Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

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 Rouviere 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

Ionospheric compensation in L-band InSAR time-series: Performance evaluation for slow deformation contexts in equatorial regions

International audienceMulti-temporal Synthetic Aperture Radar Interferometry (MT-InSAR) is the only geodetic technique allowing to measure ground deformation down to mm/yr over continuous areas. Vegetation cover in equatorial regions favors the use of L-band SAR data to improve interferometric coherence. However, the electron content of ionosphere, affecting the propagation of the SAR signal, shows particularly strong spatio-temporal variations near the equator, while the dispersive nature of the ionosphere makes its effect stronger on low-frequencies, such as L-band signals. To tackle this problem, range split-spectrum method can be implemented to compensate the ionospheric phase contribution. Here, we apply this technique for time-series of ALOS-PALSAR data, and propose optimizations for low-coherence areas. To evaluate the efficiency of this method to retrieve subtle deformation rates in equatorial regions, we compute time-series using four ALOS-PALSAR datasets in contexts of low to medium coherence, showing slow deformation rates (mm/yr to cm/yr). The processed tracks are located in Ecuador, Trinidad and Sumatra, and feature 15 to 19 acquisitions including very high, dominating ionospheric noise, corresponding to equivalent displacements of up to 2 m. The correction method performs well and allows to reduce drastically the noise level due to ionosphere, with significant improvement compared with a simple plane fitting method. This is due to frequent highly non-linear patterns of perturbation, characterizing equatorial TEC distribution. We use semivariograms to quantify the uncertainty of the corrected time-series, highlighting its dependence on spatial distance. Thus, using ALOS-PALSAR-like archive, one can expect a detection threshold on the Line-of-Sight velocity ranging between 3 and 6 mm/yr, depending on the spatial wavelength of the signal to be observed. These values are consistent with the accuracy derived from the comparison of velocities between two tracks in their overlapping area. In the case studies that we processed, the time-series corrected from ionosphere allows to retrieve accurately fault creep and volcanic signal but it is still too noisy for retrieving tiny long-wavelength signals such as slow (mm/yr) interseismic strain accumulation

Reassessing the seismic hazard in the Cusco area, Peru: New contribution coming from an archaeoseismological survey on Inca remains

International audienceDevastated by two earthquakes in historical times (1650 and 1950 CE), the Cusco Basin is now characterized by dense and chaotic urbanization that makes it even more vulnerable. Unfortunately, the large recurrence intervals of the local crustal earthquakes, the shortness of the historical record (∼500 yr) and the persistent lack of palaeoseismological studies hamper considerably the seismic hazard assessment. In such context, the outstanding archaeological heritage of the Cusco area turns out to be a relevant marker of past seismic activity.We carried out a systematic archaeoseismological survey in nine Inca sites close to Cusco and registered almost 3,000 Earthquake Archaeological Effects. Thanks to a semi-quantitative approach, we show a clear anisotropic seismic deformation on the Inca fine stonework, consistent at the regional scale. In Cusco, the architecture exhibits the impact of two different and strong ancient seismic events (M.M. intensity > VII).By combining these results with the analysis of historical photographs, our work supports, the occurrence of an unreported event during Inca times (∼1400–1533 CE). More broadly, by providing new data on the destructive potential of past earthquakes, this study urges us to conduct further research on the faults near Cusco

Reassessing the seismic hazard in the Cusco area, Peru: New contribution coming from an archaeoseismological survey on Inca remains

International audienceDevastated by two earthquakes in historical times (1650 and 1950 CE), the Cusco Basin is now characterized by dense and chaotic urbanization that makes it even more vulnerable. Unfortunately, the large recurrence intervals of the local crustal earthquakes, the shortness of the historical record (∼500 yr) and the persistent lack of palaeoseismological studies hamper considerably the seismic hazard assessment. In such context, the outstanding archaeological heritage of the Cusco area turns out to be a relevant marker of past seismic activity.We carried out a systematic archaeoseismological survey in nine Inca sites close to Cusco and registered almost 3,000 Earthquake Archaeological Effects. Thanks to a semi-quantitative approach, we show a clear anisotropic seismic deformation on the Inca fine stonework, consistent at the regional scale. In Cusco, the architecture exhibits the impact of two different and strong ancient seismic events (M.M. intensity > VII).By combining these results with the analysis of historical photographs, our work supports, the occurrence of an unreported event during Inca times (∼1400–1533 CE). More broadly, by providing new data on the destructive potential of past earthquakes, this study urges us to conduct further research on the faults near Cusco

Evidence of a large “prehistorical” earthquake during Inca times? New insights from an indigenous chronicle (Cusco, Peru)

International audienceA colonial chronicle written by the indigenous Peruvian author Pachacuti Yamqui Salcamaygua ([1613?]) relates a legend of the sudden appearance of a huge animal – kilometres in length and approximately 4 m in width – and described as the Andean snake-like deity amaru. Pachacuti Yamqui alleged that this fantastic event occurred on the day that the sovereign Pachacuti Inca Yupanqui’s eldest son was born around 1440 CE, and was named “Amaru”. We suggest that the underlying event was an earthquake, and that the propagation of the surface rupture across the landscape resembled a sudden appearance of a snake-like being wriggling over the mountains and leaving an undulating surface trace. The concordance between the snake’s route and the layout of a major fault complex above Cusco, as well as several ethnographic testimonies, support this hypothesis. Although little is known about pre-1532 CE seismicity, the current tectonic settings of the Cusco area point to seismic awareness of the Incas (ca. 1300–1532 CE). Independent results from architectural and paleoseismological fields in the Cusco area corroborate a significant impact of large earthquakes on local societies. In Peru, without pre-Hispanic written sources, the oral folklore and traditions preserved in Spanish chronicles offer a relevant, but still underexploited resource for identifying paleo-extreme events. Combining multidisciplinary geomorphic observations, archaeological evidence and historical sources, we revisit this legendary episode and its possible implications

Can we observe North Andean Sliver motion using long InSAR time-series analysis?

International audienceIn Northern Andes, oblique subduction of the Nazca plate below the South America Plate induces a northward motion of the North Andean Sliver, at a rate of ~10 mm/yr with respect to Stable South America. In Ecuador in particular, the associated strain is mainly accomodated along the large Chingual-Cosanga-Puna-Pallatanga (CCPP) fault system, which hosted several 7+ magnitude earthquakes in the historical period. Recent studies using block-modeling of GNSS data raise important questions about the partitioning and the localization of the deformation both inside and at the limits of the North-Andean sliver. Therefore, time-series analysis of InSAR data, allowing a large spatial resolution, would complement the existing geodetic dataset of observation of low-rate crustal motions in this region. Taking advantage of 7 to 8 years of Sentinel-1 archive, we compute long time-series of InSAR data for the whole Interandean region of Ecuador (~100 by 400 km), using the NSBAS processing chain. Because processing of InSAR data in this ecuatorial region raises several challenges, such as low-coherence due to vegetation, ionospheric and troposheric noise, and fading signals,we develop strategies to mitigate the noise terms. By using an optimized interferogram network, improvedweighting during multilooking, and a temporal decomposition of the time-series, we produce the first InSAR velocity maps of the Ecuadorian Cordilleras. We then compare these results to the existing block-model derived from GNSS horizontal data in order to evaluate the possibility of characterizing the motion of North Andean Sliver with an increased spatial resolution

Réactivation lors du séisme du Teil (11/11/2019, Mw4.9) : modèle géologique 3D et série temporelle InSAR

International audienc

Réactivation lors du séisme du Teil (11/11/2019, Mw4.9) : modèle géologique 3D et série temporelle InSAR

International audienc

Réactivation lors du séisme du Teil (11/11/2019, Mw4.9) : modèle géologique 3D et série temporelle InSAR

International audienc