Crustal deformation and fault slip during the seismic cycle in the North Chile subduction zone, from GPS and InSAR (ERS and ENVISAT) observations

Jean-Bernard de Chabalier(1) , Jaime CAMPOS(2) , Sylvain BONVALOT(3) , Mohamed CHLIEH(4) , Jean-Claude RUEGG(1) , Rolando ARMIJO(1) , Olivier CHARADE(1) , and Alexandre NERCESSIAN(1)

(1) Institut de Physique du Globe de Paris, 4, Place Jussieu, 75252 Paris cedex 05, France
(2) Universidad de Chile, Blanco Encalada 2002, Santiago de Chile, Chile
(3) IRD, Blanco encalada 2002, Santiago de Chile, Chile
(4) California Institute of Technology, California MC 100-23, Pasadena, CA 91125, United States

Abstract

The different phases of the earthquake cycle can produce measurable deformation of the earth’s surface. This work is aimed at describing the evolution of that deformation in space and time, as well as the distribution of causal slip on the faults at depth. We have applied GPS and InSAR (ERS and ENVISAT) techniques to northern Chile, where fast plate convergence rates are associated with large earthquakes and extensive crustal deformation. The region of Northern Chile between 18S and 23S is one of the most important seismic gaps in the world, with no rupture having occurred since 1877. In 1995, the Mw=8.1 Antofagasta earthquake ruptured the subduction interface over a length of 180 km in the region immediately to the south of this 450 km long gap. The deformation associated to this event have been extensively studied with ERS and GPS data to determine the coseismic slip distribution. We have used GPS position time series for 40 benchmarks (measured between 1996 and 2000) and ERS SAR interferograms (for interval between 1995 and 1999) to map both the post-seismic deformation following the 1995 event and the ongoing interseismic deformation in the adjacent gap region. In the sesimic gap, the interseismic velocities of 20-30 mm/yr to the east with respect to South America are mapped. Both GPS and InSAR measurement can be modelled with 100 per cent coupling in the thrust interface of the subduction to a depth of 35 km, with a transition zone extending down to 55 km depth. South of the gap inteferometric map shows interseismic deformation surimposed with deformation following the 1995 earthquake and covering the same area as the 1995 event. Some 40 per cent of this deformation is related to a Mw=7.1 earthquake that occured in 1998, and most of the signal (60 per cent) correspond to post-seismic deformation resulting to widespread aseismic slip in the subduction interface. In june 2005, a Mw=7.7 intra-plate slab pull earthquake occured in the oceanic plate, at 100 km depth, in the center of the 1877 seismic gap. Using the permanent GPS network, and ENVISAT interferograms we have mapped the coseismic deformation as well as the post-seismic deformation. These results contribute to shed new lights on the mechanical behaviour of the lower seismogenic zone of faults where large earthquakes commonly nucleate.

 

Workshop presentation

 

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