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 po rtable analogue and digital stations. A total of 197 reliable microear thquake locations and 19 focal mechanisms were determined. It is propo sed that it is possible to estimate the maximum depth of the coupled-u ncoupled transition of the subducting lithosphere using local data, de fined by the depth of the expected change of the stress field from com pressional to tensional along the slab. This change is observed at abo ut 70 km depth in the Antofagasta field work. Two estimations of the w idth of the seismogenic interplate contact are discussed: (1) the maxi mum depth of the coupled zone defined by the observed maximum depth of the shallow-dipping thrust events recorded during the experiment of 4 7 km, corresponding to a width of the seismogenic contact zone of abou t 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 co ntact 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 eve nt associated with the Atacama fault system was observed during the ex periment.