The Valparaiso 2017 sequence occurred in the Central Chile megathrust, an active zone where the last mega‐earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 mainshock, a slow trenchward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater‐type seismicity. To characterize the rupture process of the mainshock, we perform a dynamic inversion using the strong‐motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semiaxis of 10 km and 5 km, with a subshear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m, and kappa value of 1.98). This earthquake could be the beginning of a long‐term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a megathrust earthquake like 1730 is expected.

International audienceThe aim of the SI-Hex project (acronym for « Sismicité Instrumentale de l’Hexagone ») is to provide a catalogue of seismicity for metropolitan France and the French marine economic zone for the period 1962–2009 by taking into account the contributions of the various seismological networks and observatories from France and its neighbouring countries. The project has been launched jointly by the Bureau Central Sismologique Français (CNRS-University/BCSF) and the Laboratoire de Détection et de Géophysique (CEA-DAM/LDG). One of the main motivations of the project is to provide the end user with the best possible information on location and magnitude of each earthquake. So far, due to the various procedures in use in the observatories, the different locations and magnitudes of earthquakes located in the SI-Hex zone were presenting large discrepancies. In the 2014 version of the catalogue, 1D localizations of hypocentres performed with a unique computational scheme and covering the whole 1962–2009 period constitute the backbone of the catalogue (SI-Hex solutions). When available, they are replaced by more precise localizations made at LDG or, for recent times, by the regional observatories within: 1) the French Alps, 2) the southernmost Alps and the Mediterranean domain including Corsica, 3) the Pyrenees, and 4) the Armorican massif. Moment magnitudes Mw are systematically reported in the SI-Hex catalogue. They are computed from coda-wave analysis of the LDG records for most Mw>3.4 events, and are converted from local magnitudes ML for smaller magnitude events. Finally, special attention is paid to the question of discrimination between natural and artificial seismic events in order to produce a catalogue for direct use in seismic hazard analysis and seismotectonic investigations. The SI-Hex catalogue is accessible on the web site www.franceseisme.fr and contains 38,027 earthquake hypocentres, together with their seismic moment magnitudes Mw

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Most of the recorded seismicity in central Chile can be linked to the subduction of the Nazca plate. To the east, a much smaller fraction is observed at 0–30 km depths beneath the western Andean thrust. Paleoseismic studies evidenced the occurrence of at least two major earthquakes (M>7) over the past 17 ka, associated with the San Ramón fault (SRF): an important tectonic feature characterizing the west Andean thrust, close the Santiago metropolitan area. To better constrain the crustal seismicity in this area, the Chilean Seismological Center (CSN) extended its permanent seismic network with seven new broadband seismometers deployed around the scarp of the SRF and farther east. The improved azimuthal distribution and reduced station spacing allowed to complete the CSN catalog with more than 900 smaller magnitude earthquakes (ML<2.5) detected and located within the study region. The use of a 3D velocity model derived from P- and S-wave travel-time tomography considerably lowered the uncertainties associated with hypocentral locations. Our results show an important seismicity beneath the Principal Cordillera located at a depth of ∼10  km, and a deeper seismicity (~15 km) aligned with the main Andean thrust more to the west, parallel to the scarp of the SRF. Regional stress inversion results suggest that the seismicity of the west Andean thrust is accommodating northeast–southwest compressional stress, consistent with the convergence of the Nazca plate. Based on our improved crustal seismicity, combined with observations from previous studies, we have been able to refine the scenario of an Mw 7.5 earthquake rupturing the SRF. Ground-motion prediction results show peak ground accelerations of ∼0.8g close to the fault scarp.

Dehydration of the oceanic subducting slab promotes the formation of magmatic arcs, intra-slab intermediate-depth seismicity, and hydration of the overlying mantle wedge. However, the complex permeability structure of the overriding plate controls the magma and fluid migration and their accumulation at shallower depths. In this regard, mapping the inner structure of the overriding crust and mantle is crucial to understand the magmatic and hydrological processes in subduction zones. We integrate 3-D P-wave, $$V_p/V_s$$ V p / V s , and electrical resistivity tomographic models of the northern Chilean subduction zone to map the magmatic and fluids derived from the subducting oceanic Nazca plate. Results show a continental crust relatively thick (50–65 km) characterized by a lower zone of high $$V_p$$ V p values (7.2–7.6 km/s), which is interpreted as the presence of plutonic rocks. The mantle lithospheric wedge is weakly hydrated ($$V_p/V_s$$ V p / V s = 1.75–1.8) while the forearc continental crust is traversed by regions of reduced electrical resistivity values ($$< 10^2$$ < 10 2 $$\Omega m$$ Ω m ) interpreted as zones of relatively high permeability/fracturing and fluid content. These regions spatially correlate with upper plate trans-lithospheric deformation zones. Ascending melts accumulate preferentially in the back-arc, whereas hydrothermal systems form trenchward of the volcanic arc. The results highlight the complex permeability structure of the upper South American plate.