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Physics of the Earth and Planetary Interiors
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Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile
J.C. Ruegg a,∗ , A. Rudloff b , C. Vigny b , R. Madariaga b , J.B. de Chabaliera , J. Campos c , E. Kausel c , S. Barrientos c , D. Dimitrov d
Institut de Physique du Globe (IPGP), Paris, France Laboratoire de Géologie, Ecole Normale Supérieure (ENS), CNRS, Paris, France Departamento de Geofísica (DGF), Universidad de Chile, Santiago, Chile d Bulgarian Academy of Sciences, Soﬁa, Bulgaria
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The Concepción–Constituciónarea [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 onenorth–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 68 mm/year oriented 79◦ N at the Chilean trench near 36◦ S. With respect to stable South America, horizontal velocities decrease from 45 mm/year on the coast to 10mm/year in the Cordillera. Vertical velocities exhibit a coherent pattern with negative values of about 10 mm/year 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 1–3 mm/year and vertical velocities around 3–6 mm/year. Surface deformation in this area of South Central Chile is consistent with a fullycoupled 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 ± 5 km and a dislocation corresponding to 67 mm/year oriented 78◦ N. 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 68 mm/year represents more than 10 m ofdisplacement 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 8–8.5, should it happen in the near future. © 2009 Elsevier B.V. All rights reserved.
Article history: Received 30 March 2007 Accepted 10February 2008 Keywords: GPS Tectonics Seismic gap Subduction Coupling
1. Introduction The coastal ranges of Chile are among the most seismically active zones in the world. On average, one major earthquake of magnitude 8 has occurred every 10 years in historical times, and most of the individual segments of the coastal ranges have been the site of at least one magnitude 8 during the last 130 years(Lomnitz, 1971; Kelleher, 1972; Nishenko, 1985). One exception is the South Central Chile region, between 35◦ S and 37◦ S, which experienced its last large subduction earthquake on 20 February 1835 (Darwin, 1851) with an estimated magnitude close to 8.5 (Lomnitz, 1971; Beck et al., 1998) (Fig. 1). This area lies immediately to the north of the rupture zone associated with the great 1960 earthquake,of magnitude
∗ Corresponding author. E-mail address: email@example.com (J.C. Ruegg). 0031-9201/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2008.02.015
9.5 (Plafker and Savage, 1970; Cifuentes, 1989) and south of the rupture zones corresponding to the 1928 Talca earthquake (Beck et al., 1998) and the 1906 and 1985 Valparaiso earthquakes (Barrientos,...