Geometry and state of stress of the subducted Nazca plate beneath central Chile and Argentina: evidence from teleseismic data

Abstract

SUMMARY Thirty new focal mechanisms of intermediate-depth earthquakes in the South American subduction zone were determined in the region between 21 and 33 S. The focal mechanisms and depths of earthquakes with magnitudes greater than 5.7 (mb) were constrained using a body-wave modelling method. The hypocentral depths of the other earthquakes, with 5.3 < mb < 5.7, were determined by identifying the arrival time of the depth phases pP and sP on seismograms from several teleseismic stations. These refined focal depths were used to draw detailed isodepth contours of the subducted Nazca plate. In the northern part of the study area (21 to 24 S), the slab dips at an angle of approximately 30seaching a maximum depth of about 300 km. Towards the south, the dip of the slab becomes horizontal from 27.5 to 30.5s and remains horizontal to -33s. This flat slab underplates South America for a distance of 250 km. The transition from steep to horizontal subduction takes place rapidly over a flexure in the subducted slab. Quaternary volcanism stops abruptly at this transition. The geometry of the shallow part of the subduction zone was defined using modelled earthquakes published by several authors. The dip of the slab from the trench to a depth of about 80 km is the same along the whole subduction zone regardless of the geometry of the slab at greater depths. All of the intermediate-depth events studied show tensional focal mechanism except for one reverse-faulting earthquake. This compressional event occurred within the slab, about 40 km beneath the sheet of tensional earthquakes. This event suggests a complex stress regime in the downgoing plate, where down-dip tensional events are on top and this compressional earthquake below. This polarity of the stresses is inverted relative to those found in double-planed seismic zones of the western Pacific, such as the Aleutians, Tonga and Honshu. The dip of the T axes of the intermediate-depth earthquakes shows a remarkable correlation with the gradient of the subducted slab. The mean difference between the dip of the T axes and that of the plate is 1.4 with a 95 per cent interval of confidence of 8.8, as estimated by Fisher statistics. This observation suggests the state of stress within the slab is controlled primarily by the slab pull induced by the geometry and gradient of the subduction zone.