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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.

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.

The North Ecuadorian–South Colombian subduction zone was the site of the 1906 Mw 8.8 megathrust earthquake. This main shock was followed by three large events in 1942, 1958, and 1979 whose rupture zones were located within the 500 km long 1906 rupture area. A combined onshore and offshore temporary seismic network covering from the trench to the Andes was deployed during 3 months in the area of large earthquakes, in order to obtain a detailed knowledge of the seismic background activity. Resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin, namely, the Interplate Seismogenic Zone, the fore-arc region which is part of the North Andean Block and the downgoing oceanic Nazca plate. The interplate seismic activity presents along strike variations, suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes. According to our results, the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth, respectively. Shallow to intermediate depth seismicity indicates a slab dip angle of ≈25°. North of the Carnegie Ridge, the Wadati-Benioff plane is defined beneath the fore arc down to ≈100 km depth. Facing the ridge, the Wadati-Benioff plane extends beneath the Andes, down to ≈140 km depth. This observation conflicts with the hypothesis of the presence of a flat slab at a depth of 100 km facing the ridge. In the overlying fore-arc region, the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly NW-SE 100 km wide stripe stretching from the coast, at about 1°N, to the Andes. The location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe. It also correspond

La localización exacta de los terremotos costa afuera y en tierra es un desafío a largo plazo para la comunidad científica, y este problema es de particular interés en las regiones de subducción como el margen activo de Ecuador-Colombia, donde grandes terremotos y tsunamis han ocurrido. En el norte de Ecuador, la sismicidad no está bien precisada debido a la ausencia de registros sísmicos entre la costa y los Andes. Por lo tanto, algunos problemas importantes, como la geometría del slab, no están resueltos aún. Nosotros proponemos aquí estudiar la sismicidad de la región de Esmeraldas (norte de Ecuador), usando datos de dos experimentos sismológicos locales, SUBLIME y ESMERALDAS, llevados a cabo en esta parte de Ecuador. Estos fueron conducidos respectivamente en 1998 y 2005, y durante ambos experimentos, estaciones en tierra y OBS (Sismómetros de Fondo Oceánico) fueron desplegados simultáneamente. Las redes de SUBLIME y ESMERALDAS cubrieron juntas una gran área, extendiéndose desde el margen hasta la Cordillera. Hemos localizado eventos registrados usando diferentes modelos de velocidad de acuerdo a las áreas geológicas y topográficas. Los modelos de velocidad usados fueron derivados a partir de perfiles sísmicos de gran ángulo a través del margen norte ecuatoriano, y de un modelo de velocidad terrestre global. Nuestros resultados muestran que el Límite Superior pendiente arriba (UdL) de la zona sismogénica está localizado a una profundidad superficial de ~10 Km, 40 Km al Este de la Fosa. Utlizando resultados anteriores de sísmica activa y nuestras localizaciones, determinamos una inclinación del slab de 10° al nivel de la fosa que aumenta a 25°-30° en la parte profunda hasta 120km de profundidad por debajo de la Cordillera Occidental. Hemos también notado la presencia de una intensa y densa deformación en la parte superior, desde los 40Km al Este de la Fosa hasta la Cordillera. El conocimiento de los patrones de sismicidad en las zonas de subducción, como la local, [EN] Accurate location of offshore and onshore earthquakes is a long-time challenge in the scientific community and this problem is of particular interest in subduction regions like the active Ecuador Colombian margin, where great earthquakes and tsunamis occurred. In the north of Ecuador, the seismicity is not well constrained because of a lack of recorded seismicity between the coast and the Andes. Therefore, some major problems, like geometry of the slab, are not resolved yet. We propose here to study the seismicity of the Esmeraldas region (north Ecuador), using data of two local seismological experiments, SUBLIME and ESMERALDAS, carried out in this part of Ecuador. They were conducted respectively in 1998 and in 2005 and during both experiments, land stations and OBSs (Ocean bottom seismometers) were deployed simultaneously. Together SUBLIME and ESMERALDAS networks cover a large area, stretching from the margin up to the Cordillera. We locate recorded events using different velocity models according to geological and topographical areas. The velocity models used were derived from wide-angle seismic profiles across the north Ecuadorian margin, and from a global earth velocity model. Our results show that the Updip Limit (UdL) of the seismogenic zone is located at a shallow ~10 km depth, 40 km east of the trench. Using active seismic tomography of previous study and our location results we show a 10° dipping slab in the trench area, increasing to 25°-30° down to 120km depth beneath the western Cordillera. We also note the presence of an intensive thick deformation in the upper, from 40km east of the trench until the cordillera. Knowledge of seismicity patterns in subduction zones, like the UdL location, is a key parameter for seismic and tsunami risk determination