DETERMINATION OF SEISMOGENIC INTERPLATE CONTACT ZONE AND CRUSTAL SEISMICITY AROUND ANTOFAGASTA, NORTHERN CHILE USING LOCAL DATA

During September and October 1988 a microseismic field experiment was carried out around the city of Antofagasta, northern Chile, with 29 portable analogue and digital stations. A total of 197 reliable microearthquake locations and 19 focal mechanisms were determined. It is proposed that it is possible to estimate the maximum depth of the coupled-uncoupled transition of the subducting lithosphere using local data, defined by the depth of the expected change of the stress field from compressional to tensional along the slab. This change is observed at about 70 km depth in the Antofagasta field work. Two estimations of the width of the seismogenic interplate contact are discussed: (1) the maximum depth of the coupled zone defined by the observed maximum depth of the shallow-dipping thrust events recorded during the experiment of 47 km, corresponding to a width of the seismogenic contact zone of about 90 km; and (2) the maximum depth of the coupled zone defined by the depth of the observed change from a compressional to tensional stress field, which is 70 km and corresponds to a width of the seismogenic contact zone of about 130 km. With both values, the maximum magnitude M(S) estimated for the region varies between 8.6 and 8.7. No shallow event associated with the Atacama fault system was observed during the experiment.