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The geodynamic complexity of the Southwestern Alps (France, Italy) comes from its strong tectonic inheritage due to the European-African plates convergence. The motion being currently mainly accommodated along the Maghrebides, this region of the Alps only registers small to moderate seismicity linked to low-deformation rates (convergence rates of 0.3-0.9 mm/yr). Hence until now, the geometry of the active faults in the Southwestern Alps remains unclear and imprecise. Yet, a better knowledge of these faults is a prerequisite for the establishment of a regional deformation model and the improvement of the seismic hazard assessment.Taking advantages of a nine-year seismicity catalog (7659 earthquakes of local magnitudes ranging between -0.73 and 5.03), recorded by the French and Italian permanent national networks presenting no major evolution since 2014, a high-resolution relocation is currently ongoing. The purposes are to (1) understand how the seismic events are linked to the mapped faults, (2) highlight unknown deep seismogenic structures and (3) finally improve the overall picture of the 3D geometry of active faults in the Southwestern Alps.We present here the preliminary analysis of the relocated catalog. The seismicity is relocated using the double-difference relative method HYPODD with both cross-correlation and catalog times. As a result, the relocation is achieved for 5828 earthquakes. The uncertainties are reduced to less than 120m in horizontal and less than 600m in vertical compared to the initial average uncertainties of less than 2 kilometers for both values, referred by previous papers.We assess the reliability of our results by comparing, at regional scale, our new relocations with those obtained by similar methods in Ubaye region. We illustrate how the double-difference relocation refines active zones imaging at multiple scales, particularly in the swarms. In Isola region located around 60 kilometers from Nice, a swarm, active since summer 2021, initially detected by the national network as a 3-kilometerlong/1-kilometer-large shape, has been precised into a 1-kilometer-long/100-meterlarge spatial activity. This relocation improvement enabled us to detect progressive activation of fault segments. On larger scale, relation between faults that may play a key role in the present-day general dynamics of the Alpine chain and deep seismogenic structures is clarified. It is the case for the High-Durance valley (France), where the precise geometry at depth of the Crustal penninic Front and High-Durance fault is determined.

On 2nd October 2020, an unusual extreme rainfall event (600 mm) associated with the devastating “Alex storm” occurred in less than 24 hours in the Tinée valley, a low strain rate area (convergence rates of 0.3-0.9 mm/yr) of the Southern French Alps, located 20 kilometers from Nice city. This transitional zone between the Argentera Mercantour exhumed Alpine massif and the Nice/Castellane Arc, mainly filled with Cenozoic sediments covering inherited structures, has no clear active faults known and displays a low seismicity rate with only 60 events recorded between 2014 and October 2020 by the national RESIF-EPOS seismic network, with local magnitudes ranging from 0.6 to 2.6. However, in the days after the “Alex storm”, a sudden increase in the seismicity rate was observed, with 114 events detected by template matching (local magnitudes between -0.8 and 2.05). After a peak activity, reached on the 8th of October with more than 60 events detected, the seismic crisis ended around mid-December 2020. Here, we investigate how the intense rainfall can explain the seismic sequences and what are the triggering processes in such a low tectonically deformation area.Basing our analysis on a precise relocation of the seismicity, using the double-difference relative method, three swarms successively activating from south to north, with focal depths around 5 kilometers have been revealed. The main swarm clearly presents a N160 alignment, which is quite consistent with the general orientation of the Southwestern Alps main faults. A geological field analysis has also shown the presence of major unmapped pluri-kilometers faults consistent with the seismicity location and orientation. Those faults may cross-cut the entire sedimentary layers, connecting more or less directly the ground surface to the deep basement with some highly-permeable channels for fluid flow. Moreover, this relocation analysis highlighted a bi-directional migration of the seismicity within the main swarm: northwestward with a velocity of 100 m/hr, compatible with aseismic slip-driven seismicity, and southeastward with a velocity of 4.5 m/hr, rather compatible with fluid diffusion-driven seismicity.On top of that, preliminary numerical models, focusing on the analysis of Coulomb stress changes in response to the recorded rainfall rate, showed a correlation between the seismicity rate and the rainfall, which may indicate a rapid saturation of the shallow porous sedimentary layers with fluids after the storm. However, models of stress changes associated with increasing fluid pressure only or including the effect of poroelasticity are not sufficient to explain the temporal evolution of seismicity and its rates. The contribution of other driving processes is necessary and aseismic slip processes could be more relevant to explain the 3 main bursts of seismicity, the migration pattern and the few-days delay with the rainfall episode. Those rainfall-induced aseismic fault slip may have triggered local seismic ruptures along small seismogenic portions of unknown inherited structures. Thus, our study reveals that the Tinée valley area is a good example to study the complexity of aseismic triggering processes of seismicity in association with shallow rainfall-driven hydraulic perturbations in an intraplate region with a low-deformation background rate.

International audience; The French–Italian Riviera faces several geophysical hazards, including recurrent earthquakes and underwater landslides that can be tsunamigenic. The stakes are high since this is a densely populated and touristic area. Several studies have already been carried out, in particular to map tsunami hazard resulting from the near-field seismicity of the North Ligurian Faults System, which is located a short distance off the coast. In our most recent study, runup maps were developed together with local analyses of tsunami-induced current fields. However, no conclusions were drawn, based on these results, as of the associated tsunami risk along the coast. Here, to this effect, we apply a recently proposed tsunami intensity scale to the simulation results obtained in our previous work (maximum values of tsunami depths and currents). This intensity scale (7 levels) is mapped over the entire coastal area, and its site-specific values are discussed. The scale allows quantifying the potential damage inland and at sea, based on a standard coastal vulnerability that has been assessed through different records. It thus represents a useful tool to help improving our preparedness to tsunami hazard.

The north Ligurian margin is a stretched continental margin located at the junction of the Western Mediterranean Sea and the Alpine belt. This region underwent several phases of contrasting deformation styles. The Ligurian basin opened from late Oligocene to early Miocene times, as a result of a back-arc extension induced by the rollback of the subducted Apulian plate. Since then, it has been evolving in the immediate vicinity of the active Alpine orogen, in a regional compressional setting between the Corsica-Sardinia continental block and mainland Europe.Nowadays, continuous seismic activity, with mainly reverse focal mechanisms, is recorded in the northeastern part of the Ligurian Basin. It is attributed to the compressional phase at work in the Gulf of Genoa since about 5 Myrs, which led to a significant uplift of the north margin documented by a vertical offset of the Messinian stratigraphic markers by more than 1000 m offshore Imperia. Although active seismogenic faults are still poorly known, a fault system outcropping at the foot of the continental slope, offshore Liguria and the French Riviera, is suspected from previous joint high-resolution seismic reflection data interpretation and sismotectonic studies.The SEFASILS project (Seismic Exploration of Faults And Structures In the Ligurian Sea) aims to better understand the mechanisms of the ongoing tectonic inversion of the margin and the crustal-scale tectonic structures –active or not– marking its evolution. We also aim to better characterize the sharp transition from the South Alpine belt to the Ligurian basin. Acquiring quality deep seismic data in the Ligurian Sea is challenging due to the complexity of structures beneath the margin and to the screening effect of the thick Messinian evaporitic series interlayered in the sedimentary cover farther seaward. To this end, joint acquisitions of deep, long-streamer multichannel seismic (MSC) reflection data and dense sea-bottom wide angle refraction data (WAS) have been carried out along a 150 km long profile offshore Nice, perpendicularly to the basin’s axis.The MCS data, thanks to pre- and post-stack migration, highlight faults at the foot of the continental slope rooting deeper than the salt decollement level. A first arrival travel time tomographic inversion of the wide angle data allowed us to build a velocity model of the study area reaching down to the uppermost mantle. Here, we present the results obtained from the joint analysis of MCS and WAS data. On the southern part of our profile some deep reflectivity, closely mirrored by the 7 km/s tomographic isovelocity, likely corresponds to the Moho. It is lost to the north, where shallower reflectivity, which could be interpreted as the base the thick sedimentary cover, coincides with the 5 km/s isovelocity. These two features are separately observed on both sides of what appears to be a major structural discontinuity between two contrasting basement domains, coinciding with an anomalously large salt diapiric complex in the sedimentary cover, also observed farther east in the basin. Such observations and their potential consequences will be discussed, in the light of previous regional studies.

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.

The Ubaye Region, where the city of Barcelonnette is settled, is the most seismically active region in the French Western Alps since at least two centuries. Seismicity in this area exhibits a dual behaviour, with mainshock-aftershock sequences alternating with abnormally high rate of seismicity associated with seismic swarms. Understanding processes triggering such a peculiar seismic behaviour is of primary importance in order to assess the seismic hazard in this region. The latest swarm activity started on February 26, 2012, with an earthquake of moment magnitude 4.2. It was followed two years later (on April 7, 2014) by a shock of magnitude Mw 4.8. From the first earthquake to the end of 2016, the seismic level has not returned to the background level and shares the same characteristics as a seismic swarm.With the aim to discuss the seismogenic processes involved in the area, we focused on the two months following the 2014 mainshock (Mw=4.8). During this period, a dense temporary network (7 stations) was operating at a maximal distance of 10km from the epicentre area. We analysed this period starting with a double-difference relocation of ~ 6,000 earthquakes previously detected by template-matching. These hypocentres did not align on the fault plane of the 2014 mainshock, but on conjugated structures belonging to the 2-km wide damaged zone of the main fault plane and on remote structures with various orientations further away. We then computed 99 focal mechanisms from a joint inversion of P polarity and S/P ratio to clarify the geometry of the active structures. Many nodal planes are inconsistent with the structures deduced from the alignments of the earthquake locations. The stress-state orientation obtained from those focal mechanisms (σ1 trending N27°± 5°, plunging 50°± 9°, a σ2 trending N215°± 5°, plunging 40°± 9°, and a sub-horizontal σ3 trending N122°± 3°) is consistent with those previously calculated in the area (Fojtíková and Vavryčuk, 2018). Nevertheless, some structures are unfavourably oriented to slip within this stress-field, suggesting that additional processes are required to explain them. As the presence of fluids was highlighted for the 2003-2004 and the 2012-2015 crisis, we calculated the fluid pressure needed to trigger slip on the planes from the focal mechanisms using Cauchy's equation. We found that a median fluid-overpressure of ~20 MPa (range between 0 to 50 MPa) is needed to cause slip. Although the origin of fluids and how they are pressurized at depth remains open. The fluid processes seem to be the most favourable additional processes and were also proposed to explain the 2003-2004 crisis.

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; We present a compilation of over 1700 focal mechanisms for nearly 1300 earthquakes in metropolitan France and bordering regions of Western Europe. It is based on both published and unpublished sources (articles, reports, observatory websites) for which the focal mechanism solutions have been verified for internal consistency, corrected in cases of minor errors and rejected in cases of major inconsistencies between the parameters. The database, labeled FMHex20, is a first version and should be regularly updated in the future as part of an ongoing effort within the Seismicity Transverse Action of the French Résif research infrastructure. We also present first-order seismotectonic analyses for the whole metropolitan France and for two regions (Western France and Northern Alps-Jura-Vosges) to illustrate how the FMHex20 database can serve as a basis for geodynamic or seismic hazard zonation studies. Combined with complementary datasets, it can improve our understanding of the kinematics of potentially active faults, including in very-low-strain-rate regions as is the case for most of France.

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.

International audience; The Tunka Basin is a broad, emerging basin situated between the Baikal Lake to the east and the the Hövsgöl Lake to the west. The basin is bounded to the north and to the south by the Tunka and the Khamar-Daban mountain ranges, respectively. The Tunka normal fault, located at the southern foothills of the Tunka mountain range, is the main structure that controlled the development of the Tunka Basin during the Neogene. Paleoearthquake-surface ruptures attest of its present activity; and show that its western and eastern terminations are undergoing a tectonic inversion characterized by left-lateral-reverse deformations. The southern edge of the Tunka Basin is classically interpreted as being tectonically controlled. In this paper, we present the results of a geomorphological and stratigraphic analysis within its southwestern and southeastern parts suggesting that there is no active fault affecting the foothills of the Khamar-Daban mountain range. The different features observed in the Quaternary deposits are interpreted to be the result of periglacial processes induced by alternating episodes of permafrost aggradation and degradation during the Holocene. Our study concludes that the Khamar-Daban Range and the Tunka Basin are uplifting together, and that the Tunka and Mondy faults are the two main triggers of regional earthquakes.

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.

The first leg of the SEFASILS cruise took place in November 2018 onboard the RV Pourquoi-Pas ? Up-to-date multichannel and wide-angle seismic data were acquired offshore Monaco, from margin to basin, aiming at providing a renewed vision of the complex North Ligurian backarc system. The compressive and extensive tectonic phases that have closely alternated in time and space over the last 45 My have yielded fairly contrasting structures, whose understanding is rendered even more challenging by the strong overprint of the Messinian salt tectonics. There is ample evidence of a compressive reactivation of the North Ligurian margin since 5 Ma at least, especially to the East, along the Gulf of Genoa. Such deformation is associated with some notable seismicity originating from faults and mechanisms that remain poorly apprehended. Yet, this seismicity peaked at one historical Mw ~6.6-6.9 destructive event (1887 Ligurian earthquake). The main objective of the SEAFASILS effort is a better characterization of the crustal structures, and chiefly, of the active crustal faults and their potential interplay with salt tectonics beneath the margin and the northernmost part of the basin, both featuring seismicity. Linking these aspects with broader-scale lithospheric processes within the Southern Alps/Northern Apennines, addressed in the AlpArray initiative, is also of great importance. Here we present preliminary results of these seismic investigations, with time and prestack depth migrated MCS data. Emphasis was put on the construction of some suitable velocity models to get optimal focusing of structures from surface to depth. Some active crustal tomographic velocities derived from the dense OBS deployment providing complementary insight will also be presented.

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.

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.

SUMMARY The Southwestern Alps and the Ligurian margin is a region of moderate seismicity with a high rate of small to moderate events. Identifying the active faults in this very densely populated region is critical to better assess the hazard and mitigate the risk. An accurate 3-D velocity model of the shallow to middle crust is a fundamental step to better locate the seismicity, and hence, the faults from which it originates. We performed ambient noise surface-wave tomography based on all available continuous seismological data from the French and Italian permanent networks (RESIF, INGV, RSNI), and current and past temporary experiments (AlpArray, CASSAT, SISVAR, RISVAL). In addition to these available data, we deployed three more stations to improve the spatial resolution in a region with sparse seismic station coverage. Overall, we used 55 inland seismic stations, 5 oceans bottom seismometers and 2 offshore cabled site/sensors. Data span the 2014–2018 time period. Time series from all available components were cross-correlated to reconstruct both Rayleigh and Love-wave Green's functions. For each station-pair Rayleigh and Love group velocity dispersion curves were semi-automatically picked using a frequency–time analysis. Then we regionalize these group velocities to build 2-D Rayleigh and Love velocity-maps between 1.5 and 9 s period. Using a two-step inversion, we estimate the best 3-D shear wave velocity model. The first step is based on a Neighbourhood Algorithm to recover the best three layers’ velocity model at each cell of the model. We then use this three-layer model as a starting model in a perturbational method based on finite elements. At periods up to 5 s, the spatial variation of the velocity is well correlated with the effective geology of the area. Lower velocities are observed in areas where the sedimentary cover is thicker, such as the Var and Paillon valley near Nice, or in the subalpine domain in the northwestern part of the region. Higher velocities are retrieved in areas where massifs are present, such as the Argentera-Mercantour massifs in the northeastern, or the Esterel massif in the southwestern part of the region.

International audience; The French Riviera is a densely populated and touristic coast. It is also one of the most seismically active areas of the Western Mediterranean. This is evidenced by the Mw 6.7–6.9, 1887 earthquake and tsunami, that was triggered nearshore, rupturing the easternmost 40 km of the 80-km-long Ligurian fault system, which runs parallel to and offshore of the Riviera. Here, coastal hazard from co-seismic tsunamis is assessed along the French and part of the Italian Riviera by simulating three Ligurian earthquake scenarios: (1) the 1887 event offshore Genoa, Italy; (2) a similar event transposed to the westernmost 40-km segment of the fault, offshore Nice, France; and (3) the rupture of the entire 80-km fault, which constitutes an extreme case scenario for the region. Simulations of tsunami propagation and coastal impact are performed by one-way coupling with the Boussinesq model FUNWAVE-TVD, in a series of nested grids, using new high-resolution bathymetric and topographic data. Results obtained in 10-m coastal grids provide the highest resolution predictions to date for this section of the French Riviera of co-seismic tsunami coastal hazard, in terms of inundation, runup, and current velocity. In general, the most impacted areas are bays (near Cap d’Antibes and Cap Ferrat), due to wave buildup and shoaling within semi-enclosed shallow areas, enhanced by possible resonances. In contrast to earlier work, which was based on coarser resolution grids, the area of Nice harbor is found to be rather well sheltered. It should be noted that uniform fault slip was used in the ruptures and runup estimates could locally be enhanced in case of more complex ruptures, such as segmented and heterogeneous ruptures.

International audience; The north Ligurian margin is a complex geological area on many accounts. It has witnessed several phases of highly contrasting deformation styles, at crustal scale and through shallow cover tectonics, simultaneously or in quick succession, and with significant spatial variability. This complex interplay is mirrored in intricate structures that make it hard to identify active faults responsible for both, the significant seismicity observed and the tectonic inversion undergone by the margin, identified on morphostructural grounds. We present here the first preliminary results of the leg 1 of SEFASILS cruise, conducted in 2018 offshore Monaco, in an effort to answer these questions by means of modern deep seismic acquisitions, using multichannel reflection and wide-angle sea-bottom records. Some first interpretations are provided and point towards an active basement deformation that focuses at the limits between main crustal domains.

International audience; The upper part of the Ubaye Valley (French Alps) is characterized by alternating mainshock‐aftershock sequences and swarms. Particularly, during the 2012–2015 crisis, four mainshocks with Ml > 3.5 occurred. We here focus on the aftershocks of the largest one (Ml = 4.8, 7 April 2014), in order to better understand the involved processes behind this peculiar seismic behavior. We use template matching detection, waveform classification, and double‐difference relocations to analyze this seismicity, on average and at the scale of the clusters that compose it. Most event sources are aligned along a plane consistent with the mainshock fault (N165, 65°W), but occurred on conjugate structures. A few clusters of seismicity are also observed far from the mainshock source. Our analysis shows that distinct, spatially separated processes are at play. While coseismic stress transfer explains the seismicity close to the mainshock source, fluid‐pressure diffusion and distant stress triggering are required to generate events farther away. The overall distributions in time and magnitude followed a slow Omori's decay and a Gutenberg‐Richter relationship, respectively. However, these classical responses arise from the superposition of very different behaviors, associated with different processes at depth.

A propos des vibrations du sol dues à un séisme Un séisme est dû à une rupture des roches en profondeur qui occasionne un déplacement rapide des deux compartiments d'une faille. C'est un phénomène bref (quelques secondes pour les séismes faibles à modérés à quelques minutes pour les séismes majeurs) qui génère des vibrations appelées ondes sismiques. Ces ondes se propagent de la profondeur vers la surface à une vitesse très importante (plusieurs kilomètres par seconde). Ce sont les vibrations superficielles du sol qui sont responsables de la plupart des dommages aux structures (immeubles, maisons, ponts, infrastructures). Les effondrements de structures sont responsables de la plupart des pertes humaines (*). Le niveau des vibrations du sol est fonction de deux paramètres de premier ordre : la taille du séisme (= la magnitude) et la distance entre la zone de rupture et le site considéré. La magnitude est une mesure de l'énergie émise par le séisme. Un séisme de magnitude modérée (4-5) peut engendrer des vibrations du sol assez fortes mais dans une zone très réduite (exemple du séisme de Barcelonnette dans la vallée de l'Ubaye du 26 février 2012 de magnitude 4.9 qui a engendré des destructions significatives dans la ville). A l'opposé, un séisme de magnitude plus forte (> 6) peut ne faire aucun dégât s'il est situé loin des zones habitées (exemple du séisme de magnitude 6 qui a eu lieu en mer entre la Corse et le continent en 1963). Il est donc primordial de connaitre où vont avoir lieu les séismes et quelle peut être leur magnitude. En plus de la magnitude et de la distance, un autre effet doit être pris en compte. Il s'agit de l'effet de site. Ce terme désigne les amplifications des ondes sismiques engendrées par les couches géologiques superficielles ou le relief topographique. Cela peut paraitre contre-intuitif, mais l'on sait maintenant que les ondes sont amplifiées par les sols mous (alluvions par exemple) en comparaison au rocher (sol formé de granite ou de calcaire par exemple). Ainsi un séisme d'une même magnitude et à une même distance génèrera des vibrations de surface plus fortes dans une vallée sédimentaire que sur un site au rocher. Un effet d'amplification peut exister aussi dans les zones escarpées (on parle alors d'effet de site topographique). Cet effet peut concerner certains villages perchés de l'arrière-pays niçois et de la vallée du Var par exemple.

International audience; A smart city must be equipped with efficient and interconnected transportation. Unfortunately, economic or operational reasons for creating an infrastructure sometimes compete with imperatives of natural hazards. The case of Nice Cote d'Azur airport is an interesting example. As air traffic increased, the airport has been progressively extended. The largest extension occurred between 1975 and 1979 when a large amount of backfill material has been accumulated on the continental shelf to reclaim land and create new tracks. In October 1979, a submarine landslide destroyed part of a seawall under construction and killed 10 people. This landslide was associated with a tsunami wave that swept the coast from Menton to Antibes: the damage was significant but much less than if it had occurred in summer during the tourist season. This disaster stopped all projects in the area for 40 years. However, recently, development projects have again been mentioned (information relayed by the press) with, in particular, the idea of a new port near the airport, able to accommodate ferries.For several years now, scientists have been working on the area to qualify natural hazard. Their results show that the airport tracks are subsiding [1] and that the surrounding offshore area is subject to recurrent small landslides [2], while geotechnical studies point to its unstable nature [3]. Recently, a seismological sensor installed under the sea during two years has recorded small local earthquakes. Our analysis of the signals revels a strong amplification effect of seismic waves related with the presence of a thick layer of sediments. Likewise, a future larger earthquake would also be amplified in this area, accentuating the potential for a significant submarine landslide. A domino effect is therefore to be feared: an earthquake, a landslide, a tsunami and maybe the only major local airport unable to operate properly in a time of crisis.A smart city should be a safe and resilient city that takes into account the contributions of all its stakeholders, including scientists. How scientific results can be taken into account into public decisions is still an open question.[1] Cavalié O., Sladen A., & Kelner M. (2015) Natural Hazards and Earth System Sciences [2] Kelner M., Migeon S. et al. (2016) Marine Geology [3] Dan G., Sultan et al. (2007) Marine Geology.

International audience; The Sayan and Tunka faults are located at the boundary between the northernmost mountain belt of Central Asia (the Sayan-Baikal ranges) and the Siberian platform. These prominent crustal structures were involved in the opening of the southern Baikal rift system since the beginning of the Cenozoic and define large-scale sharp morphotectonic features. Despite low instrumental seismic activity, Late Pleistocene-Holocene morphotectonic features along the two faults indicate that the faults are active and have the capacity to produce strong earthquakes. A careful mapping of the most recent trace of activity, within the southeastern parts of the two faults where they merge within a relay zone, demonstrates that they correspond now to left-lateral-reverse faults, suggesting a recent inversion of their vertical component. We also show that the two faults are now structurally connected via a young surface rupture and that no obvious post-Last Glacial Maximum ruptures are observed along the central part of the Sayan Fault beyond its junction zone with the Tunka fault. This suggests that the left-lateral strike-slip deformation is transferred from the eastern Sayan fault to the Tunka fault. A detailed morphotectonic study along the southeastern Sayan fault allows estimating a left-lateral slip rate between 1.3 (min) and 3.9 mm/year (max). Finally, a critical review of Russian paleoseismic data, combined with our paleoseismological investigations, allows us to propose that the mean recurrence time along the two faults is on the order of 4 kyr and that they may have either ruptured together or during seismic clusters.

International audience; A new seismically active zone is found in the southern part of the Ligurian basin, 80-km west of Corsica (western Mediterranean). The activity began in February 2011 with a foreshock (ML 4) and a mainshock (ML 5.3) 5 days later, followed by numerous aftershocks. We first analyze the fore- and mainshock in detail. We compare the results obtained using classical methods (linear location in a 1D medium and focal mechanisms from P and S polarities) and new approaches (non-linear location in a 3D medium and waveform modeling for determining the seismic moment and the focal mechanism). Both methods provided similar results for location, depth (in the range of 6–13 km) and focal mechanisms, which reveal reverse faulting with nodal planes oriented N–S and NE–SW. We then locate 27 of the aftershocks in the 3D model and find a 10-km-long NE–SW alignment with a depth between 7 and 16 km. In 2012 and 2013, three other moderate-size events (ML 3.8, 4 and 4.5) occurred and confirm that this zone is still active. The epicentral area is located in the oceanic domain of the Ligurian Basin. From analysis of the bathymetry and high-resolution multi-channel seismic profiles, no morphologic anomaly at surface and no inherited fault in the shallow ~ 4 km depth were imaged, which suggest that no significant deformation occurred in the area since 5 Ma. Thus, the structure(s) activated during the 2011–2013 sequence remain unknown. In light of these results, we point out a notable difference on both sides of the Ligurian Basin: the northern margin, close to the alpine chain, suffered strong earthquakes and large cumulated deformation since 5 Ma, while the southern margin, close to the Corsica–Sardinia continental block, is poorly deformed since 5 Ma.

After each large destructive earthquake in the world, shocking images are presented to the adults and children on the TV and through the Internet. All sorts of questions arise: Some of them are very rational, whereas others are driven by fear of a global catastrophe or even a curse. For example, in March 2011, after the giant Tohoku, Japan, earthquake, seismologists everywhere were asked to provide explanations and often to give their opinion on what had happened. How could there be such a disaster? Can it happen to us? Many took the time to talk to students in classrooms to make clear what we know and what we are still trying to understand and also tried to limit irrational fears. Teachers, as well as researchers, were on the front line, particularly those who actively participate in networks of educational seismology in the United States (e.g., Levy and Taber, 2005), England (e.g., Denton, 2009), Italy (e.g., Cantore et al. , 2003; Solarino and Eva, 2009), Switzerland (e.g., Sornette and Haslinger, 2009), and other countries in the world. Thanks to Sismos a l’Ecole (SaE) network stations, accessible in real time in various parts of the world, students saw the waves of the many aftershocks passing through their school and through other schools in the world. Thus, they immediately understood that the waves of Japanese earthquakes first arrived at the high school stations in Taipei, then in Canberra, and then in Istanbul before arriving under their feet at their school. Let us see how this network is structured and how it allows …

SUMMARY Early in the morning of 1887 February 23, a damaging earthquake hit the towns along the Italian and French Riviera. The earthquake was followed by a tsunami with a maximum run-up of 2 m near Imperia, Italy. At least 600 people died, mainly due to collapsing buildings. This ‘Ligurian earthquake’ occurred at the junction between the southern French–Italian Alps and the Ligurian Basin. For such a historical event, the epicentre and the equivalent magnitude are difficult to characterize with any degree of precision, and the tectonic fault responsible for the earthquake is still under debate today. The recent MALISAR marine geophysical survey allowed the identification of a large system of active faults. We propose that the rupture of some of the segments belonging to this 80-km-long northern Ligurian Faults system connected to a shallow-dipping major thrust plane at depth was the source of the 1887 Ligurian earthquake. We investigated the macroseismic data from the SISFRANCE-08 and DBMI-04 historical databases using several models of intensity attenuation with distance and focal depth. The modelling results are consistent with the off-shore location, with an epicentre around 43.70°–43.78°N and 7.81°–8.07°E, and with a magnitude Mw in the range of 6.3–7.5. Numerous earthquake source scenarios have been tested on the tide gauge record at Genoa harbour. As a result, we present seven characteristic source earthquake scenarios for a shallow strong earthquake occurring below the northern Ligurian margin. The modelled tide gauge records were analysed with the help of basic statistical tools and a simple harmonic analysis, to extract the wave spectrum characteristics. This analysis indicates that scenarios of a magnitude Mw of 6.8–6.9 along a reverse N55°E striking fault are the best candidates to explain the known characteristics of the tsunami that followed. The best-fitting scenarios comprise a 70°-dipping southward fault plane with Mw 6.8 and a 16°-dipping northward fault plane with Mw 6.9, both with reverse kinematics. Taking into account the geometry of the active faults, the location of the macroseismic epicentre and the morphotectonic evolution of the continental slope, we propose that the 1887 Ligurian earthquake corresponded to the reverse faulting of a N55°E striking fault plane dipping to the north with a coseismic slip of 1.5 m.

In places where sedimentation and erosion compete at fast rates, part of the record of past earthquakes on faults may be buried, hence hidden, in the first few metres below the surface. We developed a novel form of palaeoseismology, of geophysical type, based on the use of a dense pseudo-3-D Ground Penetrating Radar (GPR) survey to investigate such possible buried earthquake traces, on a long, fast-slipping strike-slip fault (Hope fault, New Zealand), at a site (Terako) where marked alluvial conditions prevail. We first used LiDAR data to analyse the ground surface morphology of the 2 km2 site at the greatest resolution. Nineteen morphological markers were observed, mainly alluvial terrace risers and small stream channels that are all dextrally offset by the fault by amounts ranging between 3 and 200 m. The measurements document about 10 past earthquake slip events with a mean coseismic slip of 3.3 +/- 1 m, with the most recent earthquake event having a slip of 3 +/- 0.5 m. We then investigated a detailed area of the site (400 x 600 m2) with pseudo-3-D GPR. We measured 56, similar to 400 m long, 510 m spaced GPR profiles (250 MHz), parallel to the fault and evenly distributed on either side. The analysis revealed the existence of a palaeosurface buried at about 3 m depth, corresponding to the top of alluvial terraces of different ages. That buried surface is incised by a dense network of stream channels that are all dextrally offset by the fault. We measured 48 lateral offsets in the buried channel network, more than twice than at the surface. These offsets range between 6 and 108 m, as observed at the surface, yet provide a more continuous record of the fault slip. The similarity of the successive slip increments suggests a slip per event averaging 4.4 +/- 1 m, fairly similar to that estimated from surface data. From the total surface and buried 67 offset collection, we infer that a minimum of 30 large earthquakes have broken the Hope fault at the Terako site in the last about 67 kyr, with an average coseismic slip of 3.2 +/- 1 m, a minimum average recurrence time of about 200 yr, and a magnitude of at least Mw 7.07.4. Our study therefore confirms that part of the record of past earthquakes may indeed reside in the first few metres below the surface, where it may be explored with geophysical, GPR-based palaeoseismology. Developing such a new palaeoseismological tool should provide rich information that may complement surface observations and help to document the past earthquakes on faults.

SUMMARY We present here a detailed analysis of a seismic data set recorded by a dense seismological network installed over 6 months in the southeast of France. This experiment was set-up at the boundary between the Ligurian basin and the southern subalpine thrust belt (the Nice arc), which is a complex tectonic region that undergoes low to moderate seismicity. We recorded more than 500 microearthquakes, among which 348 occurred exactly in the centre of the network during a very active seismic sequence that lasted mainly over 2 months. We performed an absolute location of all of the events and calculated the magnitudes. Then we applied a cross-correlation technique to gather similar events and to relocate relatively few of them. This method revealed a very clear alignment of 19 events in a direction N120 ◦ oblique to the N20 ◦ general trend of seismicity. Focal mechanisms were determined for the four largest events and composite solutions for 32 smaller ones. Both the alignments of the earthquakes and the focal solutions revealed that two oblique segments of the fault were activated during the crisis. The main segment (8 km long) that was oriented NNE with a left-lateral strike slip movement is called the Blausasc fault. Taking into account the tectonic evolution and the relationships between surface structures and the distribution of earthquakes, and through a paleaoreconstruction of the tectonic evolution, we propose that the Blausasc fault is the hidden root of the Peille-Laghet fault, which has a mapped length of at least 15 km. The smaller segment (0.6 km long) that was activated during the crisis could be interpreted as an antithetic Riedel fracture. The active Blausasc fault is located in a densely populated zone, at only 10 km from the crowded cities of Monaco and Nice. It is thus particularly interesting to analyse it for hazard assessment. In the last section, we present a simulation that is aimed at predicting what the ground motion in the city of Nice would be like if an earthquake of magnitude 5.7 occurs on this fault. For this, we used the recordings of the largest event of the seismic sequence (Ml = 3.2) and an empirical Green’s function summation scheme to simulate the ground motion at two stations situated in urban environments. The values obtained show that especially on soft soil sites, the effects of such an earthquake would be considerable in the city of Nice.

We used satellite imagery and field data to investigate the south-westernmost Baikal rift zone. We focus our study in the Mondy and Ikhe Ukhgun valleys, site of an Mw ¼ 6.9 seismic event in 1950. Surface deformations are observed along the E–W-trending Mondy strike-slip fault and along the Ikhe Ukhgun thrust. The Mondy fault system is 80 km long and is composed of four segments 10–15 km long. These segments are characterized by subvertical planes with left-lateral movements. The Ikhe Ukhgun thrust is 20 km long, dips 40 to the south and shows reverse movement with a left-lateral component. These observations are consistent with the present-day regional NNE–SSW compression and with the focal mechanism of the 1950 Mondy earthquake that was recently re-evaluated. These features, like those observed in the Tunka basin, demonstrate a recent change of regional strain regime from transtension to transpression that we place before the Late Pleistocene.

With the assistance of the ASTER team (M. Arnold, G. Aumaître, D. Bourlès, and K. Keddadouche); International audience; The Bulnay earthquake of 23 July 1905 (Mw 8.3–8.5), in north‐central Mongolia, is one of the world’s largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the twentieth century. The 375 km long surface rupture of the left‐lateral, strike‐slip, N095°E‐trending Bulnay fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike‐slip surface ruptures worldwide. Along the 80 km stretch, between 97.18° E and 98.33° E, the fault zone is characterized by several meters width and the mean left‐lateral 1905 offset is 8.9±0.6 m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault allow us to estimate a mean horizontal slip rate of 3.1±1.7 mm/yr over the Late Pleistocene–Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event, with recurrence intervals of ∼2700–4000 yrs.

The precise nature of the transition between the present-day compressional tectonics in central Mongolia and extensional deformation in the central Baikal rift has still to be determined. For that purpose we have built a comprehensive earthquake focal mechanism data base for the Mongolia – southern Siberia area, from which we map the variations of the stress field. We focus our detailed investigations on the largest seismic event in the transition zone, the 1950 (Mw 6.9) Mondy earthquake, for which several discordant focal mechanisms have been proposed. Using a new approach in source inversion, we resolve the focal mechanism (left-lateral strike slip type on a steep south-dipping fault) and depth (14 ± 3 km) of the Mondy earthquake with a satisfactory accuracy. This seismological information, combined with the geological observations, allows us to decipher the connections between the 1950 mainshock, the local stress tensor and the active faults, which strongly suggest a partitioning of the deformation between two faults, namely the Mondy and Ikhe–Ukghun faults.

International audience; The Ligurian coast, located at the French–Italian border, is densely populated as well as a touristic area. It is also a location where earthquakes and underwater landslides are recurrent. The nature of the local tsunamigenesis is therefore a legitimate question, because no tsunami warning system can resolve tsunami arrival times of a few minutes, which is the case for the area. As far as the seismicity of the area is concerned, the frequent recurrent earthquakes are generally of moderate magnitude: most of them are lower than M w 5. However, the relatively large M w 6.9 earthquake (Larroque et al., in Geophys J Int, 2012. doi: 10.1111/j.1365-246X.2012.05498.x) that occurred on the February 23, 1887, offshore of Imperia (Italian Riviera) is quite emblematic. This unusual event for the region merits a complete study: the quantification of its rupture mechanism is essential (1) to understand the regional active deformation, but also (2) to evaluate its tsunamigenesis potential by deriving relevant rupture scenarios obtained from our knowledge of the event; for that purpose the event is extensively described here. The first point has been the subject of quite a few studies based on the seismotectonics of the area. The last documented approach has been completed by Larroque et al. (Geophys J Int, 2012. doi: 10.1111/j.1365-246X.2012.05498.x) who proposed a rupture scenario involving a reverse faulting along a north dipping fault and favoring a M w 6.9 magnitude. In the present paper (1) we study the accuracy of their solutions in relation to the computational grid spacing and the dispersive/nondispersive parameterization, (2) based on an uncertainty on the recorded wave amplitude of the Genoa tide gauge they used, we propose a M w 6.7 earthquake magnitude solution for the event (the kinematics is unchanged), co-existing with the M w 6.9, (3) we evaluate the tsunami coastal impact of the 1887 event, and (4) we test a range of possible ruptures that local faults may undergo in order to propose a synoptic mapping of the tsunami threat in the area. The spatial distribution of the maximum wave height (MWH) is provided with a tentative identification of the processes that are responsible for it. This latter issue is imperative in order to make our mapping as generic as possible in the framework of our deterministic approach (based on realistic scenarios and not on ensemble statistics). The predictions suggest that the wave impact is mostly local, considering the relatively moderate size of the rupture planes. Although the present-day seismicity in this region is moderate, stronger earthquakes (M > 6.5) have occurred in the past. The studied scenarios show that for such events specific localities along the French–Italian Riviera may experience very significant MWH related to the shallow focal depth tested for such scenarios. We may reasonably conclude that the tsunami threat is relatively significant and uniform at the Italian side of the Riviera (from Ventimiglia to Imperia), while it is more localized (sporadic) at the French side from Antibes to Menton with, however, higher local level of inundation, e.g., Nice city center.

The Southern Alps – Ligurian basin junction is one of the most active seismic areas in Western Europe countries. The topographic and the structural setting of this region is complex because of (i) its position between the high topography of the Southern Alps and the deep, narrow Ligurian oceanic basin, and (ii) the large number of structures inherited from the Alpine orogeny. Historical seismicity reveals about twenty moderate-size earthquakes (up to M=6.0), mostly distributed along the Ligurian coast and the Vésubie valley. A recent geodetic experiment shows a significant strain rate during the last 50 years in the area between the Argentera massif and the Mediterranean coastline. Results of this experiment suggest a N-S shortening of about 2-4 mm/yr over the network, this shortening direction is consistent with the seismological (P-axes of earthquakes) and the microtectonic data. The Pennic front (E-NE of the Argentera massif) and the northern Ligurian margin are the most seismically active areas. In the Nice arc and in the Argentera massif, some seismic lineaments correspond to faults identified in the field (such as theTaggia-Saorge fault or the Monaco-Sospel fault). In the western part of the Alpes Maritimes, no seismic activity is recorded in the Castellane arc. In the field, geological evidence, such as offsets of recent alluvial sediments, recent fault breccia, speleothem deformations, radon anomalies and others indicates recent deformation along these faults. Nevertheless, to this date active fault scarps have not been identified: this probably results from a relatively high erosion rate versus deformation rate and from the lack of Quaternary markers. We also suspect the presence of two hidden active faults, one in the lower Var valley (Nice city area) and the other one at the base of the Argentera crustal thrust-sheet. Offshore, along the northern Ligurian margin, the seismic reflection data shows traces of Quaternary extensional deformation, but the accuracy of the data does not yet allow the construction of a structural map nor does it allow the determination of the continuity between the offshore and onshore structures. From these data set we propose a preliminary map of 11 active faults and we discuss the questions which remain unsolved in the perspective of seismic hazard evaluations.

The Tunka basin was initiated during Oligocene, under transtensional regime (normal-sinistral) as shown by large-scale structures and geomorphology. Nevertheless, a preliminary analysis of the most recent deformations allows us to evidence transpression on several sites within the basin. These tectonic features together with focal mechanisms and preliminary GPS data, suggest that the kinematics of the Tunka basin has undergone a very recent change, which could be due to the northward propagation of the India–Eurasia collisional strain field.

Active tectonics in the Western Alps is revealed by a moderate level of seismic activity and geological evidences for Quaternary deformation. We present new geodetic determinations of the current strain rates in the southern part of the Western Alps, that complement existing results in Provence, the Jura, and the Subalpine and Belledone massifs around Grenoble. We combined first- and second-order triangulation data collected in 1948 with global positioning system (GPS) data collected in 1998. We estimate shear strain rates of 0.1‐0.2 mrad yr’1 over distances on the order of 30 km, significantly diVerent from zero in most of the network. We obtain NW‐SE to N‐S compressive shear strain directions over most of the area, in agreement with the seismological and geological data. These geodetic estimates correspond to long-term overall N‐S to NW‐SE shortening of 2‐4 mm yr’1 over the study area, on the same order as the far-field shortening measured by continuous GPS on the Grasse‐Torino baseline (’2.0±0.5 mm yr’1), that incorporates our geodetic network. These results provide new constraints on interseismic strain in the Western Alps. © 2000 Elsevier Science B.V. All rights reserved.

Hassoun, V. ... et al.-- 6th International Symposium on Submarine Mass Movements and Their Consequences (ISSMMTC), 23-25 September 2013, Kiel, Germany, Submarine landslides on continental slopes are triggered by diverse mechanisms such as sea-level variations, climate-driven sediment supply fluctuations, slope steepening related to long term tectonics, earthquakes and human activities. In the present work, we try to discriminate the origin of major gravity events recorded in three sediment cores collected on the Var Sedimentary Ridge (NW Mediterranean) by means of X-ray imaging, grain-size distributions and 210Pb-based chronologies supported by 137Cs and AMS 14C dating. An alternation of hemipelagic mud and sandy turbidite layers is apparent in all cores. In two of them, the topmost turbidite can be correlated to the well-known 1979 Nice-Airport landslide. A sub-surficial sandy layer is identified in all three cores with an approximate age of 120 years, suggesting for the first time a relationship with the largest earthquake in the region (1887; Mw = 6.9). Below the depth of that major event, sand beds are tentatively related to older local earthquakes and centennial floods

We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

International audience; During one month, in October and November 2010, the region of Sampeyre (Western Alps, Italy) was struck by a seismic activity of several hundred events. The location of 287 events recorded at least by three stations showed a diffuse swarm ranging between 8 and 13 km depth. The number of earthquakes in the database was increased thanks to a detailed analysis of the continued recordings of the nearest station (DOI) which allowed identifying 2612 earthquakes. The temporal distribution of the seismicity is characterized by the alternation of active and quiescent periods. Moment magnitude was computed for 730 events by a P wave spectral analysis of the DOI station data. The estimated magnitudes range between 0.7 and 3.15. The b-value of the cumulated frequency–magnitude distribution is around 1.4. Spatio-temporal analysis of the located earthquakes highlighted an overall northward migration of the seismicity. From spatio-temporal criteria, the seismicity was divided in three clusters. Each cluster is characterized by its own focal mechanism family. The stress tensor determined from the focal mechanisms indicates probable stress heterogeneities at small spatial scale. The observed migration of the seismicity could be compatible with earthquake triggering by fluid diffusion processes.

The Sicilian accretionary prism shows a large, southward imbricated system of thrust sheets. The frontal part of the belt has been tectonically emplaced in Plio-Pleistocene times. Major decollement surfaces and sole thrusts at the base of and inside the prism consist of melanges with a “blocks in matrix” fabric. In the frontal part, two superposed decollement levels are recognized, the upper one located at the base of the Paleogene-Neogene sedimentary series, and the basal one at the base of duplexes (Triassic-Miocene sedimentary series). Syntectonic dewatering of the sedimentary sections occurred along decollements and thrust faults, involving hydrofracture and mineralized vein development. Inside the deep sole thrusts (rooted in the basal decollement); hydrofractured blocks show different generations of syntectonic quartz and calcite extensional veins, while in the upper decollement only calcite veins have been observed. Both quartz and calcite veins show aqueous primary fluid inclusions of low salinity, with low trapping temperature for fluids in the upper decollement ( If we assume that, at the time of deformation, the main heat source was the regional conductive geothermal gradient, the trapping temperatures of the fluids inside veins of the deep sole thrusts are higher than those predicted by the Pilo-Pleistocene gradient for this subduction zone and correspond to a thermal anomaly. We suggest that the frontal Sicilian prism supported localized transient fluid flow of deep freshwater sourced from two different origins: a shallow one for fluids now trapped in the upper decollement, and a deep internal origin (6–10 km minimum depth) for the hot fluids channelized along the basal decollement and now trapped inside the sole thrust of the Mt. Scalpello duplex area.

International audience; We present here new data from the MALISAR surveys that focus on two areas of the northern Ligurian margin where we reveal recent and active deformation. A set of N60°E scarps that are oblique to the margin is seen at the foot of the continental slope. These correspond to cumulated reverse-strike slip faulting that is consistent with the present-day kinematics of earthquakes. At the north-western margin, the Marcel Fault appears at the surface as a 10-km-long scarp trending N60°E. Several earthquakes of moderate magnitude (3.8 < ML < 4.6) have focal mechanisms consistent with the geometry and the kinematics of, and have been located around, the Marcel Fault. At the north-eastern margin, offshore of Imperia, the major feature is a large promontory that is bound by a network of N60°E faults on its southern side. The structures and the deflection of the drainage network are consistent with a recent uplift of the Imperia Promontory. No surface ruptures have been identified in the epicentre area of the 1887 Ligurian earthquake (the major historical regional event: MW ~6.5-6.7), although the Imperia Fault network has the necessary characteristics to account for this earthquake. Therefore, from the present study, we propose that the rupture, as a reverse-strike slip faulting of a few or all of the segments belonging to the Imperia Fault network, was the source of the 1887 event. These MALISAR data have allowed identification of an 80-km-long transpressive system of Plio-Quaternary faults. The question of the potential activation of all of these faults during a single event is now of major concern, which has led to reappraisal of the regional seismic hazard as moderate to high.

The lower Var valley is the only large outcropping zone of Plio-Quaternary terrains throughout the southwestern Alps. In order to assess the seismic hazard for the Alps – Ligurian basin junction, we investigated this area to provide a record of earthquakes that have recently occurred near the city of Nice. Although no historical seismicity has been indicated for the lower Var valley, our main objective was to identify traces of recent faulting and to discuss the seismogenic potential of any active faults. We organized multidisciplinary observations as a microseismic investigation (the PASIS survey), with morphotectonic mapping and imagery, and subsurface geophysical investigations. The results of the PASIS dense recording survey were disappointing, as no present-day intense microseismic activity was recorded. From the morphotectonic investigation of the lower Var valley, we revealed several morphological anomalies, such as drainage perturbations and extended linear anomalies that are unrelated to the lithology. These anomalies strike mainly NE-SW, with the major Saint-Sauveur – Donareo lineament, clearly related to faulting of the Plio-Pleistocene sedimentary series. Sub-surface geophysical investigation (electrical resistivity tomography profiling) imaged these faults in the shallow crust, and together with the microtectonic data, allow us to propose the timing of recent faulting in this area. Normal and left-lateral strike-slip faulting occurred several times during the Pliocene. From fault-slip data, the last episode of faulting was left-lateral strike-slip and was related to a NNW-SSE direction of compression. This direction of compression is consistent with the present-day state of stress and the Saint-Sauveur–Donareo fault might have been reactivated several times as a left-lateral fault during the Quaternary. At a regional scale, in the Nice fold-and-thrust belt, these data lead to a reappraisal of the NE-SW structural trends as the major potentially active fault system. We propose that the Saint-Sauveur–Donareo fault belongs to a larger system of faults that runs from near Villeneuve-Loubet to the southwest to the Vésubie valley to the north-east. The question of a structural connection between the Vésubie – Mt Férion fault, the Saint-Sauveur–Donareo fault and its possible extension offshore through the northern Ligurian margin is discussed.The Saint-Sauveur–Donareo fault shows two en-échelon segments that extend for about 8 km. Taking into account the regional seismogenic depth (about 10 km), this fault could produce M ~6 earthquakes if activated entirely during one event. Although a moderate magnitude generally yields a moderate seismic hazard, we suggest that this contribution to the local seismic risk is high, taking into account the possible shallow focal depth and the high vulnerability of Nice and the surrounding urban areas.

Although numerous transfer zones of deformation have been observed in thrust wedges, their mechanisms and conditions of development are poorly understood. In this paper three basic questions are addressed: 1. (1) what are the parameters controlling the genesis of transfer zones? 2. (2) what are the kinematics of thrust propagation during transfer zone development? 3. (3) which kinds of geological structure link the two compartments on either side of the transfer zone? We attempted to solve these problems using a 3-D experimental approach. Sand models were used to simulate the foreland, a rigid, but mobile, backstop was used to produced a thrust Coulomb wedge which was studied during the various stages of its formation. Shortening led to the development of transfer zones in the wedge, separating two domains of homogenous behaviour and connecting thrust planes in different domains. The experimental results show that the development of transfer zones is controlled by mechanical and/or geometrical parameters such as: 1. (1) variations in the sedimentary thickness in the foreland. 2. (2) the steepness of the backstop. 3. (3) domains with different basal friction in the foreland. 4. (4) kinked backstop geometry. During deformation of the models, two types of thrust sequence were identified: alternative thrust propagations on both sides of the transfer zone and synchronous thrust propagations which are anastomosed within the transfer zone. 3-D reconstructions revealed that the thrust deformations observed in the transfer zone are oblique ramps. The frontal thrust bending of the Barbados accretionary complex, as well as the imbrication of thrust in the “Castellane” arc and the oblique ramp in the Zanskar Range, are discussed in relation to the mechanisms and structures revealed by the models. The results of analogue modelling suggest that the development of transfer zones is directly linked to variations in the boundary conditions (mechanical and/or geometrical) between two adjacent parts of a thrust wedge.