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Crustal deformation and fault slip during the seismic cycle in the North Chile subduction zone, from GPS and ERS-SAR observations

Jean-Bernard De Chabalier(1) and K Feigl(2)

(1) Institut de Physique du Globe, 4, place Jussieu, F- 75252 Paris cedex 05, France
(2) CNRS-OMP, 14, avenue E. Belin, 31400 Toulouse, France

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 possible causative distribution of slip on the fault at depth. We have applied GPS and SAR interferometry techniques to Northern Chile, where fast plate convergence rates are associated with large subduction earthquakes and large crustal deformation. The Northern Chile region between 18°S and 23°S 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 the 450km-long gap. The coseismic deformation associated with this event has been documented earlier. Here we use the GPS vectors determined for 40 points (measured between 1996 and 2000) and ERS SAR interferograms (for the period 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 seismic gap an interseismic velocity gradient perpendicular to the arc with 20-30 mm/yr eastward displacement with respect to South America is mapped. Both GPS and InSAR measurements can be modelled with 100% coupling of the thrust interface of the subduction to a depth of 35 km, with a transition zone down to 55 km depth. The slip in that zone increases linearly from zero to plate convergence rate. South of the gap, the interferometric map shows interseismic deformation superposed with deformation following the 1995 earthquake and covering the same area as the coseismic deformation. 40% of this deformation is related to seismic activity in the 3.3 years following the 1995 event, in particular slip during a Mw = 7.1 earthquake in 1998. However, most of the signal (60%) corresponds to post-seismic deformation resulting from widespread aseismic slip in the subduction interface. The afterslip appears to have occurred down-dip in the transition zone at 35-55 km depth and to have propagated laterally northwards at 25-45 km depth under the Mejillones Peninsula, which is a prominent geomorphological feature at the boundary between the 1877 and 1995 rupture zones. We propose a simple slip model for the seismic cycle associated with the Antofagasta earthquake, where the transition zone has alternating behaviour with both aseismic shear and seismic slip.

Key words: earthquake cycle, subduction zone, northern Chile, InSAR, GPS

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry