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The Mw 6.4 Petrinja earthquake (2020, Croatia) is among the strongest continental earthquakes that occurred in Eastern Europe for decades. In such low-strain contexts, the sparse terrestrial-monitoring (few seismic and geodetic stations) of rare but strong earthquakes often prevents a detailed analysis of their seismic source. Here, we take advantage of > 160 geodetic benchmarks and optical image correlation to obtain a dense near-field coverage of the coseismic surface displacements. The geodetic dataset is obtained by repeated measurements of benchmark networks designed for civilian purposes and constitutes a unique dataset of coseismic displacements in the near-field of the fault. The optical image correlation is based on pre-earthquake (December 2017) WorldView and post-earthquake (February 2021) Pleiades satellite images with a 50 cm resolution. We also complete these displacement fields with unwrapped coseismic interferograms based on Sentinel-1 products, except in the near field affected by decorrelation. These displacement fields are consistent and thus suitable for modeling the slip distribution of the Petrinja earthquake. The elastic inversion of the geodetic benchmarks revealed interesting characteristics of this event: the rupture occurred on a near-vertical strike-slip fault, at a shallow depth (< 10 km), with significant slip reaching the surface. It also suggests that the deformation was partly accommodated by a subparallel strand 2.5 km from the main source northward. The aim of this research is to improve the source model of Petrinja 2020earthquake sequence, with a joint inversion of the geodetic benchmarks, optical image correlation, and InSAR data. Nevertheless, the comparison of the geodetic and coseismic offsets measurement on the field, shows that > 70% of the slip is likely distributed at the surface. Moreover, the coseismic strain maps derived from the unique benchmark data set helped us to identify zones where deformation appears distributed. Finally, the new data raises questions about whether such moderate earthquakes are accompanied by subsurface off-fault deformation or residual elastic strain.

Europe has experienced over the last years earthquakes of moderate magnitude (Mw 5-6), yet destructive, reminding us of the seismogenic potential of slowly deforming regions. Among them, the 2020 Mw 6.4 Petrinja earthquake ruptured the Petrinja-Pokupsko Fault (PPKF) in Central Croatia, about 50-km southeast of Zagreb, a region in which the caracterisation of seismogenic faults had been insufficiently studied before that event. Understanding the strain accommodation through time and space is critical for accurate assessment of the regional seismic hazard.Using field observations and high-resolution topographical data derived from airborne LiDAR (~10 cm resolution) and tri-stereo satellite images (Pléiades, resolution 50 cm), we accurately mapped the fault trace, underlined at several sites by geomorphic markers such as valleys, terrace risers, and alluvial fans that have recorded cumulative displacements ranging from 5 to > 50 m and potentially up to ~180 m. Along the studied section, our fault mapping is composed of a clear NW-SE-trending 10-km-long strand between Donja and Cepelis, and of 1-4-km-long right-stepping segments marked by a non-negligible vertical component. The southern strand is composed of 2-3 sub-parallel segments that accommodate the deformation within a < 500 m wide fault zone.We have identified several sites on the main southern strand where offsets have been accurately measured and where displaced markers have been sampled for cosmogenic nuclide exposure dating and radiocarbon datings. This will allow to estimate the slip-rate for this fault at different sites and over several time spans.The mapped fault appears very discontinuous with the deformation absorbed by a series of small fault sections rather than on a single fault strand. This likely reflects a recent transpressive deformation, with immature faults, in agreement with the source parameter of the 2020 Petrinja earthquake derived from seismology.Finaly, the 2020 coseismic surface ruptures affected the northern section of the PPKF, while the mapped cumulative displacements appears more prominent along the southern section. A better knowledge of the seismic history of this entire fault system is thus crucial for seismic hazard assessment of this area.

The eastern Rhine Graben Boundary fault (eastern RGBF) constitutes the eastern margin of the Upper Rhine Graben (URG), the most seismically active area in the plate interiors of Europe. Our recent paleoseismic studies have revealed Late Pleistocene-Holocene surface-rupturing paleoearthquakes with magnitudes M 6–6.5 and cumulative surface displacements in the order of 1–1.2 m vertically and 4–6 m horizontally. Based on the empirical relationships of Wells and Coppersmith, these parameters suggest that the plausible rupture scenarios of those paleoearthquakes are linked to shorter fault segments within the 300 km long eastern RGBF rather than an entire rupture of the fault. Up to date, segmentation on faults of the URG has yet to be evaluated. We aim to define fault segments within the eastern RGBF and their relative tectonic activity to understand how deformation is distributed along the marginal faults and within the graben. To achieve this, we integrate seismicity data, morphotectonic observations (from SRTM, TanDEM-X and LiDAR-based DEMs), geology (Plio-Pleistocene sediment thickness), and interpretation of commercial seismic lines.We define up to seven segments of varying lengths based on geometric and structural fault trace discontinuities (bend, gaps, and changes in strike and dip) and the occurrence and degradation state of tectonic landforms (triangular facets, beheaded channels, hanging valleys and offset alluvial fans), which we also take into account to define the relative level of tectonic activity at each segment. The most active segments are the South-Kraichgau and Freiburg segments, with potential magnitudes of M 7–7.5 (including the historical M 6.7 Basel eq of 1356). The northern area, comprising the Frankfurt-Darmstadt and Odenwald segments, constitute presently a seismic gap with quiescence in historical and instrumental seismicity but with tectonic expression in the landscape and thickest Late Tertiary-Pleistocene deposits, suggesting a potential hazard. Our results provide a basis to propose plausible rupture scenarios for the eastern RGBF for future PSHA studies.

Analysis of lithospheric deformation is key to understanding current tectonics and other active deformation processes. The Alceste project, conducted in the framework of the Résif Seismicity Transverse Action, aims at proposing an updated seismic hazard model in metropolitan France built from the most recent data and academic consensus. One of the contributions will come from geodetic observations through strain rate integration in seismotectonic zoning and seismic hazard models. Most of Western Europe in general, and metropolitan France in particular, is located within the Eurasian Plate, which has very low deformation and seismicity rates. These GNSS-derived secular velocity field could be related to the combination of different deformation processes, with a minor contribution from plate tectonics (relative plate motions, mantle convection, etc), while most of the measured velocities could be explained by non-tectonic long-term or transient processes (gravitational motions, Glacial Isostatic Adjustment, erosion, anthropogenic deformation, etc). Some of these physical processes causing surface deformation also reflect stress changes at depth that may be associated with loading on active faults and seismicity. Properly mapping this deformation is therefore a key to better assess seismic hazard in slow straining areas. In order (i) to assess the variability due to the diversity of the strain-rate calculation methods used in the scientific community and (ii) to test their capacity to resolve low-amplitude consistent surface deformation, we conduct a benchmark exercise. We build sets of synthetic velocity fields sampled at the existing GNSS permanent stations from the RENAG (REseau NAtional GNSS Permanent), RGP (Réseau GPS Permanent) and other permanent and non-permanent benchmarks. Our synthetic velocity fields have the same characteristics (noise, uncertainties) as the observed velocities in metropolitan France but they contain surface deformation signals from known physical processes (block rotations, fault elastic loading, large scale flexure, etc). We compare the strain rate invariants derived independently by nine different RENAG research teams (using different software) to the expected strain rate patterns and discuss drawbacks and advantages of each approach. In a second step, we analyzed the strain rate tensors derived from the synthetic velocity fields to discuss potential regional style of the deformation in metropolitan France. Previous studies have shown that the computation of strain rate tensors is impacted by the user-defined parameters and the algorithm specificity used. Exploring these different biases in the strain rate solutions represent the opportunity to improve the understanding of the conventional problem of the standard interpolation.

Long-term fault escarpments are built by the accumulation of individual earthquakes producing incremental surface displacements on the fault releasing crustal tectonic loading. Cumulative escarpment studies have revealed a spatial slip variability along active faults as well as a temporal variability with the alternation of phases of intense seismic activity over a short period of time followed by long periods of quiescence. Understanding this spatial and temporal slip variability on individual faults and over a complex fault system provide a better knowledge of co-seismic rupture extents, essential for estimating past earthquakes magnitude and for seismic hazard assessment.Up to now, most studies have focused on a timeframe over few seismic cycles, making it difficult to apprehend the rupture barriers persistence and cumulative slip distribution. Here, we aim at quantifying the slip variability over several timescales ranging from a few months to a few million years on the same fault.Our study focusses on the ~50 km-long Liri fault, SW of the Fucino basin. The fault is located at the contact between Cretaceous limestone and patches of Quaternary deposits locally convering Mio-Pliocene flysch sediments. Detailed mapping of the fault trace on high-resolution Digital Elevation Model (DEM) from UAV-acquired images, Pleiades images and Lidar together with field observations revealed changes in the morphological expression of the fault north and south of an important wind gap located at Capistrello. To the north, the faut trace is ~16 km-long located on the eastern side of ~2km-wide limestone ridge, reaching ~1300m asl elevation. Two bends in the fault trace, made of ~5km long segments, can be observed with the fault strike varying between N115° and N140°. In this northern section, the fault scarp appears subtle and we did not observe Quaternary deposits on the hanging wall. In the 30 km-long section, south of Capistrello, the cumulative scarp composed of numerous splays is evidenced by a sharp trace, offsetting several morphological surfaces and associated Quaternary sediment packages. Three major bends are observed in this section of the fault, separating 10 to 30 km-long segments striking between N110° and N160°. An alluvial surface offset by ~14 m of cumulative displacement was dated at ~35kyr using 36Cl cosmogenic exposure dating suggesting a minimum slip rate of 0.4 mm/yr. Other morphological markers that have accumulated displacement between ~10 and 70 m-high have also been sampled for 36Cl cosmogenic exposure dating. Moreover, we excavated two small trenches at the base of the fault scarp within the Quaternary deposits affected by the fault revealing 3 rupture-surfacing earthquakes over the last 2500 yr, the last one occurring after 1226 CE. We will present those results and will discuss how the displacement varies along the fault both in time and space.

SUMMARYDue to the steady moderate seismicity observed along the Briançon seismic arc, in the south-western French Alps, three temporary GNSS (Global Navigation Satellite System) surveys took place in 1996, 2006 and 2011, across a ∼50 × 60 km² wide area, to investigate the surface deformation field. The horizontal velocity field computed from these three surveys showed an east–west extension in the network. A fourth campaign was led in 2016, creating a 20 yr observation span, resulting in measurements which reach a sufficient accuracy to assess whether extension found within the Briançon network is localized onto any particular tectonic feature. Several faults in this area are known to be active normal faults. Assessing the localization of the deformation may lead to a better understanding of the active tectonics of the Alpine belt. To address this issue, a robust velocity field was computed from the combination of the different campaign and permanent GNSS data. Strain rate tensors were derived for the first time in this area on a 0.1 × 0.1 deg grid to assess the distribution of the deformation. The regional deformation appears localized in the Briançon area and reaches up to 20 ± 5 nanostrain yr−1 in the centre of the network. The observed velocities were projected on a profile across the network and compared with modelled interseismic deformation to characterize the behaviour of the major active faults known in the study zone. While a two-fault model provides the best fit to the data, a single fault model has only marginally higher residuals, with parameters which are more consistent with the seismotectonics of the region. The localization of the single modelled fault is consistent with the location of the High Durance Fault (HDF). Therefore, we used the known geological location of this structure as a priori information in a block model to compute a fault slip rate at the interface between the two blocks. The velocities on the interface indicate 0.4–0.5 mm yr−1 of extension, and therefore strain accumulates along the HDF throughout the seismic cycle. The geodetically derived fault slip rate is converted into an equivalent seismic moment release rate, which is consistent within its uncertainty bounds with the known historical and instrumental seismicity of the Briançon area.

Paleoseismology is a fundamental method to characterize the activity of faults in low to moderate strain regions such as SE Spain. Among the different parameters to characterize such activity, the slip rate is one of the most crucial for fault-based probabilistic seismic hazard assessments (PSHA) as it controls the rates of earthquake occurrence and ultimately the hazard levels likely to be exceeded in a given time period.The Alhama de Murcia Fault (AMF) is the most active structure within the Eastern Betics Shear Zone (EBSZ), a transpressive fault system that accommodates the largest part of the Africa-Eurasia convergence in SE Iberia. The AMF has caused some of the most important earthquakes in the EBSZ since historical times, including the damaging 2011 Mw 5.2 Lorca event. In this setting, paleoseismic studies in the EBSZ have paid special attention to this fault, and particularly to its central segment (Lorca-Totana) as this is one of the most geomorphologically prominent. Despite this, the segment comprises a wide deformation zone where the fault splays into five subparallel slip-partitioned branches, four of these still unstudied to date. We present a comprehensive paleoseismic study that integrates paleoseismic data from four out of the five branches that compose the segment. Our aim is to improve the representativeness of the geological slip rates by accounting for a nearly complete transect of the fault zone: we excavated eight new trenches across the four branches including seven fault-perpendicular and one parallel trench to measure vertical and lateral displacements, respectively. Fault slip analysis combined with OSL and radiocarbon dating allowed the calculation of slip rates for each branch and for the whole transect, as well as their variability over time.A total net slip rate of 1.60 +0.16/-0.11 mm/yr for the past 18-15 ka is obtained, which is almost twice the previous estimations from a single fault branch (0.9±0.1 mm/yr). This points out the relevance of accounting for all structures of a fault zone for a more reliable characterization. The slip rate variability analysis depicts cyclic patterns of short slip rate accelerations followed by longer quiescence periods, some of which are interestingly similar to those identified in the neighboring Carrascoy Fault in previous studies. This may, for the first time, suggest potentially synchronous activity among faults in Iberia. The present study is therefore an important step to improve the representativeness of the slip rate estimations in the AMF, and ultimately for subsequent PSHA studies in the area. Despite this, two main challenges still need to be assessed; first, the intermittent deposition of alluvium in the area makes it difficult to have correlative time periods between sites to integrate slip rates. Second, the lack of data in one of the five fault branches and the lack of detailed 3D trenching in most branches suggests that the obtained slip rate values could be a minimum. In this sense, integrating data from new paleoseismic sites and refining the existing data would likely allow to refine the current estimations and potentially fill the present knowledge gaps.

Within the low-deforming western European Alpine belt, GNSS measurements show that uplift is the main signal characterizing current surface deformation in the range, reaching up to 2 mm/yr, while no shortening is observed across the belt. Based on the huge amount of satellite data available today, it now appears possible to constrain new high resolution surface velocities in the western Alps, which is of primary importance to better understand the links between surface deformation and neotectonics processes in this region.Relying on ~ 170 radar acquisitions from Sentinel-1 satellite over four years, we propose for the first time an InSAR-based mapping of the uplift pattern affecting the Western Alps on a ~350x175 km-wide area. Their processing is challenging due to the high noise level inherent to mountainous areas and the low expected deformation signal. We thus use in this study the NSBAS small baseline approach (Doin et al., 2011) for interferograms corrections, unwrapping, and time-series inversion. Atmospheric corrections are made using ERA5 reanalysis model (Hersbach et al., 2020). We estimate regional line-of-sight (LOS) velocities by correcting the resulting time-series from outliers and by separating seasonal and linear signals through different approaches which all yield similar results, thus highlighting the robustness of the obtained LOS velocity field. Based on several assumptions, we finally convert LOS velocities to uplift rates using local incidence angles. The corresponding InSAR-derived velocity field is validated by the comparison with GNSS solutions. They both show uplift in the core of the belt, with higher rates in its northern part, and subsidence at its periphery. Our approach however provides a denser spatial distribution of vertical motions compared with GNSS. Higher uplift rates are found within the external crystalline massifs compared with surrounding areas, in agreement with the variations expected from recent deglaciation and long-term exhumation data.These results bring new insights into active tectonics in the Western Alps. While several distinct wavelength patterns can be identified within the uplift signal throughout the western Alps, we suggest that they may originate from common geodynamic processes, with differential surficial responses explaining their localization. These processes may involve glacial isostatic adjustment, erosion, and/or slab break-off.

The western European Alps display measurable surface deformation rates from levelling and GNSS data. Based on the time series analysis of 4 yrs of Sentinel-1 data, we propose for the first time an ...

The Petrinja-Pokupsko fault-system is a NW-SE right-lateral fault system that ruptured during the 29 December 2020 Mw 6.4 earthquake (~40km south-east of Zagreb, Croatia). Field analysis revealed opening of cracks and offsets of several centimeters (3 to 40 cm) along a ~20 km long fault zone extending from the Kupa river (in the northwest) to the Petrinjčica river (in the southeast). Optical image correlation based on WorldView satellite images has been used to document the first-order near-field rupture signal. The pre-event image was acquired on 7th December 2017, and the post-event image on 15th January 2021. The first results indicate a right-lateral displacement of ≈75 cm with a small (6 earthquakes. Since the fault extends farther NW and SE, from the Vukomeričke Gorice hills to Mount Kozara (Bosnia), for a total length of about 100 km, it could generate potentially larger events. It is also noteworthy that the 2020 Petrinja event occurred only 9 months after the Zagreb March 2020 (Mw 5.3) earthquake. This event occurred on an ENE-WSW-trending thrust fault, broadly orthogonal to the right-lateral Petrinja-Pokupsko fault system, ~45 km north of Petrinja, and raises the prospect of potential interplay between strike-slip and thrust faults in moderate strain-rate intra-plate settings. To address this problem, future works will aim at constraining the geometry of this fault network and its seismogenic potential.

The 29 December 2020, Mw 6.4 Petrinja earthquake nucleated at a depth of ~10 km in the Sisak-Moslavina County in northern Croatia, ~6 km WSW of the Petrinja town. Focal mechanisms, aftershocks distribution, and preliminary Sentinel-1 InSAR interferogram suggest that the NW-SE right-lateral strike-slip Pokupsko-Petrinja fault was the source of this event.The Croatian Geological Survey, joined by a European team of earthquake geologists from France, Slovenia and Italy, performed a prompt systematic survey of the area to map the surface effects of the earthquake. The field survey was guided by geological maps, preliminary morphotectonic mapping based on 1:5,000 topographical maps and InSAR interferogram. Locally, field mapping was aided by drone survey.We mapped unambiguous evidence of surface faulting at several sites between Župić to the NW and Hrastovica to the SE, in the central part of the Pokupsko-Petrinja fault, for a total length of ~6.5 km. This is probably a minimum length since several portions of the fault have not been explored yet, and in part crossing forbidden uncleared minefields. Surface faulting was observed on anthropic features (roads, walls) and on Quaternary sediments (soft colluvium and alluvium) and Miocene bedrock (calcarenites). The observed ruptures strike mostly NW-SE, with evidences of strike-slip right-lateral displacement and zones of extension (opening) or contraction (small pressure ridges, moletracks) atlocal bends of the rupture trace. Those ruptures are interpreted as evidences of coseismic surface faulting (primary effects) as they affect the morphology independently from the slope direction. Ground failures due to gravitational sliding and liquefaction occurrences were also observed, mapped and interpreted as secondary effects (see Amoroso et al., and Vukovski et al., this session). SE of Križ, the rupture broke a water pipeline with a right-lateral offset of several centimetres. Measured right-lateral net displacement varies from a few centimetres up to ~35 cm. A portion of the maximum measured displacement could be due to afterlisp, as it was mapped several days after the main shock. Hybrid surface ruptures (shear plus opening and liquefaction), striking SW-NE, with cm-size left-lateral strike-slip offsets were mapped on the northern side of the Petrinja town, ~3 km NE of the main fault.Overall, the rupture zone appears discontinuous. Several factors might be inferred to explain this pattern such as incomplete mapping of the rupture, inherited structural discontinuities within the Pokupsko-Petrinja fault system, or specific mechanical properties of the Neogene-Quaternary strata

Fault displacement hazard assessment is based on empirical relationships derived from data of historical surface rupturing earthquakes. This approach is used for land use planning, sizing of lifelines or major sensitive infrastructures located in the proximity of active faults. These relationships provide the probability of occurrence of surface rupture and predict the amount of displacement, both for the main ruptures (principal) and for distributed ones appearing beyond.Following the first version of the global database SURE 1.0 (Baize et al., 2019), we are continuing the effort to compile observations from well-documented historical and recent surface faulting events in order to feed and improve empirical relationships. The new SURE2.0 global database consolidates the previous version SURE 1.0 data, rejecting some poorly constrained cases, reviewing some cases already in, and adding well-documented new ones (e.g. Ridgecrest sequence, USA, 2019). In total, the SURE 2.0 database has 46 earthquakes, including 15 normal fault cases, 16 reverse fault cases and 15 strike-slip cases from 1872 to 2019. The magnitude range is from M4.9 to 7.9, with ruptures from 5 to 300 km long.SURE 2.0 provides the geometric location and attribute information of rupture segments in a GIS environment and a spreadsheet reports the amplitude and characteristics of deformation, including data sources and its eventual geometric refinement during analysis. In this new version, we completed an essential task to derive attenuation relationships, by classifying each rupture segment and each slip measurement point, using a ranking scheme based on the pattern and amplitude of the observed rupture traces, and considering the structural context and the long-term geomorphology. This distinguishes the principal rupture (class 1), which is the main surface expression of the source of the earthquake. Typically, in the siting study, this class is assigned to the identified active fault. Class 2 features (distributed ruptures) are characterized by shorter lengths and smaller displacements that appear randomly close and around the main rupture. We introduced the distributed main fracture category (class 1.5), which corresponds to the relatively long minor fractures recognized on cumulative structures secondary to the main fault. Class 3 represents triggered slip evidences on remote active faults, clearly not connected with the earthquake causative fault (sympathetic ruptures).As was done with reverse fault cases (Nurminen et al., 2020), this new SURE 2.0 version will be used to derive probabilities associated with the rupture distribution during any type of earthquake.

The Belledonne region, located on the western edge of the French Alps, behaves as a deformation transfer zone between the inner part of the western Alps, where geodesy and seismicity show extensional deformation, and its compressional surrounding basin (the Rhône Valley). Seismological and geodetic networks are less dense and younger in the Rhône Valley, which makes it more difficult to characterize its deformation. Nevertheless, these two regions have a moderate historical and instrumental seismicity. A large part of these earthquakes is concentrated on the Belledonne range and accommodated by the active NE–SW Belledonne fault, located at the western foot of this chain. The fault characteristics, such as its connection at depth with surrounding fault systems (e.g. Cléry fault), still need better constraints. The dense seismological network present in the Alpine region has made it possible to highlight its dextral strike-slip kinematics. To complete these observations, we present here an update of the geodetic velocity field around this fault from GNSS data recorded over the last two decades.To do so, we first computed daily positions for a total of about 200 stations provided by different European networks (IGS, RENAG, RGP, GAIN, DGFI networks) over a period of 23 years (from 1997 to 2020), by using a double-difference processing with the GAMIT software (Herring et al. 2015). Then, we constrained a velocity field with the Kalman filter GLOBK with respect to the fixed European plate. We finally analyzed the residual motions in our area of interest with respect to stable Europe, as provided by our updated velocity field.Across the Belledonne range, our results show a deformation pattern consistent with the dextral strike-slip mechanism observed by the current seismicity. Methodological studies concern the expected decrease of uncertainty on the velocity field thanks to the increase of recordings through time. These tests aim at quantifying the Belledonne fault present-day slip rate, including a well-constrained velocity uncertainty. We also exploit the new 3D velocity field to confirm and precise the local amplitude, in the Belledonne area, of the general uplift of the Alpine belt, as observed by previous geodetic studies.

Based on geomorphological criteria, large-scale slow gravitational deformation affecting entire mountain flank, often being referred as Deep-Seated Gravitational Slope Deformation (DSGSD), have been shown to affect most of the reliefs worldwide. For instance in the European Alps, these deformation patterns were identified in several areas such as the Aosta Valley (Martinotti et al., 2011) or the Mercantour massif (Jomard, 2006). DSGSD inventories based on visual interpretation of scarps and field mapping were then compiled (e.g. Crosta et al., 2013) revealing the widespread occurrence of DSGSD. However, many aspects of these large-scale gravitational processes remain unclear and in particular their present-day activity and temporal evolution remain largely unknown.The present study aims at characterizing the spatial extent of DSGSD, and their velocity, at the scale of Western Alps through InSAR time series analysis using NSBAS processing chain (Doin et al., 2001). We used the whole SAR Sentinel-1 archive, between 2014 and 2018, with an acquisition every 6 days, on an ascending track. The processing was adapted to fit the specific conditions of the Alps (seasonal snow cover, strong local relief, vegetation and strong atmospheric heterogeneities). In particular we implemented a correction using the ERA 5 weather model and we used snow masks in winter allowing to select long temporal baseline interferograms with as little snow as possible. As we specifically aim to study deformation patterns at the scale of valley flanks, an average high-pass filter on moving subwindows has been applied to the interferograms prior to the implementation of time-serie inversions. This step strongly reduced the impact of residual atmospheric delays.The resulting velocity map in the line of sight (LOS) of the satellite reveals ubiquitous gravitational deformation patterns over the whole Western Alps, with localized patches of moving slopes showing sharp discontinuities with stable surrounding areas. We used radar geometry and InSAR measurement quality factors as indicators to identify the most trusted areas and to extract an inventory of potential DSGSD with their spatial extent. Doing so, we identified more than two thousands slowly deforming areas characterized by LOS velocities from 4 to 20 mm/year. We then compared the geometries of our “InSAR-detected-deforming-slopes” with previously published DSGSD inventories. Good agreements were found for example in the Aosta valley where most of the deforming areas from our velocity map are falling into the DSGSD outlines of Crosta et al. (2013). Currently, we continue to investigate the potential of this large-scale velocity map for DSGSD understanding and we plan to use artificial intelligence to search for possible generic properties between the detected sites.

The earthquake with magnitude ML=6.2 that occurred on 29th December 2020 has caused significant material damage to objects and infrastructure in the towns of Petrinja, Sisak,Glina and the surrounding area. According to the satellite interferometry data, the coseismic and postseismic deformation area covers around 500 square kilometers. The existing geodetic benchmarks have been set in the affected towns, and their coordinates have been determined based on previous GPS campaigns. The GPS network was set up and adjusted at the State Geodetic Administration's request for geodetic monitoring of infrastructure and cadastral projects. These points are not primarily intended for high accuracy measurements at the level of a few millimeters, so their accuracy and the absolute shift concerning geodynamic processes in the region should be taken into account. Nevertheless, the data obtained by their observation after the earthquake can provide valuable information about the horizontal and vertical displacements with a certain level of confidence. The field survey has detected disappearance of a large number of benchmarks and some valuable information has been lost. Still, 58 points were found and observed and it has been concluded that 52 points are reliable and can be used for future research. Because the network of benchmarks is not developed in rural areas, there is a gap in the distribution of benchmarks in affected area. Therefore, the additional data was collected using the benchmarks established for the engineering and cadastral projects and studies. From a total of 67 points that have been found and observed, 42 points will be used. Along with the data collected in urban areas, there will be a total of 94 benchmarks. The accuracy of the geodetic benchmark measurements is at the centimeter level, while the values of deformation are at the level of a few decimeters. Therefore, the obtained data can be used to better assess the displacement recorded during the 29 December 2020 event. In the future, field research will focus on finding additional benchmarks to reach a better spatial distribution.

The 11-11-2019 Le Teil earthquake (Mw4.9), located in the Rhône river valley occurred along the La Rouvière fault (LRF) within the NE termination of the Cévennes faults system (CFS). This very shallow moderate magnitude and reverse-faulting event inverted an Oligocene normal fault which was not assessed to be potentially active, causing surface rupture and strong ground shaking. Its morphology shows no evidence of cumulative reverse faulting during the Quaternary. All of this information raises the question of whether the fault was reactivated for the first time since the Oligocene during the Teil earthquake, or if it had broken the surface before, during the Quaternary period, but could not be detected. In addition, it poses the question of the potential reactivation of other faults of the CFS and other faults in metropolitan France as well.To tackle those issues, we launched paleoseismic investigations along the LRF to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. Twelve trenches were dug along the section that broke in 2019. The trenches were dug in aeolian deposits and slope colluvium lying against the ancient LRF normal fault mirror carved in the Barremian limestones. Five trenches yielded favorable Quaternary deposits to document deformation suggesting that one paleo-event, maybe more, occurred with kinematic characteristics (sense of movement, amount of displacement) similar to the 2019 event. The radiocarbon dating of the deformed units (“bulks” collected from the colluvium clayey-silty matrix) suggests, in particular, that at least one event occurred in the past 13 Ka (i.e. penultimate event prior to the Teil earthquake) . The fact that these events are not preserved in the morphology is explained by the small amount of displacement and a long return period, consistent with the low strain rate measured by GPS in this region (~10-9 yrs-1). Our study shows that it is therefore fundamental to carry out more detailed paleoseismological investigations in metropolitan France, especially along ancient faults favorably oriented with respect to the present stress field. Those are already planned in the next coming months along other segments of the CFS.

After the earthquake of 29/12/2020 in Petrinja (ML6.2, ImaxVIII-IX EMS), an attempt was made to characterize the active structure associated with the earthquake. As a first step towards this goal, we performed a geomorphological analysis in order to contribute to the identification and characterization of the surface expression of the active Pokupsko dextral strike-slip fault. We focused on the area between the southernmost parts of Vukomeričke Gorice and the southernmost parts of Hrastovica Mountain, where the NW-SE striking Pokupsko fault has slipped during and after the recent earthquake (Ganas et al., 2021). Using available 1 : 5 000 scale topographic maps and various 10 m resolution digital elevation model visualizations, we mapped lineaments that could represent relatively recently active fault segments. We used a quantitative approach to perform stream sinuosity analysis (e.g., Leopold et al., 1964; Zamolyi et al., 2010) on major streams crossing the structure to identify distinct changes in channel patterns that may be associated with vertical movement along the predominantly strike-slip fault. We observed changes in the shape of the valleys, especially the changes in width, height, and direction. By summarizing various geomorphological indicators of active fault segmentation at the surface with available geological data (Pikija, 1987) and so far limited field observations, we provide insights into the structure of the Pokupsko fault.Preliminary results show good agreement between lineament mapping, changes in valley shape, changes in the stream sinuosity index, and (to some extent) previously mapped faults. In addition, some of the changes in stream sinuosity correspond to locations where coseismic surface ruptures occurred during the December 29 earthquake (Budić et al., this session; Pollak et al., 2021). Results suggest that the several-kilometer-wide zone of uplifted Neogene deposits results from the dextral-transpressive structure, which at the surface consists of a series of subparallel fault strands branching off the main fault that runs along the SE slopes of the Hrastovica Mountain. The SW-most fault strands are associated with significant changes in the shape of the valleys: the wide valleys of Petrinjčica, Utinja and Šanja change to narrow and deeply incised as they cross the uplifted structure. Paleocene and Eocene rocks, which otherwise underlie the Neogene, outcrop in the NE parts of the fluvial breakthrough valleys, indicating the uplift of the Hrastovica Mountain. Topographic data show a decrease of the mountain range elevation towards the SW. This evidence suggests that the main fault runs on the NE side of the mountain, strikes NW-SE and dips steeply towards the SW. The fault strike deviates between Župić and Farkašić. The fault plane solution for the December 29 earthquake suggests a nearly pure strike-slip fault, while geomorphic evidence strongly indicates areas of active uplift along the fault, further supported by the general antiformal structure. We interpret this as an indication of either a general current transpressional character of the fault or as local kinematic variations due to segmentation and changes in the strike of the fault; further analyses are pending.

Probabilistic fault displacement hazard analysis (PFDHA) estimates the probability of occurrence and the expected exceedance of on-fault (principal fault rupturing; PF) and off-fault (dist ributed rupturing; DR) surface displacement during an earthquake. Here we concent rate on off-fault rupturing on dip-slip earthquakes, and present an original probability model for the occurrence of DR and for the expected exceedance of displacement dist ribution based on an approach named “slicing” (an alternative to the “gridding” approach commonly used). The method is developed based on the compilation and reappraisal of surface ruptures from 32 historical crustal dip-slip earthquakes, with magnitudes ranging from Mw 4.9 to 7.9. A ranking scheme is applied to distinguish PF (rank 1) from simple DR (rank 2) and t riggered faulting (rank 3). Thus modellers can use prediction equations based on or excluding ruptures st rongly related to local st ructural setting depending on the site of concern. In the case of a st ructural setting at a site where large-scale bending (rank 21, 22) and pre-existing faults (rank 1.5, 3) is considered irrelevant, modelling can be performed considering only the unpredictable DR (rank 2). To minimize bias due to the incomplete nature of the database, we int roduce the “slicing” approach, which considers that the probability of having a surface rupture within slices parallel to the PF is homogeneous along the st rike of each slice. “Slicing” probabilities, computed as a function of magnitude of the earthquake and distance from the PF, are then combined with Monte Carlo simulations that model the dependence of the probability of occurrence of rupture and exceedance of displacement with the dimensions and position of the site of interest with respect to the PF. Finally, both probabilities are combined with existing predictive equations of exceedance of displacement on the PF to calculate fault-displacement hazard curves for sites of interest.

Due to the low to moderate seismicity of the European Western Alps, few focal mechanisms are available to date in this region, and the corresponding current seismic stress and strain fields remain partly elusive. The development of dense seismic networks in the past decades now provides a substantial amount of seismic records down to low magnitudes. The corresponding data, while challenging to handle due to their amount and relative noise, represent a new opportunity to increase the spatial resolution of seismic deformation fields.The aim of this study is to assess spatial variations of the tectonic regimes and corresponding stress and strain fields, which will provide new insights into active seismic deformation in this area. The dataset comprises more than 30,000 earthquakes relocalized in a 3D crustal velocity model, and more than 2200 focal mechanisms newly computed in a consistent manner. We inverted this new set of focal mechanisms through several strategies, including a seismotectonic zoning scheme and a Bayesian inversion, which provides a probabilistic 3D reconstruction of the faulting style in the Western Alps. The global distribution of P and T axes plunges confirms a majority of transcurrent focal mechanisms in the overall alpine realm, combined with pure extension localized in the core of the belt. Extension is found clustered, instead of continuous along the backbone of the belt. Compression is robustly retrieved only in the Po plain, which lays at the limit between the Adriatic and Eurasian plates. High frequency spatial variations of the seismic deformation are consistent with surface horizontal GNSS measurements as well as with deep lithospheric structures, thereby providing new elements to constrain homogeneously deforming zones. We interpret the ongoing seismotectonic and kinematic regimes as being controlled by the joint effects of far-field forces –imposed by the counterclockwise rotation of Adria with respect to Europe- and of buoyancy forces in the core of the belt, which together explain the high frequency patches of extension and of marginal compression overprinted on an overall transcurrent tectonic regime.These results shed new lights on seismicity distribution and tectonic regime variations both regionally and at depth. They appear complementary to geodetic constraints on active faults and to existing structural studies, thus allowing us to bring new insights into future seismogenic zoning schemes.

International audience; The analysis of the seismicity catalog (1996 to 2019) covering the region from the Juramountains to Corsica provides a first-order image of the distribution of earthquakes, highlightinglarge structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepeningof the focal depths through the Western Alps, several large active faults (e.g. Belledonne, MiddleDurance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of themain events display a diversity of seismic behaviors that can be explained by the complexity of thedifferent geological domains with a more or less strong structural inheritage, by variable rheologicalcharacteristics at the scale of the crust and by the joint action of different mechanisms of deformation.The distribution of the historical events is in fairly good agreement with the instrumental seismicity,but several earthquakes of M > 6 are highlighted since the 14th century until the beginning of the 20th.

International audience; The Billecocha plateau (4000 m a.s.l.) lies in the high elevation Ecuadorian Andes volcanic arc. It overhangs by 2000 m above the interandean valley. Both the plateau and surrounding volcanoes are heavily affected by active faulting characterized by straight, sharp and discontinuous scarps within a 6 km wide and 24 km long corridor. Contrasting interpretations have been proposed to explain the expression at surface of the so-called Billecocha fault system (BFS), from normal faulting related to postglacial elastic rebound or gravitational processes, to right-lateral faulting compatible with the North-Andean Sliver tectonic regime. The instrumental seismicity recorded around the BFS is low, however, a M~7 earthquake heavily struck the region in 1868.With the aim to discuss the kinematic and coseismic nature of the encountered deformations as well as the seismogenic character of the BFS, we performed (1) morphological analysis to map and quantify evidence of active faulting and (2) paleoseismological investigations across the longer segment of the fault system. In three trenches, we show that surface deformations are at least partly coseismic in origin during the Holocene with a minor lateral component, the last paleoseismic event being compatible in date with the 1868 earthquake. In addition, some of the enlightened paleoseismic events could have occurred in relationship with volcanic eruptions of the surrounding volcanoes.While field evidence of reverse and strike slip faulting suggests that regional tectonics could be involved, the geomorphological signature of the BFS at the mountain scale, as seen on the digital surface model, can partly be related to the development of deep seated gravitational deformations, hence suggesting an interaction between boundary (i.e. tectonic, volcanic) and body forces (i.e. gravity, post-glacial rebound). Further studies are however mandatory to better address the influence of each process at the BFS, in particular geodetic and seismological surveys.Given the available data, we suggest that the BFS could actually correspond to the distributed surface expression of the tectonic reactivation of the inherited Pujilí suture, enhanced by gravitational phenomenon. In this light, paleoearthquakes identified along the BFS may help evidencing the recurrence of major events in the region. However, it also imply that surface deformations along the BFS shall not be used without a careful and more detailed field work to derive fault slip rates for seismic hazard calculations.

Comptes rendus geoscience 353(S1), 381-412 (2021). doi:10.5802/crgeos.98 special issue: "Seismicity in France = Sismicit�� en France / guest editors/r��dacteurs en chef invit��s: Carole Petit, St��phane Mazzotti, Fr��d��ric Masson", Published by Institut de Frances, Acad��mie des sciences, Paris

International audience; On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE–SW La Rouvière normal fault in reverse faulting in agreement with the present-day E–W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics.

The Rhône River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric synthetic-aperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouvière fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates.

This study analyzes surface displacements generated by a low magnitude crustal earthquake in the Ecuadorian Andes by combining analysis of SAR Interferometry, geological field investigations and seismological data. In March 2010, a significant surface faulting event occurred in the Pisayambo area (Eastern cordillera), along the major dextral fault zone bounding the North Andean Sliver and the South-America Plate. Interferograms were inverted to determine fault plane geometry and slip displacement distribution. The event affected a 9 km-long previously unknown fault, referred as the Laguna Pisayambo Fault (LPF), with purely dextral movement reaching 45 cm and concentrated in the top 3 km of the crust. Geological investigations confirm both the fault mechanism and the amplitude of displacements. While these large displacements would be related to an event with a magnitude of 5.44 if using a standard crustal rigidity, we show that they can be convincingly associated with an Mw5.0 earthquake, that occurred on 2010/03/26. Reconciling the apparent differences in magnitude requires the existence of a low rigidity medium at shallow depths and/or postseismic activity of the fault. However, considering only the latter hypothesis would imply an unusually active postseismic process, in which 400-500% of the coseismic moment is released in the 6 days following the earthquake. Our observations highlight that the scaling laws relating surface observations to earthquake magnitude, classically used for seismic hazard assessment, should be carefully used. This study also illustrates how systematic InSAR analysis, even in places where no clues of ground deformation are present, can reveal tectonic processes.

The Alhama de Murcia Fault (AMF) is one of the most seismically active faults in the Iberian Peninsula, with important associated historical and instrumental seismicity (e.g. the 1674 IEMS VIII and 2011 Mw 5.1 Lorca earthquakes), and numerous geomorphic and paleoseismic evidence of paleoearthquakes. It is an oblique left-lateral strike slip fault within the Eastern Betics Shear Zone (EBSZ), a nearly 500 km long fault system that absorbs a great part of convergence between the Nubian and Eurasian plates. Previous paleoseismic studies have mainly focused on the southwestern and especially the central segment of the fault and yielded slip rate values ranging from 1.0 up to 1.7 mm/yr. In the central segment (Lorca-Totana), the fault splays into several branches, the two frontal ones forming a pressure ridge. Paleoseismic trenches have exclusively been dug in the northwestern fault of the pressure ridge, where most of the displacement is along strike, while the expected reverse southeastern branch has never been directly observed.We present the first results of paleoseismic trenching across a complete transect of the pressure ridge in the Lorca-Totana segment of AMF. To do so we excavated an exceptionally large trench (7 m deep) in the NW branch and 5 trenches in the SE branch. We have been able to: a) extend the paleoearthquake catalogue in the NW branch by interpreting a total of 13 paleoearthquakes, 6 of which were not identified in previous studies. A restoration analysis has been performed; b) unveil the existence and recent activity (Holocene) of the thrust that bounds the pressure ridge to the SE. We have interpreted at least 5 surface ruptures, with the last one being younger than 8-9 kyr BP, based on new radiocarbon dates.The study of these two sites allows for the refinement of the seismic parameters of the fault, formerly inferred from the study of a single branch. In this sense, the more complete paleoearthquake catalogue will allow for reassessment of the recurrence intervals assigned to the fault and new slip rate estimates will be inferred by combining data from the two studied sites. Furthermore, forthcoming OSL dates may allow us to prove or reject the synchronicity of surface ruptures on both sides of the pressure ridge, shedding light on the rupturing style of this fault system during the Late Quaternary. We discuss how these new data on fault-interaction may affect several seismic parameters and their repercussion in source modelling for fault-based probabilistic seismic hazard assessments (PSHA) of the region.

On November 11th 2019, a Mw 4.9 earthquake shook the Rhone River Valley in southern France, a rather densely populated area with many industrial facilities including several nuclear power plants. The “Le Teil” earthquake was felt as far as Montpellier and Grenoble, 120 km from the epicenter. Seismological data promptly showed that the earthquake corresponded to a reverse faulting event along a NE-SW trending fault with a focus at a very shallow depth (~1 km). In parallel, satellite-based radar observations (InSAR) showed the uplift of the SE compartment (up to 10 centimeters) along a sharp NE-SW trending ~4.5-km-long discontinuity. Field investigations conducted in the following days and weeks in the epicentral area uncovered several evidences of surface ruptures across roads and paths where the InSAR discontinuity is mapped. We also carried out airborne LiDAR surveys to map the rupture below the dense forest cover. Characteristics of surface deformations are fully consistent with InSAR and seismological data, and allow concluding to the reactivation of an Oligocene normal fault segment (i.e. La Rouvière fault) that belongs to the Cévennes fault system, a 120 km long polyphased system bounding the southern rim of the Massif Central. The absence of clear cumulative compressional deformation along the fault rupture, which on the contrary displays inherited extensional deformation (most likely Oligocene in age), suggests that the fault has not moved significantly since millions of years. These observations relaunch the question of seismic hazard assessment in stable continental regions such as continental France and most of Western Europe, where strain rates are very low.

We propose a new analysis of the W-Alpine seismicity based on space and time distributions along the Alpine arc. The overall area bears witness of a relatively important seismic activity localized along the so-call Briançonnais and Piemontais seismic arcs, but also along alignments corresponding to individualized active fault, e.g. in front of the Belledonne massif, and locally in form of important seismic swarms (Ubaye, Maurienne, Mont-Blanc). The regional tectonic regime is well analyzed (see for instance Mathey et al., this session), with detailed mapping of both the stress and strain fields. However, actual available studies do not take into account the time and space distributions. Our study is developed using several available datasets covering various time spans and various strategies (local and regional seismic networks, template matching, historical seismic catalogue). We focus firstly on the space distribution of the activity along the arc, taking into account: (i) the simple occurrence of seismic events to calculate regional density maps, also investigating the B-value mapping; and (ii) the energy density, using the seismic moment fluxes per surface unit as a proxy. On this basis, we secondly analyze the time evolution of the seismicity, which is actually limited by the available dataset’s time span. Our integrated analyze focusses on 3 primary targets: (i) to compare the information arising from the different databases; (ii) to compare the most active zones in terms of earthquake occurrence vs. seismic energy released; (iii) to unravel potential evolutions or establish relative steadiness in alpine seismicity through time. This work will finally allow to better understand and discuss the Alpine seismicity’s mechanisms, in relation with the actual dynamics of this orogen.

Paleoseismic data on the eastern central Rhine Graben Boundary Fault, as part of the Upper Rhine Graben (URG) fault system, revealed Holocene earthquake activity with surface rupturing faults. The URG is one of the most seismically active areas in the stable continental interiors of Central Europe north of the Alps. We opened the first paleoseismic trenches N of Basel and S of Frankfurt along the ca. 300 km long eastern Rhine Graben Boundary Fault (RGBF). After extensive shallow geophysical and morphotectonic investigations and analyses, we discovered that the eastern central RGBF consists of several parallel fault strands that are marked by topographic steps, by varying hydrogeologic conditions, moisture content and by geophysical anomalies in the subsurface (GPR and ERT data). Some of the scarps close to the alluvial plain of the river Rhine have been identified as erosional features. We opened six trenches perpendicular and parallel to the second topographic scarp and strand of the main RGBF in Ettlingen area. Trenching the main RGBF was precluded due to forest cover and the presence of big blocks of rock in the colluvium at the base of the slope (red Triassic sandstones). Trenches were up to 20 m in length and 2 m in width, and up to 3 m in depth. None of the trenches reached the Triassic Buntsandstein “basement”, and all exposed Pleistocene and Holocene strata. Some strata are interpreted as blocky/gravelly colluvium of the Glacial periods, Loess, redeposited gleyey Loess, soli-/gelifluction layers and deposits and organic paleosols. Most of these layers are clearly displaced by faults and downthrown to the west, although some strata appear to warp or fold over faults. Massive liquefaction and periglacial features have been found, the relation to the sedimentary sequences in the trenches need to be elaborated in future. The process is interpreted to be instantaneous, as massive colluvium is placed against clayey/silty Loess deposits, and therefore we attribute these displacements to earthquake-related faulting. Creep along the strand can be ruled out. The displacement on free faces is on the order of 30 – 50 cm per event vertically, and considerable horizontal offset (ca. 2 m), and we found evidence for two of such events. Applying the commonly used empirical relationships, these findings are interpreted as two events with a magnitude M larger than 6. These results show the bias between the seismogenic landforms (scarps, hanging valleys, triangular facets, etc.) in the eastern UGR margin and seismicity recorded by seismic stations in the area, as currently most of the activity is found in the southern URG near Freiburg. Our findings contribute significantly to the completeness of the earthquake history in the eastern central URG.

The Mw 4.9 earthquake that occurred near Montelimar on November 11, 2019 showed peculiar characteristics: a very shallow hypocenter (1km depth) with unexpected surface ruptures for such a moderate magnitude, and only few aftershocks showing low magnitudes (ML < 2.7). This event occurred in the industrialized Rhone Valley (including nuclear power plants and chemical industry) where several historical earthquakes with similar intensities and magnitudes took place (e.g. 1773, 1873, 1934).The earthquake broke a ~5-km-long segment of the northern tip of the Cevennes fault system (La Rouvière Fault Segment). This ~100 km-long fault network has a NE-SW orientation trend and is inherited from the Variscan orogeny (~300 Ma). It first registered an extensive and transtensive tectonic phase ending at the Oligocene age (~30 Ma) before being inverted, as revealed by the reverse focal mechanism of the Le Teil event.To date, this fault network has been poorly investigated in terms of seismic hazard, likely due to the low Mw expected on such short structures. Therefore, we started a new study to document its paleo-earthquake record in the framework of the new French RGF program (Alps and surrounding basins, BRGM).Our first target was to map the cumulative trace of the fault. A first airborne LiDAR survey was acquired by helicopter and UAV (unmanned aerial vehicle) just after the earthquake. They allowed the identification of a continuous inherited scarp of 1 – 2 m in height over ~4 km along the preexisting Oligocene fault. In order characterize the post-Oligocene deformation along this fault, we performed a detailed analysis of geomorphological field observations, as well as a geophysical study by acquisition of seismic, electrical and ground-penetrating radar profiles. These profiles aimed to better understand how the 11/11/19 earthquake surface rupture is connected at depth to the Oligocene structure (La Rouvière Fault).Each step of the analysis aims at eventually locating sites for further paleoseismological trenches, accounting for fault location, sediment preservation with favorable age determination potential and accessibility. This kind of investigation will provide information on the evolution over time of the seismic activity of this fault network, as well as relevant data on the current hazard they present in the specific context of the French Rhone Valley.

In the Western Alpine arc, GNSS measurements indicate that the far field convergence responsible for the Oligo-Miocene continental collision is now over. However, seismicity and slow deformation are still active. Former collisional tectonic features, such as the Penninic Front, are nowadays reactivated as normal faults. Indeed, geodetic and seismotectonic studies show that the inner part of the chain is undergoing transtensional deformation, although local compression is observed in the foothills at the periphery of the arc. Due to the low to moderate seismicity of the Western Alps, the stress and strain fields remain partly elusive.The aim of the present study is to quantitatively assess the current seismic stress and strain fields within the Western Alps, from a probabilistic standpoint. We used a new set of more than 30,000 Alpine earthquakes recorded by the dense local Sismalp seismic network since 1989. We first computed well-constrained focal mechanisms (f.m.) for more than 2,000 events with Ml ranging from 0.5 to 4.9 based on first motion (P-wave) polarity. This is the first time that such a huge focal mechanism dataset can be analyzed in the Alps. Corresponding events have been localized using a 3D velocity model (B. Potin, 2016). The global distribution of P and T axes dips confirms a vast majority of dextral transtensive focal mechanisms in the overall Alpine realm. We interpolated these results based on a Bayesian interpolation method, providing a probabilistic 2D map of the styles of seismic deformation in the Western Alps. Compression is robustly retrieved only in the Pô plain where seismicity depth differs from the shallow seismicity of the Western Alps. Extension is localized at the center of the belt. Importantly, extension is clustered instead of continuous along the belt. We then summed seismic moment tensors in homogeneous volumes of crust, to obtain seismic strain rates directly comparable to geodetic ones. Last, we inverted f.m. together in specific areas to obtain principal stress directions. A major outcome is the orientation of the extension, which is surprisingly oblique to the arc, rather than normal, as commonly thought.These results bring new insights on the geodynamic processes driving the seismotectonic activity of the Western Alps, such as the relative contributions of crustal tectonics and deep processes.

SUMMARY Quito, the capital city of Ecuador hosting ∼2 million inhabitants, lies on the hanging wall of a ∼60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3–5 mm yr–1 of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (

International audience; The Holocene sedimentary record of the flood plain of the Saint-Ciers-Sur-Gironde marsh was examined on sediment cores from the right bank of the Gironde estuary with regard to the evolution of the marsh and its potential to preserve high-energy deposits. Sedimentological, geochemical, geophysical and micropaleontological methods were applied. Radiocarbon ages in the central part of the investigation area reach back to 7,971 ± 44.5 BP. The sediments of the Saint-Ciers-sur-Gironde marsh reflect the conditions of a Holocene estuarine salt marsh before human activity and draining. The interpretation of the core data showed that the study area includes different facies during development from the pre-Holocene to recent times Tidal mudflats in the northwest at Mortagne and Beaumont, and a fluvial facies developing into an estuarine facies in the southeast near Camp and Saint-Ciers-Sur-Gironde. The last stage is the formation of the saltmarsh. The changes in grain size reflect different transport mechanisms during the development from a fluvial environment to the recent marsh. First gravel, then sand and finally muddy sediments were deposited in the estuary and finally in the marsh area. Below the salt marsh deposits in Mortagne-Sur-Gironde, there is some evidence of deposits from energy-rich events in tidal mudflats. The southern and central part, in which estuarine clays were deposited, was probably most of the time outside the range of storms. On the marshland surface, erosion, pedogenesis and bioturbation processes destroy storm relevant layers in a very short time. © 2019 Gebrüder Borntraeger Verlagsbuchhandlung, Stuttgart, Germany.

International audience

The 2016 Mw 6.1 Parina earthquake ruptured a shallow-crustal normal fault within the high Andes of south Peru. We use high-resolution DEMs and field mapping of the surface ruptures generated by the earthquake, in combination with co-seismic and post-seismic InSAR measurements, to investigate how different features of the geomorphology at Parina are generated by the earthquake cycle on the Parina Fault. We systematically mapped 12 km of NW-SE trending surface ruptures with up to similar to 27 cm vertical displacement and similar to 25 cm tensional opening along strike, separated by a gap with no observable surface ruptures. Co- and post-seismic InSAR measurements require slip below this gap in surface ruptures, implying that surface offsets observed in paleoseismic trenches may not necessarily be representative of slip at seismogenic depths, and will typically yield an underestimate of paleo-earthquake magnitudes. The surface ruptures developed along 10-20 m high cumulative scarps cutting through late Quaternary fluvio-glacial deposits and bedrock. The 2016 Parina earthquake did not rupture the full length of the late Quaternary scarps, implying that the Parina Fault does not slip in characteristic, repeat earthquakes. At Parina, and across most of the Peruvian Altiplano, normal faults are most-easily identified from recent scarps cutting late Quaternary moraine crests. In regions where there are no recently-deposited moraines, faults are difficult to identify and lack time constraints to quantify rates of fault slip. For this reason, current fault maps may underestimate the seismic hazard in the Altiplano.

International audience; The past thermokarst activities in valleys of northern France and Belgium covered the Upper Weichselian and the Upper Saalian periods. To develop in western Europe, thermokarst first requires an accumulation of ground ice close to the surface progressively stored along the glacial-time permafrost aggradation it is regionally uncommon during the early glacials, especially on the plateau, but frequent during the Upper Pleniglacials in valleys. These features mostly relate to various frost mounds created by injection and segregated ices. The role of ice wedges is really very limited in this zone of southern extent of the European palaeo-permafrost on plateau and terraces. Thermokarst events are mostly susceptible to occur during the coldest part of the glacial. With a more progressive warming or a retrogressive thermokarst triggered by erosion, as in Arctic today, deformations are more gradual, in direct relation with the rheological properties of the sediments and usually local drainage. They are in concurrence with the vegetation dynamic that will limit its expression. Thermokarst events are in first order orbitally forced under control of a maximum insolation and a minimum in precession, as well as during the Weichselian and the Saalian. They are moreover, associated with abrupt warming transmitted by Dansgaerd Oeschger events. Snowiness and mild winter temperatures are probably the main triggers for thermokarst activity as of today. Other events can be triggered by solar activity as at 20 ka or perhaps enhanced by major ash splay as during the MIS 6b Zeiffen interstadial. Thermokarst events are usually followed on the continent by a reorganization of the rivers from braided to meandering systems. © 2018 Elsevier B.V.

International audience; The Asian plate interiors are known to have host strong earthquakes with magnitudes up to M ≈ 8 in recent history, especially around the border area between Mongolia, Kazakhstan, China and Russia (e.g., M7.3 Chuya earthquake, 2003). Their recurrence times seem to be long, because of the relative low slip rates (

International audience; We integrate paleoseismic data sets along the Mt. Vettore‐Mt. Bove normal fault system rupturing at the surface in the 30 October 2016 Norcia earthquake. Through the analysis of new trenches from this work and a review of the preexisting data, we correlate events among trench sites along antithetic and synthetic fault splays. We recognize seven M 6.5, 2016 Norcia‐type (or larger) surface‐faulting events in the last ~22 kyr, including 2016. Before 2016, one event occurred in the past two millennia (260–575 CE) and possibly corresponds to the event damaging Rome in 443 or 484/508 CE. Three previous events occurred between 10590 and 415 BCE, whereas the two oldest ones date between 19820 and 16540 BCE. The average recurrence time is 3,360–3,640 years for the last ~22 kyr and 1,220–1,970 years for the last ~4 kyr. We infer a minimum dip‐slip rate of 0.26–0.38 mm/year on the master fault in the central portion of the Mt. Vettore–Mt. Bove normal fault system and a dip‐slip rate of at least 0.10 mm/year on the southernmost portion. We infer a Middle–Late Pleistocene inception of the long‐term scarp of the investigated splays. The along‐strike variation of slip rates well reproduces the trend of the 2016 surface slip; thus, the time window exposed in the trenches is representative for the present fault activity. Based on trenching data, different earthquake rupture scenarios should be also considered for local hazard assessment.