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In the northern Andes, partitioning of oblique subduction of the Nazca plate beneath the South American continent induces a northeastward motion of the North Andean Sliver. The strain resulting from this motion is absorbed by crustal faults, which have produced magnitude 7+ earthquakes historically in the Andean Cordillera of Ecuador and southern Colombia. In order to quantify the strain in that area, we derive a high-resolution surface velocity map using InSAR time-series processing. We analyzed 6–8 yr of Sentinel-1 data and combined different satellite line-of-sight directions to produce a reliable velocity map in the east direction. We use interpolated GNSS data to express the velocity map with respect to Stable South America and remove the long-wavelength pattern due to the postseismic deformation following the 2016 Mw 7.8 Pedernales earthquake. The InSAR velocity map finds high east–west shortening strain rates along north–south trending structures within the Western Cordillera and the Interandean valley, with little deformation taking place east of them. This result strengthens the previous proposition of a ∼350 km long Quito-Latacunga tectonic block, forming a restraining bend in the overall right-lateral strike-slip fault system accommodating the northeastward escape motion of the North Andean Sliver. However, the high spatial resolution provided by InSAR indicates that previously proposed boundaries for this block need to be revised. In particular, InSAR results highlight high strain rates (>300 nstrain yr−1) along undescribed active structures, south and west of the proposed limits for the Quito-Latacunga block, respectively, in Peltetec and Ibarra regions. Interestingly, the two areas with the largest strain rates spatially correlate with the proposed areas of large historical earthquakes. Modeling of the InSAR and GNSS velocities in these areas suggests shallow coupling and high slip rates on structures which, previously, were not identified as active. We also demonstrate a slow-down of the shallow aseismic slip on the Quito fault after the Pedernales earthquake, suggesting that stress changes following large megathrust events might trigger transient slip behaviors on crustal faults. The high-resolution strain map provided by this work provides a new basis for future tectonic models in the Ecuadorian and southern Colombian Andes, and will contribute to the seismic hazard assessment in this highly populated area of the Andes. [ABSTRACT FROM AUTHOR]

The 2019-11-1, Mw4.9 Le Teil earthquake occurred within the NE termination of the Cévennes faults system (CFS) in southern France, along the La Rouvière fault (LRF), an Oligocene normal fault which was not known to be potentially active. This shallow moderate magnitude reversefaulting event produced a 5 km-long surface rupture and strong ground shaking. No evidence of previous quaternary activity was observed in the morphology, raising the question whether the fault had been reactivated for the first time since the Oligocene or had broken the surface in the past without being detected in the morphology. To address this issue, we carried out paleoseismological investigations to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. We discovered that at least one event prior 2019, occurred between 13.5 and 3.3 ka within the central part of the fault segment that broke in 2019, and that a possible earlier surfacerupturing event occurred within the northern part of this segment during the 16th century. Further investigations coupling sub-surface geophysical investigations and trenching are now carried out within the southern and northern segments of the LRF as well as along the other fault segments of the CFS.

Fragile geological features must undergo frequent structural health assessments to prevent catastrophic failure events. The mechanical behavior of natural sites is largely guided by vibrations of the earth and environmental exposure, but damage is rarely assessed, except empirically. The Chauvet‐Pont d'Arc cave, a UNESCO World Heritage Site, represents a shining example of fragility that would benefit from monitoring. It is overhung by a rock column known as Abraham's pillar that extends out from the cliff like a natural tuning fork. For this study, we monitored dancing movements of this pillar for over 2 years to analyze its elastic response to weather conditions. Using ambient‐seismic‐noise‐based methods, we identified the pillar's first two natural resonance modes. Through extensive monitoring of the site, we observed the striking temporal evolution of these two resonance frequencies on hourly, daily, seasonal, and pluriannual scales in response to changes in air temperature and insolation. Based on thermo‐acousto‐elastic modeling with a simplified 3D geometric structure, we determined how thermally‐induced stress stiffening affects the rock material, by applying convective and radiative heat fluxes to the model. From the results obtained, we suggest a novel quantitative method based on daily observations that can estimate the level of damage within the rock material. Our work provides a foundation for distinguishing between reversible processes and damage for hazard studies in the frame of climate change. Such knowledge is crucial not only for the preservation of heritage sites but also for enhancing risk assessment protocols and informing conservation efforts worldwide. Plain Language Summary: Rock columns are prominent geological features detaching from rock cliffs. The Chauvet‐Pont d'Arc cave, a UNESCO World Heritage Site, is overhung by such a rock column called Abraham's pillar. By seismic sensors, we monitored vibrations of this pillar that continuously oscillates like a natural tuning fork. These vibrations are particularly amplified at two certain frequencies, called natural resonance frequencies, that are related to fixation conditions and health state of the pillar. Their values fluctuate along days and years, and this wandering is strongly linked with air temperature and solar heating over time. We modeled how temperature affects the mechanical properties of the rock material. Based on these results, we suggest a novel method that can estimate the daily level of damage within the rock material, paving the way to enhance rockfall hazard assessment and prevent catastrophic failures. Key Points: The Chauvet‐Pont d'Arc UNESCO site is overhung by a rocky pillar that has been monitored during more than 2 yearsResonance frequencies of pillar's bending modes evolve with thermal forcings at daily and seasonal time scalesThermo‐acoustoelasticity is used to model thermally‐induced stresses and acoustoelastic parameters are proposed as a proxy of rock damage [ABSTRACT FROM AUTHOR]

The 2019, M w 4.9 Le Teil earthquake occurred in southeastern France, causing substantial damage in this slow deforming region. Field observations, remote sensing and seismological studies following the event revealed that coseismic slip concentrates at shallow depth along a ∼5 km long rupture associated with surface breaks and a thrusting mechanism. We further investigate this earthquake by combining geological field mapping, 3-D geology, InSAR time-series analysis and a coseismic slip inversion. From structural, stratigraphic and geological data collected around the epicentre, we first produce a 3-D geological model of the region surrounding the rupture using the GeoModeller software. Our model includes the geometry of the geological layers and the main faults, including the La Rouvière Fault, (LRF) the Oligocene normal fault that ruptured during the earthquake. We generate a time-series of surface displacement from Sentinel-1 SAR data ranging from early 2019 January to late 2020 January using the NSBAS processing chain. The spatio-temporal patterns of surface displacement for this time span show neither a clear pre-seismic signal nor significant post-seismic transient deformation. We extract the coseismic displacement pattern from the InSAR time-series, highlighting along-strike variations of coseismic surface slip. The maximum relative displacement along the line of sight is up to ∼16 cm and is located in the southwestern part of the rupture. We invert for the slip distribution on the fault from the InSAR coseismic surface displacement field. Constraining our fault geometry from the geological model, acceptable fault dip ranges between 55° and 60°. Our model confirms the reactivation of LRF, with reverse slip at very shallow depth and two main slip patches reaching, respectively, 30 and 24 cm of slip, both around 500 m depth. We finally discuss how the 3-D fault geometry and geological structure may have impacted the slip distribution and propagation during the earthquake. This study is a step to reassess the seismic hazard of the many faults similar to the La Rouvière one along the Cévennes fault system, in a densely populated area hosting several sensitive nuclear sites. [ABSTRACT FROM AUTHOR]

Deciphering the impact of short-term or long-term forcing on fluvial incision, as well as understanding the influence of local (channel lithology and morphology) and global (tectonic motions) parameters in the spatial variation of incision efficiency, are ongoing geomorphological research fields. To shed new light on these issues, we chose to study the “Hautes Gorges du Verdon” (High Verdon Gorges or HVG), located in the foreland of the Southwestern Alps. We collected 24 samples along three polished surfaces for Cosmic Ray Exposure (CRE) $^{36}$Cl dating, which allowed us to constrain short-term incision rates ranging from 0.06 to 0.2 mm/yr between 60 and 15 ka. Compared to known regional uplift and denudation rates, incision rates obtained in the HVG suggest tectonic or isostatic uplift as the main driver of Verdon River incision in the Late Quaternary. This comparison allows us to propose that the downcutting of the Verdon Gorges started approximatively 1.5 to 2 Ma ago, even if the drainage network of the Verdon catchment area could have been shaped earlier, during the Messinian salinity crisis.

Quito, the capital city of Ecuador hosting ∼2 million inhabitants, lies on the hanging wall of a ∼60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3–5 mm yr–1 of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (<1 km) locking depth for the central segment, increasing to a few kilometres south and north of it. Including InSAR results in the inversion requires locking to vary both along dip and along strike. 3-D spatially variable locking models show that shallow creep occurs along the central 20-km-long segment. North and south of the central segment, the interseismic coupling is less resolved, and data still allows significant slip deficit to accumulate. Using the interseismic moment deficit buildup resulting from our inversions and the seismicity rate, we estimate recurrence time for magnitude 6.5 + earthquake to be between 200 and 1200 yr. Finally, PS-InSAR time-series identify a 2 cm transient deformation that occurred on a secondary thrust, east of the main Quito fault between 1995 and 1997. [ABSTRACT FROM AUTHOR]

Mainland France is part of a plate interior with a strong structural heritage, undergoing a low rate of deformation, where destructive earthquakes can nevertheless occur. In this paper, we emphasize that the knowledge of active faults is still largely fragmentary, and that significant efforts are needed to generate robust data, in particular on the numerous faults, that still lack any study. This is the aim of the “Failles ACTives France” (FACT) axis launched in the framework of the Transverse Seismicity Action (ATS) of the Resif-Epos consortium. We present some recent investigations carried out along suspected active faults in mountainous areas, their forelands and remote lowlands, which implement new approaches and new tools, and allow characterizing their Quaternary activity.

This study analyzes surface displacements generated by a low-magnitude crustal earthquake in the Ecuadorian Andes by combining analysis of synthetic aperture radar (SAR) interferometry, geological field investigations, and seismological data. In March 2010, a significant surface faulting event occurred in the Pisayambo area (Eastern Cordillera), along the major dextral fault zone bounding the North Andean Sliver and the South America Plate. Interferograms were inverted to determine fault plane geometry and slip displacement distribution. The event affected a 9 km long previously unknown fault, referred as the Laguna Pisayambo Fault, with purely dextral movement reaching 45 cm and concentrated in the top 3 km of the crust. Geological investigations confirm both the fault mechanism and the amplitude of displacements. While these large displacements would be related to an event with a magnitude of 5.44 if using a standard crustal rigidity, we show that they can be convincingly associated with an M w5.0 earthquake, that occurred on 26 March 2010. Reconciling the apparent differences in magnitude requires the existence of a low-rigidity medium at shallow depths and/or postseismic activity of the fault. However, considering only the latter hypothesis would imply an unusually active postseismic process, in which 400-500% of the coseismic moment is released in the 6 days following the earthquake. Our observations highlight that the scaling laws relating surface observations to earthquake magnitude, classically used for seismic hazard assessment, should be carefully used. This study also illustrates how systematic InSAR analysis, even in places where no clues of ground deformation are present, can reveal tectonic processes. [ABSTRACT FROM AUTHOR]

On November 11, 2019, a M$_{\mathrm{w}}$ 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.