L'escarpement de faille de la Têt est-il le résultat de la tectonique active Plio-Pléistocène ou d'une exhumation Pléistocène ?

International audienceL'expression morphologique de la faille de la Têt ressemble à celle d'une faille active. Son fonctionnement, attesté pour le Pliocène, reste cependant débattu pour le Pléistocène. Un schéma d'évolution de l'escarpement de faille est proposé. Il se base sur les relations entre morphologie de l'escarpement et remplissage sédimentaire des bassins. Il souligne l'importance des phénomènes d'exhumation par incision du réseau hydrographique au cours du Pléistocène. Ainsi, les mouvements tectoniques pléistocènes apparaissent faibles et leur impact sur les caractéristiques du drainage limité. Le schéma proposé conduit également à minorer le rejet au cours du Pliocène, celui-ci étant réévalué entre 150 et 300 m. Une grande partie des mouvements tectoniques responsables du relief actuel sont donc antérieur

Surface rupture associated with a moderate intraplate earthquake: the Mw 6.2 Parina event (December 1st, 2016) in the Peruvian Altiplano

[ESP] Los análisis de desplazamiento de falla y riesgo sísmico emplean relaciones empíricas para predecir la magnitud potencial del terremoto ("relaciones de escala"; por ejemplo, Wells y Coppersmith, 1994), deslizamiento de la superficie), funciones de probabilidad de ruptura de la superficie y cantidad de deslizamiento de la superficie (por ejemplo, "probabilidad condicional de ruptura" "Y" probabilidad de superación ", respectivamente; ver Youngs et al; 2003). Esas relaciones comparten el problema común de que dependen de un número limitado de casos de magnitud moderada a grande (> = 6.5) y anteriores a 2000. Los terremotos del oeste de EE. UU. y Japón están ampliamente representados, y los casos intraplaca son pocos. Aquí, informamos evidencia de fallas en la superficie que ocurrió durante un terremoto moderado que ocurrió en el Altiplano del sur del Perú. Presentamos datos de campo y de alta resolución que mejoran el conocimiento geodinámico de la región y proporcionan pistas para actualizar las herramientas de riesgo sísmico. El terremoto de falla normal Parina 2016 Mw 6.2 ocurrió dentro de los altos Andes del sur del Perú en una región con escasa sismicidad reciente y sin tensión horizontal geodésica observable. Las observaciones de campo y los DEM de alta resolución de las rupturas de la superficie permiten investigar la relación entre el deslizamiento en la falla de Parina, la geomorfología local y la tectónica regional. Mapeamos un segmento principal de tendencia NW-SE y 6 km de largo, con deslizamiento vertical de hasta ~ 27 cm (hacia abajo hacia el SW) y apertura tensional de ~ 25 cm. El deslizamiento de la superficie no se distribuye fuera de la falla principal, con la excepción de un cordón paralelo a 200 m de la falla principal en su extremo norte. Un punto llamativo es un segmento menor de tendencia NW-SE y un segmento roto de 1.5 km de largo con valores de deslizamiento más pequeños (hasta 8 cm) distantes por 5 km al norte, a lo largo de la misma zona de falla. Las dos trazas de ruptura mapeadas coinciden directamente con la proyección ascendente del plano de falla co-sísmica inferido de las mediciones de InSAR y, por lo tanto, pueden representar dos secciones superficiales distintas de la falla sísmica primaria, separadas por un espacio de superficie. Esta brecha ocurre donde la geología superficial está constituida por sedimentos sueltos. Las rupturas coinciden con escarpes de 10-20 m de altura que atraviesan depósitos fluvio-glaciales que son arrojados hacia el SW, y forman la extensión hacia el sudeste del sistema de fallas Lagunillas-Mañazo más grande que tiende el NO-SE en el Altiplano peruano. Una estimación preliminar lleva a inferir un deslizamiento de sentido normal repetido en la falla de Parina desde la última glaciación mayor (~ 10-30 ka), lo que implica una tasa de deslizamiento vertical ~ 1 mm / a. Además de su interés regional en términos de tectónica activa y geodinámica (Wimpenny et al., 2018), la ruptura de la superficie de Parina 1) constituye un nuevo caso para enriquecer la base de datos SURE pendiente con nuevos datos precisos, especialmente para eventos intraplaca, 2) geología de superficie es un parámetro clave que influye en el deslizamiento de la superficie, 3) ilustra una vez más que los terremotos moderados pueden romper la superficie en un patrón complejo, 3) muestra que las técnicas de alta resolución permiten mejorar la caracterización de las rupturas de la superficie (longitud de la ruptura y desplazamiento máximo / medio) y 4) cuestiona potencialmente los parámetros de falla que se infirieron en el pasado cuando dichos enfoques no estaban disponibles. Esos son argumentos que apoyan la idea de la necesidad de una revisión profunda de las relaciones empíricas, basadas en catálogos de terremotos modernos.[ENG] Fault displacement and Seismic hazard analyses employ empirical relationships to predict potential earthquake magnitude ("scaling relationships"; e. g., Wells and Coppersmith, 1994), surface slip), probability functions of surface rupture and surface slip amount (e. g., “conditional probability of rupture” and “probability of exceedance”, respectively; see Youngs et al; 2003). Those relationships share the common issue that they rely on a limited number of moderate-to-large magnitude (>=6.5) and pre-2000 cases. Earthquakes from western US and Japan are largely represented, and intraplate cases are few. Here, we report surface faulting evidence that occurred during a moderate earthquake that occurred in the Altiplano of Southern Peru. We present field and high-resolution data that improve the geodynamic knowledge of the region and provide clues to upgrade seismic hazard tools. The 2016 Mw 6.2 Parina normal-faulting earthquake occurred within the high Andes of southern Peru in a region with sparse recent seismicity and no observable geodetic horizontal strain. Field observations and high-resolution DEMs of the surface ruptures allow investigating the relationship between slip on the Parina Fault, local geomorphology and the regional tectonics. We mapped one major NW-SE-trending and 6-km-long segment, with up to ˜27 cm vertical slip (downthrown to the SW) and ˜25 cm tensional opening. Surface slip is not distributed off the main fault, with the exception of a parallel strand 200-m off the major one at its northern tip. One striking point is a minor NW-SE-trending and 1.5-km-long ruptured segment with smaller slip values (up to 8 cm) distant by 5 km to the north, along the same fault zone. The two mapped rupture traces directly coincides with the up-dip projection of the co-seismic fault plane inferred from InSAR measurements, and they therefore may represent two distinct surface sections of the primary earthquake fault, separated by a surface gap. This gap occurs where surface geology is constituted of loose sediments. The ruptures coincide with 10-20 m high scarps cutting through fluvio-glacial deposits that are downthrown to the SW, and they form the southeastward extension of the larger Lagunillas-Mañazo fault system that trends NW-SE across the Peruvian Altiplano. A preliminary estimation leads to infer a repeated normal-sense slip on the Parina Fault since the last major glaciation (˜10-30 ka), implying a vertical slip rate ˜1 mm/y. Besides its regional interest in terms of active tectonics and geodynamics (Wimpenny et al., 2018), the Parina surface rupture 1) constitutes a new case to enrich the pending SURE database with new accurate data, especially for intraplate events, 2) surface geology is a key parameter influencing the surface slip, 3) illustrates once again that moderate earthquakes can rupture the surface in a complex pattern, 3) shows that high-resolution techniques allows improving the characterization of surface ruptures (rupture length and max/mean displacement) and 4) potentially questions the fault parameters that were inferred in the past when such approaches were not available. Those are arguments that support the idea of the need for a deep revision of empirical relationships, based on catalogues of modern earthquakes

Earthquake surface ruptures on the altiplano and geomorphological evidence of normal faulting in the December 2016 (Mw 6.1) Parina earthquake, Peru

The 2016 Mw 6.1 Parina earthquake ruptured a shallow-crustal normal fault within the high Andes of south Peru. We use high-resolution DEMs and field mapping of the surface ruptures generated by the earthquake, in combination with co-seismic and post-seismic InSAR measurements, to investigate how different features of the geomorphology at Parina are generated by the earthquake cycle on the Parina Fault. We systematically mapped 12 km of NW-SE trending surface ruptures with up to ~27 cm vertical displacement and ~25 cm tensional opening along strike, separated by a gap with no observable surface ruptures. Co- and post-seismic InSAR measurements require slip below this gap in surface ruptures, implying that surface offsets observed in paleoseismic trenches may not necessarily be representative of slip at seismogenic depths, and will typically yield an underestimate of paleo-earthquake magnitudes. The surface ruptures developed along 10–20 m high cumulative scarps cutting through late Quaternary fluvio-glacial deposits and bedrock. The 2016 Parina earthquake did not rupture the full length of the late Quaternary scarps, implying that the Parina Fault does not slip in characteristic, repeat earthquakes. At Parina, and across most of the Peruvian Altiplano, normal faults are most-easily identified from recent scarps cutting late Quaternary moraine crests. In regions where there are no recently-deposited moraines, faults are difficult to identify and lack time constraints to quantify rates of fault slip. For this reason, current fault maps may underestimate the seismic hazard in the Altiplano

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics 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

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics 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

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

Temporal and spatial analysis of the 2014-2015 Ebola virus outbreak in West Africa

West Africa is currently witnessing the most extensive Ebola virus (EBOV) outbreak so far recorded. Until now, there have been 27,013 reported cases and 11,134 deaths. The origin of the virus is thought to have been a zoonotic transmission from a bat to a two-year-old boy in December 2013 (ref. 2). From this index case the virus was spread by human-to-human contact throughout Guinea, Sierra Leone and Liberia. However, the origin of the particular virus in each country and time of transmission is not known and currently relies on epidemiological analysis, which may be unreliable owing to the difficulties of obtaining patient information. Here we trace the genetic evolution of EBOV in the current outbreak that has resulted in multiple lineages. Deep sequencing of 179 patient samples processed by the European Mobile Laboratory, the first diagnostics unit to be deployed to the epicentre of the outbreak in Guinea, reveals an epidemiological and evolutionary history of the epidemic from March 2014 to January 2015. Analysis of EBOV genome evolution has also benefited from a similar sequencing effort of patient samples from Sierra Leone. Our results confirm that the EBOV from Guinea moved into Sierra Leone, most likely in April or early May. The viruses of the Guinea/Sierra Leone lineage mixed around June/July 2014. Viral sequences covering August, September and October 2014 indicate that this lineage evolved independently within Guinea. These data can be used in conjunction with epidemiological information to test retrospectively the effectiveness of control measures, and provides an unprecedented window into the evolution of an ongoing viral haemorrhagic fever outbreak.status: publishe