Abstract

During September-October 1988, 13 analog and 16 digital seismographs were installed in northern Chile within 100 km around the city of Antofagasta (22.5-24.5-degrees-S; 68.5-70.5-degrees-W). The purposes of this study were to observe the microseismicity, to describe the morphology of the subducting slab near the southern edge of the rupture of the last great 1877 earthquake (M(w) = 8.8) in the northern Chile seismic gap, and to monitor the seismic activity probably associated with the Atacama fault system that is roughly parallel to the coast. The analysis of the analog records provides a total of 552 reliable events (2.0 M 5.0), whose hypocentres delineate the morphology of the subducting plate in the region. The Nazca plate subducts to the east with a dip of 10-degrees along the trench from 22-degrees-S to 25-degrees-S down to 30 km depth. At 30-60 km depth a slight variation in the dip angle is observed from 17-degrees (22-23.5-degrees-S) to 14-degrees (24-25-degrees-S). Downplate, from 60 to 100 km in depth, the dip angle increases more rapidly to the north of 23.5-degrees-S than to the south of this latitude, where an almost constant dip (14-16-degrees) is observed and the subducting plate becomes more subhorizontal. For greater depths (100-150 km), the dip of the subducting Nazca plate gradually varies from 36-degrees to 18-degrees between 22-degrees-S and 24.5-degrees-S. South of 24-degrees-S and below 100 km depth, an absence of seismicity is observed. However, a cluster of intermediate depth activity is located near the hypocentre of the December 9, 1950 (M(w) = 8.2) intraplate normal fault earthquake, around 500 km inland from the trench. Shallow seismicity (depth less-than-or-equal-to 30 km) is located near the Atacama fault system. Focal mechanisms show normal faulting with slight left-lateral motion along an average strike in the north-northeast-south-southwest direction, which is in agreement with the observed superficial orientation of the fault. Shallow seismicity is also observed on the Mejillones Peninsula, the main irregularity along the coastline. Focal mechanisms of microearthquakes located near the Cerro Moreno fault, which is in the Mejillones Peninsula show left-lateral motion along a north-south fault plane, similar to the fault orientation observed in the field. South of Antofagasta, the Coastal Scarp presents shallow seismicity. Focal mechanisms were possible to obtain only for events with depths between 20 and 30 km which are characterized by thrust faults and are probably associated with the interplate subducting zone.