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International audience; Fault slip induced by fluid perturbation in shale formations may only lead to as sparse seismicity. However, fault slip may strongly impact the integrity of shale formations that serve as caprocks for geological reservoirs holding buoyant fluids such as CO2, natural gas, or hydrogen. A better understanding of the fluid reactivation processes of fault and the seismic triggering process is therefore critical for reservoir monitoring and fault stability. Here we analyze the seismic responses of a shale fault exposed to fluid pressurization during an in situ field-scale injection experiment at ∼340 m depth in the Mont Terri underground research laboratory (Switzerland). Two main types of seismic signals are observed as the fault was activated and started to slowly slip. After an aseismic phase, we observed tremor signatures and an increase in noise amplitude, which were directly associated with the slowly propagating fault slip in response to fluid injection. These signatures were later followed by micro-earthquakes that seem to occur further away from the fluid-pressurized area. We interpret these micro-earthquakes to be triggered by stress perturbations from the main slip growth. These two classes of seismic responses therefore highlight two different processes. Tremors seem to be a more direct observation for the fluid-induced slip propagation than micro-earthquakes. Even hidden in the noise, they precede earthquake failures, thus providing a useful tool for monitoring fluid leakage activated by slow deformation on low permeable shale faults, with applications for sealing integrity of caprocks.

Tectonic pseudotachylytes are thought to be unique to certain water‐deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid‐rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km‐long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m‐thick (ultra)cataclastic strands hosted in a damage‐zone made of chlorite‐epidote‐rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite‐, epidote‐ and calcite‐bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s−1) resulted in the production of vesiculated pseudotachylytes both at dry and water‐pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid‐deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co‐seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust., Key Points The Bolfín Fault Zone shows the first pseudotachylytes described in the Atacama Fault System giving insights of past seismic activityRotary shear experiments at seismic slip velocities resulted in formation of vesiculated pseudotachylytes in all tested conditionsNatural pseudotachylytes formed in a hydrothermal fluid‐rich environment where vesiculation is related to CO2 degassing from calcite veins

International audience; The low frequency earthquakes (LFEs) that constitute tectonic tremor are often inferred to be slow: to have durations of 0.2-0.5 s, a factor of 10-100 longer than those of typical MW 1-2 earthquakes. Here we examine LFEs near Parkfield, CA in order to assess several proposed explanations for LFEs' long durations. We determine LFE rupture areas and location distributions using a new approach, similar to directivity analysis, where we examine how signals coming from various locations within LFEs' finite rupture extents create differences in the apparent source time functions recorded at various stations. We use synthetic ruptures to determine how much the LFE signals recorded at each station would be modified by spatial variations of the source-station traveltime within the rupture area given various possible rupture diameters, and then compare those synthetics with the data. Our synthetics show that the methodology can identify interstation variations created by heterogeneous slip distributions or complex rupture edges, and thus lets us estimate LFE rupture extents for unilateral or bilateral ruptures. To obtain robust estimates of the sources' similarity across stations, we stack signals from thousands of LFEs, using an empirical Green's function approach to isolate the LFEs' apparent source time functions from the path effects. Our analysis of LFEs in Parkfield implies that LFEs' apparent source time functions are similar across stations at frequencies up to 8-16 Hz, depending on the family. The interstation coherence observed at these relatively high frequencies, or short wavelengths (down to 0.2-0.5 km), suggest that LFEs in each of the seven families examined occur on asperities. They are clustered in patches with sub-1-km diameters. The individual LFEs' rupture diameters are estimated to be smaller than 1.1 km for all families, and smaller than 0.5 km and 1 km for the two shallowest families, which were previously found to have 0.2-s durations. Coupling the diameters with the durations suggests that it is possible to model these MW 1-2 LFEs with earthquake-like rupture speeds: around 70 per cent of the shear wave speed. However, that rupture speed matches the data only at the edge of our uncertainty estimates for the family with highest coherence. The data for that family are better matched if LFEs have rupture velocities smaller than 40 per cent of the shear wave speed, or if LFEs have different rupture dynamics. They could have long rise times, contain composite sub-ruptures, or have slip distributions that persist from event to event.

International audience; Travelling seismic waves and Earth tides are known to cause oscillations in well water levels due to the volumetric strain characteristics of the ground motion. Although the response of well water levels to S and Love waves has been reported, it has not yet been quantified. In this paper we describe and explain the behaviour of a closed artesian water well (Gomè 1) in response to teleseismic earthquakes. This well is located within a major fault zone and screened at a highly damaged (cracked) sandstone layer. We adopt the original Skempton approach where both volumetric and deviatoric stresses (and strains) affect pore pressure. Skempton's coefficients < tex - mathid = "IM0001" > B and < tex - mathid = "IM0002" > A couple the volumetric and deviatoric stresses respectively with pore pressure and < tex - mathid = "IM0003" > BKu and < tex - mathid = "IM0004" > N are the equivalent coupling terms to volumetric and deviatoric strains. The water level in this well responds dramatically to volumetric strain (P and Rayleigh waves) as well as to deviatoric strain (S and Love waves). This response is explained by the nonlinear elastic behaviour of the highly damaged rocks. The water level response to deviatoric strain depends on the damage in the rock; deviatoric strain loading on damaged rock results in high water level amplitudes, and no response in undamaged rock. We find high values of < tex - mathid = "IM0005" > N= 8.5 GPa that corresponds to -0.5 < A < -0.25 expected at highly damaged rocks. We propose that the Gomè 1 well is located within fractured rocks, and therefore, dilatency is high, and the response of water pressure to deviatoric deformation is high. This analysis is supported by the agreement between the estimated compressibility of the aquifer, independently calculated from Earth tides, seismic response of the water pressure and other published data.

International audience; In this article, we develop a system of equations describing fluid migration, fault rheology, fault thickness evolution and shear rupture during a seismic cycle, triggered either by tectonic loading or by fluid injection. Assuming that the phenomena predominantly take place on a single fault described as a finite permeable zone of variable width, we are able to project the equations within the volumetric fault core onto the 2D fault interface. From the basis of this "fault lubrication approximation", we simulate the evolution of seismicity when fluid is injected at one point along the fault to model induced seismicity during an injection test in a borehole that intercepts the fault. We perform several parametric studies to understand the basic behaviour of the system. Fluid transmissivity and fault rheology are key elements. The simulated seismicity generally tends to rapidly evolve after triggering, independently of the injection history and end when the stationary path of fluid flow is established at the outer boundary of the model. This self-induced seismicity takes place in the case where shear rupturing on a planar fault becomes dominant over the fluid migration process. On the contrary, if healing processes take place, so that the fluid mass is trapped along the fault, rupturing occurs continuously during the injection period. Seismicity and fluid migration are strongly influenced by the injection rate and the heterogeneity.

International audience; A number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field. The existence of active low angle normal faults is much debated because (1) the classical theory of fault mechanics implies that faults are locked when the dip is less than 30° and (2) shallow dipping fault planes do not produce large earthquakes (M > 5.5). To reconcile observations and theory, we propose a new model for fault reactivation by introducing an elasto-plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties. Contrary to the classical model which implies that the dilation angle ψ equals the friction angle φ, our model accounts for ψ < φ and permits ψ < 0 in the fault gouge as deduced from laboratory and field observations. Whilst the predicted locking angles differ in most cases by less than 10° from the classical model, a significant amount of plastic strain (strain occurring in elasto-plastic regime) is predicted to occur on badly oriented faults prior to locking in a strain-hardening regime. We describe four modes of reactivation which include complete/partial reactivation with/without tensile failure in the surrounding medium. This paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking. We conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime.

International audience; Autonomous hydrophones arrays are an excellent tool for monitoring mid-ocean ridge seismic activity. The major advantage of using arrays of autonomous hydrophones for recording deep-ocean ridge earthquakes is its low magnitude detection thresholds achievable using hydroa-coustic techniques. Regional analysis of the detection thresholds of the different autonomous hydrophones arrays deployed along the Mid-Atlantic Ridge reveals the strong influence of the detection threshold in the number of recorded events and it must be taken into account in any further analysis. In this study, the analysis of both autonomous hydrophones and teleseismi-cally detected Mid-Atlantic Ridge seismicity reveals that the background seismicity from the relatively short recording periods of the autonomous hydrophones mimic the results of the much longer teleseismic recording. It also reveals that seismicity generally cluster at both the segment scale and on Mantle Bouguer Anomaly maxima. The big majority of these clusters seem to be related to dyke intrusions and propagation along the Mid-Atlantic Ridge. These dyke intrusions interact with the mainshock-aftershock sequences. The seismic sequences mainshock-aftershock analysis reveals that the strength of the faults is highly influenced by the mode, or style, of faulting. Detachment faults, which are ubiquitous along the Mid-Atlantic Ridge, can produce more prolific shorter duration seismic sequences revealing faster and reduced strain releases in comparison to higher angle normal faults. This reduced strain release is most likely to occur due to the presence of higher levels of serpentinization on detachment faults. Higher levels of serpentenisation can also promote an aseismic transient slip on the mainshock-aftershock sequences.

Autonomous Hydrophones (AuH) arrays are excellent to seismically monitor mid oceanic ridges. One of their advantages is the ability of estimating, both for specific array geometries and specific oceanic regions around the globe, the error of the seismic locations. This has been implemented for all the AuH arrays deployed on the Mid-Atlantic Ridge (MAR). Another advantage of the AuH is its low detection threshold. An analysis of the detection thresholds of the AuH that were deployed in the MAR revealed that this feature can greatly impact the number of recorded events, and that this must be considered for further analyses. Transmission Loss analysis shows that the AuH Source Level of an earthquake is mainly due to the conversion from seismic to acoustic energy while the propagation paths play a smaller contribution. An analysis of MAR seismicity reveals that the AuH recorded seismicity mimics the longer time span of teleseismically recorded seismicity and that both are influenced by crustal thermal structure variations along the ridge. AuH recorded seismic clusters are directly linked with teleseismically recorded events and the sections where they occurred are the most active sections of the MAR. Seismicity generally clusters at segment extremities and at the segment scale, on MBA maxima. Size-frequency and mainshock-aftershock analyses of the clusters reveal that aftershock decay rate is influenced by the mode of faulting. Detachment faults produce seismic sequences with faster decay rate associated with a reduced strain release in comparison to normal faults. This implies the presence of higher levels of serpentinisation on detachment faults.; Le principal avantage des réseaux d'AuH, pour la surveillance sismique des dorsales océaniques, est leur faible seuil de détection. Cependant, les variations de seuil de détection entre les réseaux d'AuH déployés au voisinage de Açores, peut influencer le nombre de séismes enregistrés. Ces variations doivent donc être analysées avant interprétation de la sismicité. L'amplitude acoustique à la source (SL) d'un séisme dépend surtout de l'efficacité de la conversion séismoacoustique mais aussi, dans une moindre mesure, des effets de propagation. Un autre avantage, la possibilité d'estimer l'erreur de localisation pour différentes géométries des réseaux et de topographie du fond, est présenté pour touts les réseaux d'AuH déployés sur la Dorsale Médio Atlantique (MAR). L'analyse de la séismicité de la MAR montre que la sismicité enregistrée par les AuH ressemble à celle enregistrée par les réseaux à terre au cours des 40 années passées. La distribution spatiale de cette sismicité est liée aux variations du régime thermique de la croute le long de la dorsale. Des essaims de séismes, enregistrés par les AuH, sont liées à des télé-séismes et les sections où ils se produisent sont les plus actives. A l'échelle du segment. les essaims se groupent en extrémité et au voisinage de maxima de la MBA. L'analyse des distributions des SL et du taux de diminution du nombre des répliques indiquent que les failles de détachement produisent des essaims avec des diminutions plus rapides du nombre des répliques, que les failles normales. Cette observation serait associée à des contraintes plus faibles sur les failles de détachement et impliquerait un niveau de serpentinisation plus important.

International audience; Dynamic rupture of a 3-D spontaneous crack of arbitrary shape is investigated using a finite volume (FV) approach. The full domain is decomposed in tetrahedra whereas the surface, on which the rupture takes place, is discretized with triangles that are faces of tetrahedra. First of all, the elastodynamic equations are described into a pseudo-conservative form for an easy application of the FV discretization. Explicit boundary conditions are given using criteria based on the conservation of discrete energy through the crack surface. Using a stress-threshold criterion, these conditions specify fluxes through those triangles that have suffered rupture. On these broken surfaces, stress follows a linear slip-weakening law, although other friction laws can be implemented. For The Problem Version 3 of the dynamic-rupture code verification exercise conducted by the SCEC/USGS, numerical solutions on a planar fault exhibit a very high convergence rate and are in good agreement with the reference one provided by a finite difference (FD) technique. For a non-planar fault of parabolic shape, numerical solutions agree satisfactorily well with those obtained with a semi-analytical boundary integral method in terms of shear stress amplitudes, stopping phases arrival times and stress overshoots. Differences between solutions are attributed to the low-order interpolation of the FV approach, whose results are particularly sensitive to the mesh regularity (structured/unstructured). We expect this method, which is well adapted for multiprocessor parallel computing, to be competitive with others for solving large scale dynamic ruptures scenarios of seismic sources in the near future.

International audience; We study theoretically and numerically the process of fault interactions within a quasi-static faulting model at long, tectonic timescale. The model handles birth, growth and sliding of multiple straight but non-coplanar interacting faults, regardless of any coseismic dynamic slip events. The study is restricted to the 2-D elastic antiplane case, an idealization of the normal faulting process. The model handles a general slip-dependent friction law for faults, to take into account a possible long-term fault weakening process. At fault tips, finite stress and progressive weakening lead to fault tip cohesive zones and ensure stability. We introduce a new numerical method based on a series development of slip profiles using a Chebyshev basis, which provides an accurate computation of large stress gradients at fault tips. Here simulations are limited to two parallel faults, which is enough to investigate many important features such as slip partitioning between faults, variable fault tip velocities and the state of stress in fault relay zones, responsible for the fault linking process. We study both a quasi-static problem and the associated spectral problem and show the link between them. We compare our quasi-static simulations with experimental results concerning the acceleration/deceleration of fault tips submitted to stress interaction and concerning the geometrical parameters that favour the linking of two normal faults. We find that the linking (coalescence) process should most likely occur during the deceleration phase of the faults tips subject to stress shadowing in the fault relay. Furthermore, for large ratios of fault lengths to separation, the linking process should begin for fault overlaps comparable to the values observed by Soliva & Benedicto in small natural fault relays (typically 2.9 times the fault separation).

International audience; The Gulf of Corinth is one of the most active extensional regions in the Mediterranean area characterized by a high rate of seismicity. However, there are still open questions concerning the role and the geometry of the numerous active faults bordering the basin, as well as the mechanisms governing the seismicity. In this paper, we use a 2-D plane strain finite element analysis to constrain the upper crust rheology by modelling the available deformation data (GPS and geomorphology). We consider a SSW–NNE cross-section of the rift cutting the main active normal faults (Aigion, West Eliki and OffShore faults). The models run for 650 Kyr assuming an elasto-viscoplastic rheology and 1.3 cm yr −1 horizontal extension as boundary condition (resulting from GPS data). We model the horizontal and vertical deformation rates and the accumulation of plastic strain at depth, and we compare them with GPS data, with long term uplift rates inferred from geomorphology and with the distribution of seismicity, respectively. Our modelling results demonstrate that dislocation on high-angle normal faults in a plastic crustal layer plays a key role in explaining the extremely localized strain within the Gulf of Corinth. Conversely, the contribution of structures such as the antithetic Trizonia fault or the buried hypothetical subhorizontal discontinuity are not necessary to model observed data.