pseudotachylytes, i.e., close to the seismic rupture, whereas at larger distance to the seismic rupture accelerated creep at hundreds of MPa occurs on a longer time scale. 

How to cite: Trepmann, C., Brückner, L., and Dellefant, F.: Fast stress-loading and -unloading below the seismogenic zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16171, https://doi.org/10.5194/egusphere-egu24-16171, 2024.

GPS positioning offers millimetric precision in measuring deformation of the lithosphere during the seismic cycle. In particular, during the post-seismic phase, long-lasting and large-scale deformation are measured. They result from the viscoelastic relaxation in the asthenosphere. Consequently, the post-seismic phase is currently modeled using viscoelastic rheologies (e.g., Maxwell or Burgers viscous models). On the other hand, the inter-seismic phase is mainly modeled using purely elastic models. In particular, coupling models, widely used to quantify the accumulation of deformation on the subduction fault, are therefore used to evaluate earthquake hazard. However, such elastic models fail to explain mid-field deformation without the use of an external hypothesis (e.g., a third plate called sliver).

The study of post-seismic deformation has provided important insights into the rheological properties of the asthenosphere during the post-seismic phase. For example, viscous creep has been found Newtonian since the cumulative post-seismic displacements normalized by the co-seismic offset, as a function of distance to the trench, superimpose very well for earthquakes of different magnitudes [Boulze et al. 2022].

By incorporating these different results and using the backslip theory [Savage 1983], we model the inter-seismic phase using viscoelastic models. We explore the impact on coupling distribution along the Chilean subduction zone, in particular discussing differences with the elastic model in terms of depth and lateral extension. We also examine the impact of viscoelastic models in a region of Chile (Taltal region, 25.2°S) where elastic models currently fail to reproduce deformation in the near-field [Klein et al. 2018]. Finally, we show that a 2-Burgers viscous model is necessary to reproduce deformation in Argentina in 2010, before the Maule earthquake.

How to cite: Boulze, H., Fleitout, L., Klein, E., and Vigny, C.: Decoding inter-seismic deformation: Insights from viscoelastic modeling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2389, https://doi.org/10.5194/egusphere-egu24-2389, 2024.

Geometrical irregularities of faults drive stress heterogeneities that strongly affect the seismic rupture. Here we analyze the effect of fault topography and remote stresses during the interseismic phase on the static stress pattern around faults and on the onset of failure. The analytical solution is derived using perturbation theory for a defined interface topography. We apply our solution for the static stress field near the East Anatolian Fault and we show that a large stress barrier is developed around the segment that ruptured during the M_w 7.8 Kahramanmaraş Earthquake. Considering stress field conditions that are associated with left-lateral strike slip on the fault, we show how the barrier location is affected by the fault geometry, while the amplitude of stress variations are sensitive to the background stress values and their directions. The solution predicts that the value of the accumulated elastic energy in the host rock around the fault is maximal in the barrier region suggesting that in this area the elastic energy available for potential slip is the largest. We therefore suggest that the length of the ruptured segment and magnitude of the strong Kahramanmaraş Earthquake were greatly influenced by the stress heterogeneity generated by the fault geometry during the long interseismic period. This example of the East Anatolian Fault shows that the geometry of the fault is crucial for the location and the extent of earthquakes along it. We further suggest that the presented analytical approach provides a simple yet powerful new tool for assessing seismic hazards before earthquakes occur.

How to cite: Sagy, A., Morad, D., and Lyakhovsky, V.: Stress, energy, and the onset of failure around geometrically irregular faults: Example from the East Anatolian Fault, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9615, https://doi.org/10.5194/egusphere-egu24-9615, 2024.

Shallow creep, as a widespread phenomenon in the earthquake cycle, plays an important role in understanding the behavior of faults and seismic hazards. However, it is still under debate whether creeping is an inherent behavior of fault or is a form of afterslip following large earthquakes. The East Anatolian Fault was recently ruptured by the 2020 Mw6.8 Elazig, and 2023 Mw7.8/Mw7.6 Kahramanmaras earthquake sequence, providing a unique opportunity to investigate the relation between shallow creep and earthquakes along strike-slip fault. Here, we show the spatial distribution and temporal evolution of creeping segments along the EAF using the InSAR phase-gradient stacking method. We derive the shear-strain rates in three periods – before the 2020 earthquake, between the 2020 and 2023 earthquakes, and after the 2023 earthquake sequence. By comparing the spatial distribution of the interseismic strain rates, the coseismic slip, and the post-seismic strain rates, we document a tight connection between creeping and coseismic slip on the two recent earthquakes. We also investigate the temporal behavior of faults following the two earthquakes using time-series shear strain analysis. The results reveal behaviors of shallow creep on different segments of the EAF with different statuses before the earthquakes. Our results shed new light on understanding the mechanism of creeping and its relation with large earthquakes during the earthquake cycle.

How to cite: Liu, Z. and Wang, T.: Shear-strain rates across the East Anatolian Fault (EAF) response to the 2020 Mw6.8 Elazig, and 2023 Mw7.8/Mw7.6 Kahramanmaras earthquake sequence, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18092, https://doi.org/10.5194/egusphere-egu24-18092, 2024.

On February 6, 2023, southern Türkiye was hit by two major earthquakes at 01:17 UTC (M_w 7.8, Pazarcık, Kahramanmaraş) and at 10:30 UTC (M_w 7.6, Elbistan, Kahramanmaraş) leading to severe damages at the complex junction of the Dead Sea Fault (DSF), the Cyprus arc and the East Anatolian fault zone (EAFZ). The ruptures propagated along several known strands of the southwestern termination of the EAFZ, the main Pazarcık and Karasu valley faults and the Çardac-Sürgü fault. The spatial extent of the impacted zone (300 x 300 km) supports the use of satellite images to map ruptures and damages and measure the co-seismic displacement over the whole region. Among the different satellite constellation available nowadays, Sentinel-2 presents the advantages of offering high-resolution images (10 m), global coverage with frequent revisit time and open access policy to the images. We here present the high-resolution mapping of the entire coseismic surface ruptures derived from image correlation of optical Sentinel-2 satellite acquisitions. We further estimated the rupture width, the total and on-fault offset, and of the diffuse deformation obtained a few days after the two mainshocks along the two main ruptures at 50 m resolution along the rupture. The mapping and the estimation of the offset are validated with the location of the rupture and the offset measurements collected on the ground. We found that the ruptures extend over lengths of 310 km and 140 km, with maximum offsets reaching 7.5±0.8 m and 8.7±0.8 m near the epicenters, for the Mw 7.8 and Mw 7.6 mainshocks, respectively. We propose a segmentation of the two ruptures based on these observations, and further discuss the location of potential supershear rupture. The use of optical image correlation complemented by field investigations along earthquake faults provides new insights into seismic hazard assessment.

How to cite: Provost, F., Karabacak, V., Malet, J.-P., Van Der Woerd, J., Meghraoui, M., Masson, F., Ferry, M., Michéa, D., and Pointal, E.: Co-seismic fault offsets of the 2023 Türkiye earthquake ruptures using Sentinel-2 satellite imagery and field observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20077, https://doi.org/10.5194/egusphere-egu24-20077, 2024.

The seismic chronicle, derived from the analysis of 14 short sediment cores and three long cores from Lake Iznik (NW Turkey), along with the identification of a subaquatic fault segment belonging to the Middle Strand of the North Anatolian Fault (MNAF), provides insights into both local seismicity and the regional seismic activity over the last 6000 years.

The integration of this seismic chronicle with ground-motion estimations at the core locations for all historical earthquakes, together with the evolution of sedimentation rate through time, allow to discuss the epicentral region and epicentral intensity of each historical earthquake in the western NAF system. This analysis also helps us to discriminate which earthquake is likely to generate an event deposit in the case of several historical earthquake candidate, especially when chronological uncertainty are larges

This approach allows a discussion of the factors influencing the threshold (sedimentation rate, ground motions at different spectral frequencies ) for triggering an event deposit in the Lake Iznik and the type of slope destabilization that can be triggered .

Thanks to these finding and through the established scaling relationship it is then possible to infer a minimum intensity for prehistoric earthquakes recorded in Lake Iznik at a given period.

Combining these data with paleoseismological data from the region allows us to propose a scenario for the long-term seismic cycle of the western NAF system.

How to cite: de Sigoyer, J., Domenge, J., Céline, B., Gastineau, R., Sabatier, P., and Duarte, E.: Information on past seismicity of the western NAF system (Turkey) combining ground-motion models with historical earthquakes and event deposits recorded in the sediments of Lake Iznik (NAF system, Turkey), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12894, https://doi.org/10.5194/egusphere-egu24-12894, 2024.

We report the first example where the timing of earthquake slip from in situ ³⁶Cl cosmogenic exposure dating of an active normal fault scarp can be verified using independently ¹⁴C dated Holocene coastal notches which are deformed along the strike of the fault. We have remodelled ³⁶Cl data from the active Pisia-Skinos normal fault, Greece, published by Mechernich et al. (2018), which indicates that the fault slip rate fluctuated through time. We model the expected coastal uplift and subsidence induced by slip on the fault using elastic half-space models and surface ruptures observed following the 1981 Pisia-Skinos earthquakes. Coastal uplift is constrained by elevation measurements of Holocene coastal notches that have previously been dated using ¹⁴C by Pirazzoli et al. (1994) and agree with time periods consistent with Holocene climate stability. We mapped the elevations and numbers of notches along the strike of the Pisia-Skinos fault, including measurements made underwater for locations where fault slip has submerged the notches below the present-day shoreline. We show that the spatial patterns and timing of uplift and subsidence from the notches agrees with the timing of periods of high slip associated with earthquake clusters and quiescence associated with anti-clusters from the slip histories derived from ³⁶Cl data, and with the uplift and subsidence derived from elastic half-space modelling. In particular, where modelled subsidence is highest, Holocene notches that formed between 6-2 ka can be preserved but are submerged. Notches could form at this time because the ³⁶Cl data show that the Pisia fault had entered a period of relative quiescence with a slip-rate of <0.1 mm/yr, accompanied by uplift from the offshore Strava fault. In contrast, rapid slip on the Pisia fault at 1.4 mm/yr between 2 ka and the present-day did not allow notches to form during this time period in the location of highest subsidence. Our example is the first that independently calibrates the timing of slip derived from ³⁶Cl on a fault plane using ¹⁴C dates on a deformed coastline, and is consistent with the idea that slip-rate variations can be measured and should be incorporated into seismic hazard assessment.

How to cite: Robertson, J., Sgambato, C., Roberts, G., Mildon, Z., Faure Walker, J., Iezzi, F., Mitchell, S., Ganas, A., Papanikolaou, I., Rugen, E., Tsironi, V., Beck, J., Mechernich, S., Deligiannakis, G., Binnie, S., Dunai, T., and Reicherter, K.: Deformed Holocene coastal notches reinforce the validity of earthquake slip histories implied by in-situ 36Cl exposure fault scarp dating., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12582, https://doi.org/10.5194/egusphere-egu24-12582, 2024.

Features such as fault geometry and slip-rates are key inputs to assess the seismic hazard imposed by either ground motion or fault displacement. However, complexities in the geology of faults, such as relay zones and along-strike fault bends, could lead to settings characterized by high segmentation, with multiple splays arranged both along and across strike. In order to assess the seismic hazard associated with such fault sectors, it is necessary to establish whether the 3D shallow deformation is equally spread over the multiple fault splays or the activity tends to localise on specific splays. This problem is enhanced when these faults are located within urban areas, and therefore their surface expression is altered by intense anthropic activity.

Within the framework of a work on the mitigation of the fault displacement hazard associated with the Mt. Marine active normal fault (Central Italy), we have performed two paleoseismological surveys within the town of Pizzoli (about 10 km NW of L’Aquila), where the fault is expressed with several splays arranged both along and across-strike. The trenches were planned to explore (i) potential fault scarps altered by human activity, identified through aerial photographs, LiDAR and fieldwork analysis, and (ii) discontinuities in the stratigraphic record highlighted by geophysical investigations (ERT, GPR) and borehole data.

The paleoseismological surveys intercepted five fault splays arranged across-strike, three synthetic and two antithetic to the main Mt. Marine fault. The fault splays show evidence of multiple Late Pleistocene/Holocene surface-rupturing seismic events, marked by colluvial wedges and infilled fractures. Moreover, we constrained the Late Pleistocene slip-rate of the Mt. Marine fault splays by dating and correlating Late-Pleistocene paleosols found (1) outcropping in the footwall of one of the inner fault splay and (2) in a borehole located just at the hangingwall of the outermost splay.

Our results show that the fault splays exhibit different and variable activity rates, suggesting that fault activity is localized on specific fault splays through space and time with the potential to rupture simultaneously during large earthquakes. Our findings have strong implications on fault-based seismic hazard assessments, as they imply that data collected on one splay may not be representative of the behaviour of the entire fault.

How to cite: Iezzi, F., Francescone, M., Pizzi, A., Blumetti, A. M., Boncio, P., Di Manna, P., Pace, B., Piacentini, T., Papasodaro, F., Morelli, F., Caciagli, M., Chiappini, M., D’Ajello Caracciolo, F., Materni, V., Nicolosi, I., Sapia, V., and Urbini, S.: Slip localization on multiple fault splays accommodating distributed deformation across normal fault complexities., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13133, https://doi.org/10.5194/egusphere-egu24-13133, 2024.

In extensional settings under Andersonian mechanics, low-angle normal faults should not form in favour of steeply dipping normal faults. However, InSAR shows that a seismic sequence including an earthquake with magnitude M_w 5.6 on August 1^st, 2023 (NEIC - National Earthquake Information Center) at the northern end of the Afar rift was caused by normal faulting on a low-angle 35° dipping plane. Our best-fit InSAR model shows that the low-angle normal fault occurred on the west margin of the rift axis, it was relatively deep (6.7 km) and it slipped fully seismically, having a geodetic magnitude of M_w 5.66 in agreement with the global seismic recordings (NEIC). Temporally, the faulting occurred at the end of a one-year period (December 2022-December 2023) of increased seismicity in the northern sector of Afar, with swarms of seismicity migrating northward along the rift. The seismic characteristics, fault location and kinematics are consistent with the low-angle normal fault being triggered by fluids that locally could be released by a deep magmatic heat source along the rift axis under high extensional stresses. Our observations show that low-angle normal faults can form in rifting settings, are activated seismically and are likely fluid-induced.

How to cite: Pagli, C., La Rosa, A., Keir, D., Hurman, G., Wang, H., Doubre, C., Viltres, R., Raggiunti, M., and Ayele, A.: Low-angle normal faulting triggered by fluids, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14326, https://doi.org/10.5194/egusphere-egu24-14326, 2024.

Nearby faults interact with each other through the exchange of stress. However, the extent of fault interaction is poorly understood. In particular, closely tied tectonic systems like crustal-scale faults that are right next to subduction zone interfaces are likely to express such interactions. Interactions may lead to slow-slip activity, resulting in episodes of transient surface motion.

Our study concentrates on Northwest Sulawesi (Indonesia), which hosts two fault zones with potential for major earthquakes and tsunamis: the strike-slip Palu-Koro fault and the Minahassa subduction zone. Both fault zones accommodate 4 cm/yr of interseismic relative motion. Thanks to a 20-year-long effort in geodetic monitoring, we are able to identify multiple periods during which surface velocities deviate from their interseismic trend. The most recent episode followed the 2018 M_w7.5 Palu earthquake.

We use a Bayesian methodology with forward predictions of slip on the two fault interfaces to match the observations following the 2018 M_w7.5 Palu earthquake, and infer that both deep afterslip on the Palu-Koro fault and slow slip on the Minahassa subduction interface have caused the observed transient surface motion. This finding represents the first recording of a slow slip event on the Minahassa subduction interface. We also speculate that the subduction interface and the strike-slip fault are likely interacting on a regular basis, affecting the seismogenic potential of both parts of this tectonic system.

How to cite: Nijholt, N., Simons, W., Broerse, T., and Riva, R.: Triggered and recurrent slow slip in North Sulawesi, Indonesia, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16276, https://doi.org/10.5194/egusphere-egu24-16276, 2024.

Detections of slow slip events (SSEs) are now common along most plate boundary fault systems at the global scale. However, no such event has been described in the south Peru - north Chile subduction zone so far, except for the early preparatory phase of the 2014 Iquique earthquake. We use geodetic template matching on GNSS-derived time series of surface motion in Southern Peru - Northern Chile to extract SSEs hidden within the geodetic noise. We detect 33 events with durations ranging from 9 to 40 days and magnitudes from $M_w$~5.6 to 6.2. The moment released by these aseismic events seems to scale with the cube of their duration, suggesting a dynamic comparable to that of earthquakes. We compare the distribution of SSEs with the distribution of coupling along the megathrust derived using Bayesian inference on GNSS- and InSAR-derived interseismic velocities. From this comparison, we obtain that most SSEs occur in regions of intermediate coupling where the megathrust transitions from locked to creeping or where geometrical complexities of the interplate region have been proposed. We finally discuss the potential role of fluids as a triggering mechanism for SSEs in the area. 

How to cite: Jara, J., Jolivet, R., Socquet, A., Comte, D., and Norabuena, E.: Detection of slow slip events along the southern Peru - northern Chile subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9782, https://doi.org/10.5194/egusphere-egu24-9782, 2024.

Geodetic data covering different phases of the earthquake cycle provide a great opportunity to improve our understanding of the processes and parameters governing the dynamics at subduction margins. However, quantifying the individual contributions of physical processes such as viscoelastic relaxation, afterslip, and (re)locking throughout the earthquake cycle remains challenging. Moreover, it is relevant to account for these processes within a rheological framework that is consistent over the entire earthquake cycle. We address this using Bayesian inference in the form of an ensemble smoother, a Monte Carlo approach that represents the probability density distribution of model states with a finite number of realizations, to estimate geodynamic model parameters. Prior estimates of the imperfect physical model are combined with the likelihood of noisy observations to estimate the posterior probability density distribution of model parameters.

We construct a 2D finite element seismic cycle model with a power-law rheology in the mantle. A priori information, such as a realistic temperature field and a coseismic slip distribution, is integrated into the model. Model pre-stresses are initialized during repeated earthquake cycles wherein the accumulated slip deficit is released entirely. We tailor the last earthquake to match the observed co-seismic slip of the 2011 Tohoku earthquake. The heterogeneous rheology structure is derived from the temperature field and experimental flow laws. Additionally, we simulate afterslip using a thin, low-viscosity shear zone with a Newtonian rheology. We focus on constraining power-law flow parameters for the mantle, and the shear zone viscosity.

We assimilate 3D GEONET GNSS displacement time series acquired before and after the 2011 Tohoku earthquake. The data require separate viscoelastic domains in the mantle wedge above and below ~50 km depth, and in the sub-slab mantle. Power-law viscosity parameters are successfully retrieved for all three domains. The trade-off between the power-law activation energy and water fugacity hinders their individual estimation. The wedge viscosity is >10¹⁹ Pa·s during the interseismic phase. Postseismic afterslip and bulk viscoelastic relaxation can be individually resolved from the surface deformation data. Afterslip is substantial between 40-50 km depth and extends to 80 km depth. Bulk viscoelastic relaxation in the wedge concentrates above 150 km depth with viscosities <10¹⁸ Pa·s. Landward motion of the near-trench region occurs during the early postseismic period without the need for a separate low-viscosity channel below the slab.

How to cite: Govers, R., Marsman, C., Vossepoel, F., van Dinther, Y., and D'Acquisto, M.: Bayesian Inference of Rheological Parameters from Observations Before and After the Tohoku Earthquake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16670, https://doi.org/10.5194/egusphere-egu24-16670, 2024.