Located in the heart of the Bolivian orocline, the Cochabamba department and its two million inhabitants are exposed to frequent seismic activity. However, the tectonic structures causing these earthquakes remain poorly identified. Indeed, Bolivia’s national seismological network does not optimally cover the area and the hypocentral locations of local earthquakes are therefore subject to large uncertainties which hinder their association with specific faults. We established a regional network consisting of 11 broadband and short-period seismic stations, spaced approximately 20 km apart. This study highlights the initial 6-month seismic bulletin made by manual and automated deep-neural-network based seismic phase picking. We also test the network's ability to resolve focal mechanisms of moderate to small events with a combined inversion of waveforms and polarities. Our preliminary results document midcrustal microseismicity located in the Main Thrust fault shear zone, and in its hangingwall, in a region affected by tectonic slivers and transverse faults impacting the sedimentary cover. These outcomes provide fresh insights into the fault system’s seismogenic behavior and potential across the Bolivian orocline.

Understanding the driving forces and nature of intraplate seismicity remains a major unsolved problem in seismology. In the western Pyrenees, seismicity is concentrated in a narrow region that follows the boundary between the Axial Zone and the North Pyrenean Zone. Despite the presence of a permanent network in the region, the geometry of active faults, and their relationship with crustal structures, remain elusive, owing to significant earthquake location uncertainties. Here, we exploit data recorded by a large-N nodal array deployed in the Chainons Bearnais region during four weeks of 2022 in order to image crustal structures and characterize active faults. We automatically detected and picked P and S waves with PhaseNet, resulting in a catalog of over 500 events, half of which are located beneath the temporary deployment. Tomographic images obtained from the inversion of P and S arrival times provide detailed insight into the geometry of folds and thrusts in the sedimentary cover, as well as the presence of a main fault in the basement which dips northward with an angle of 65 degrees (Chainons Bearnais normal fault). Seismicity relocation within the 3D model obtained by tomography shows that earthquakes are concentrated along this main active fault, extending from the top of the basement to a depth of approximately 16 km. These results demonstrate that passive imaging approaches can offer cost-effective alternatives to traditional controlled source imaging for seismotectonic studies and natural resource exploration in regions with active seismicity.

Characterizing groundwater responses to natural drivers is cost effective and offers great potential in hydrogeological investigations. However, there is a lack of method development and evaluation, for example by comparing results with those derived from using conventional methods. This paper presents a modified method to calculate the hydraulic conductivity ( K ) of confined aquifers using the well water response to atmospheric tides. The approach separates the Earth and atmospheric tide influences on filtered well water-level records in the time domain. The resulting ill-posed regression deconvolution problem can be overcome by constraining the well water response to atmospheric tides in order to follow a physically realistic semi-diurnal barometric response function (S 2 -BRF), or to follow directly a modified hydraulic model (BE-Hvorslev) similar to a slug test evaluation. An analysis with synthetic data shows that K up to 10 -4 m/s can be estimated when pressure records with short sampling intervals are available. Application to a field dataset from Cambodia and Benin, with 20-minute to 60-minute sampling intervals, respectively, results in K values of 5.82∙10 -7 m/s and 2.9·10 -7 m/s. This agrees with results independently derived from pumping tests for both confined sediments and semi-confined hard-rock conditions. This method offers a promising and low-cost approach to derive K solely from monitoring datasets in confined aquifers. This is especially advantageous for low-conductivity formations where hydraulic testing takes time.

In the South Andes western edge, a very active seismic contact, with earthquakes up to magnitude $9.5$ and ca. $4000\thinspace\textnormal{km}$ extension threatens cities and very large populations. The existence of modern seismological networks along the contact allowed the observation of unprecedented earthquake cycle characteristics, which can improve our ability to estimate earthquake hazard, a main objective of seismology. Using dimensional and similarity analysis techniques, we show precise mechanical conditions under which the earthquake generation process unfolds, and derive a set of scaling equations linking renormalized variables. Later on, we test our theoretical results using a curated earthquake point-catalog by using gridding, box-counting, statistical bootstrap and fixed-point iteration collapse techniques. We found non-trivial scaling laws valid across multiple orders of magnitude capable of describing a complex interplay between renormalized earthquake occurrence and renormalized moment release rate. We discuss finite-strain and seismic-moment release-rate conditions; declustering, foreshock, mainshock, aftershock notions; cutoff magnitudes, earthquake hazard implications and a possible large-scale tectonic energy transfer mechanism.

SUMMARY An unusual seismic activity has recently occurred in the Gripp valley, located in the central part of the French Pyrenees. Since spring 2020, two new swarms appeared, clearly outside the usual location of the seismicity in this area. On 20 September 2020, almost concomitantly with the activation of the second seismic swarm, a hole suddenly opened in the bed of a local river, the Adour de Payolle. This hole drained the water from the river, which dried up over 500 m. We follow and study the spatial and temporal evolution of these clusters, using four temporary stations deployed a few days or months after the beginning of the crisis to complete the regional network. These additional data lead to the construction of a comprehensive catalogue of more than 4900 earthquakes, using both a template matching approach and a deep-learning based phase picking method to complete and improve the initial catalogue available from the French seismological agency. This allows highlighting a slow and clear migration of the seismicity during 1 yr. Precise absolute and relative event locations reveal a dipping faulting structure, confirmed by the focal mechanism estimated for the highest magnitude event of the sequence (ML 3). We propose to explain the observed migration of the seismicity by deep fluids going up through a newly discovered faulting structure.

Estimating subsurface hydraulic properties using the groundwater (GW) response to Earth tides (ET) and atmospheric pressure is an alternative approach to pumping or slug tests (passive vs. active methods). Yet testing the applicability of models under different subsurface conditions and comparing results with traditional hydraulic methods are lacking. We first review the assumptions of analytical models used to evaluate the GW response to ET and their applicability in unconfined to confined conditions. Second, we develop a robust approach to select the right model based on amplitude and phase pattern of the diurnal and semi‐diurnal tides. Third, based on earlier works we develop an approach to derive the hydraulic conductivity of the screened interval using the GW response to atmospheric tides, here named “atmospheric slug test” (AST). We estimate transmissivity and storativity at three shallow aquifers in Cambodia and compare the results with subsurface properties derived from pumping tests (PT). Transmissivity values from AST and PT are in good agreement. However, we show that storativity values derived from ET show large discrepancies if borehole skin effects are ignored. Further, the GW response to ET exhibits a strong decay in amplitude with frequency while maintaining close to zero and positive phase shifts. When supplemented with the calculated transmissivity values, none of the analytical models was able to reproduce this frequency dependent behavior. Our work emphasizes the need for evaluating passive methods robustness under different subsurface conditions. Further work is required to understand the frequency dependent GW response to natural or artificial forces.

The M W 8.8 Maule earthquake is the largest well-recorded megathrust earthquake reported in South America. It is known to have had very few foreshocks due to its locking degree, and a strong aftershock activity. We analyze seismic activity in the area of the 27 February 2010, M W 8.8 Maule earthquake at different time scales from 2000 to 2019. We differentiate the seismicity located inside the coseismic rupture zone of the main shock from that located in the areas surrounding the rupture zone. Using an original spatial and temporal method of seismic comparison, we find that after a period of seismic activity, the rupture zone at the plate interface experienced a long-term seismic quiescence before the main shock. Furthermore, a few days before the main shock, a set of seismic bursts of foreshocks located within the highest coseismic displacement area is observed. We show that after the main shock, the seismic rate decelerates during a period of 3 years, until reaching its initial interseismic value. We conclude that this megathrust earthquake is the consequence of various preparation stages increasing the locking degree at the plate interface and following an irregular pattern of seismic activity at large and short time scales.

The FMNEAR method (for determination of Focal Mechanism using NEAR-source records) has already proved its efficiency for continental earthquakes recorded by a large number of well-distributed stations. Installed in January 2015, it has been running in real time in Chile, in the specific tectonic conditions of the active convergent margin of the Nazca plate, where most of the earthquakes occur offshore. About 3 yr of data were collected, containing thousands of earthquakes, for which 80% have low-to-moderate moment magnitudes (M-w < 4.6). Here, we show results of the FMNEAR inversion, processed in real time at the National Seismological Center of Chile. We compare our solutions with the solutions provided by the U.S. Geological Survey and the Global Centroid Moment Tensor. Our results tested on all the inverted earthquakes common to the three methods show that the main characteristics of the source are well retrieved by the FMNEAR inversion. With the present-day seismic network installed in Chile, FMNEAR was able to provide more than 6200 automatic event solutions, from which about 1000 were considered as reliable. All the inversions were conducted a few minutes after the earthquake origin time, with a calculation time varying from a few seconds for the smallest events to half an hour for the biggest and most complicated earthquakes. The FMNEAR approach showed its efficiency in inverting even small earthquakes in Chile (M-w < 4.5), if the azimuthal station coverage and the number of near stations are sufficient. By implementing FMNEAR in other tectonic settings and continuing to probe its results in the active seismic context of Chile, we expect to improve its capacity to provide viable and meaningful information in real time on the local seismic activity.

Seismicity induced by fluid injection in a natural fault is investigated in situ in the near field of the source. We present synchronous seismic and hydromechanical measurements directly recorded in the decametric injection zone. The three main types of seismic events were recorded during injection and shut‐in: high‐amplitude and short duration seismic events (SE) (i.e., microearthquakes), low to constant amplitude and 5 to 17 s long tremor‐like signals (TLS), and long period events (LP) with a narrow‐frequency band content. Seismicity first initiates with a sequence of SE and TLS, when pressure is high (~3.5 MPa), slip is activated on the fault, which experiences a twentyfold increase of permeability. Then LP events appear to be associated to fluid leakage in the fault caused by dilation during slip. During shut‐in, residual pressures as low as 0.6 MPa still trigger SE events. We show that the initial TLS sequence triggers when a progressive transition occurs from rupture controlled by effective stress variations close to the injection source to a large friction weakening‐dominated slip on the fault. We conclude that the combination of these different seismic signal types may be a proxy to monitor fault instability associated to fluid pressure perturbations.

We present in situ measurements of fluid pressure, deformation and seismicity in natural fractures together with coupled hydromechanical simulations. We conducted a step-rate water injection (~3.5MPa and 1200 s) to induce the local pressurization of a critically stressed fractured carbonate reservoir layer located at 250m-depth in the Low Noise Underground Laboratory (LSBB), southern France. An observed factor-of-3 increase in the fracture permeability was associated with the injection-induced fluid pressure increase and about 100 triggered seismic events. Both normal opening (a few microns) of the fluid-injected fracture and the associated tilt (<1 micro-radian) of the fracture near field displayed inelastic behavior highlighting an irreversible fracture shear and dilatant failure, amounting to about 1/3–1/2 of the maximum measured deformations.Using a plane-strain finite-difference coupled hydromechanical model, our calculation shows that tensile failure first occurred in the injection zone and then shear failure spread along fractures into the surrounding unsaturated rock through stress transfer from the injection zone. The most striking result of these model simulations is that the mechanical weakening of the fractures in the near field induced a 2–5×105Pa release of the normal stress across the fluid-injected fracture that provoked fracture slip and increase in permeability. A geological exploration of the fracture zone after the experiment showed that no major failure had occurred, and we therefore relate these strength and permeability variations to the slight reactivation (~microns) of pre-existing fractures.

Landslides are sensitive to fluid pressures generated by rainfall and snowmelt. Correlations between landslides accelerations and seasonal infiltrations have been widely observed, and models have shown that fluid pressurization in fractures is a dominant driving factor. Nevertheless the precise mechanisms relating fluid pressures and rupture are poorly constrained. Here, we quantified in-situ the effect of fluids and strength loss related to pressure increase and deformation in fractures. Then, using hydromechanical modeling, we analyzed these data and explained the evolution of permeability with deformation. Finally, based on these novel observations, we used improved hydromechanical simulations to model the behaviour of the Schilienne landslide in France.