El Terremoto de Tocopilla de Mw=7.7 (Norte de Chile) del 14 de Noviembre de 2007: Resultados preliminares de la geodesia especial (InSAR)

A Mw 7.7 subduction earthquake occurred on November 14, 2007 in Tocopilla (northern Chile). This region (between 16.5ºS and 23.5ºS) had been identified as major seismic gap (~1000 km length) since the South Peru (Mw= 9.1, 16 August 1868) and the Iquique (Mw=9.0, 10 May 1877) megathrust earthquakes. This gap was reduced to 500 km after the Arequipa (Mw = 8.3, 23 June 2001) and the Antofagasta (Mw = 8.1, 30 July 1995) earthquakes. We compute interferograms using Envisat ASAR images acquired before and after the Tocopilla earthquake to infer the location, geometry and slip of the rupture. Elastic modeling of this data allows us to infer that the 2007 main rupture extended over an area of ~150 x 60 km2, between 35 and 55 km depth, with a mean displacement of ~ 1.3 m. That means that the Tocopilla earthquake ruptured the deeper part of the seismogenic interface, probably within the transition zone. This earthquake released a little portion of the slip deficit accumulated in the seismic gap during the last 130 years (~ 10m). Hence the Tocopilla event may constitute a precursor of a future large thrust event in the current 500 km seismic gap that continues accumulating elastic strain from 1877.Un terremoto de subducción de Mw 7.7 tuvo lugar el 14 de Noviembre de 2007 en Tocopilla (norte de Chile). Esta región (entre 16.5ºS y 23.5º S) había sido identificada como una gran laguna sísmica (de ~ 1000 km de longitud) desde los terremotos del Sur de Perú (Mw = 9.1, 16 de Agosto de 1868) y de Iquique (Mw = 9.0, 10 de Mayo de 1877). La extensión de la laguna se redujo después de los terremotos de Arequipa (Mw = 8.3, 23 de Junio de 2001) y de Antofagasta (Mw=8.1, 30 de Julio de 1995). Hemos calculado interferogramas a partir de imágenes ASAR Envisat adquiridas antes y después del terremoto de Tocopilla para deducir la localización, geometría y deslizamiento asociados a la rotura. La modelización elástica de estos datos indica que la ruptura principal de 2007 se propagó sobre un área de ~150 x 60 km2, entre 35 y 55 km de profundidad, con un deslizamiento medio de ~1.3 m. Esto significa que el terremoto de Tocopilla rompió la parte profunda de la interfase sismogéncia, probablemente dentro de la zona de transición. Este terremoto relajó una porción muy pequeña del déficit de deslizamiento acumulado en la laguna sísmica durante los últimos 130 años (~10 m). Por lo tanto, el evento de Tocopilla podría constituir un precursor de un gran terremoto de subducción en la laguna sísmica actual de 500 km que continua acumulando deformación elástica desde 1877.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEFrench National Research Agencypu

Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile

International audienceThe Concepción-Constitución area [35-37°S] in South Central Chile is very likely a mature seismic gap, since no large subduction earthquake has occurred there since 1835. Three campaigns of Global Positioning System (GPS) measurements were carried out in this area in 1996, 1999 and 2002. We observed a network of about 40 sites, including two East-West transects ranging from the coastal area to the Argentina border and one North-South profile along the coast. Our measurements are consistent with the Nazca/South America relative angular velocity (55. 9°N, 95.2°W, 0.610 °/ Ma) discussed by Vigny et al., 2008 (this issue) which predicts a convergence of 68mm/yr oriented 79°N at the Chilean trench near 36°S. With respect to stable South America, horizontal velocities decrease from 45mm/yr on the coast to 10mm/yr in the Cordillera. Vertical velocities exhibit a coherent pattern with negative values of about 10mm/yr on the coast and slightly positive or near zero in the Central Valley or the Cordillera. Horizontal velocities have formal uncertainties in the range of 1-3mm/yr and vertical velocities around 3 to 6mm/yr. Surface deformation in this area of South Central Chile is consistent with a fully coupled elastic loading on the subduction interface at depth. The best fit to our data is obtained with a dip of 16° +/- 3°, a locking depth of 55 +/- 5km and a dislocation corresponding to 67mm/yr oriented N78°. However in the Northern area of our network the fit is improved locally by using a lower dip around 13°. Finally a convergence motion of about 68mm/yr represents more than 10 m of displacement accumulated since the last big interplate subduction event in this area over 170 years ago (1835 earthquake described by Darwin). Therefore, in a worst case scenario, the area already has a potential for an earthquake of magnitude as large as 8 to 8.5, should it happen in the near future