Seismotectonic and stress distribution in the central Chile subduction zone

Abstract

We determined the stress tensor from focal mechanisms and the shape of the downgoing Nazca Plate subducted beneath central Chile (26–35°S), based on accurately located hypocenters of local and teleseismic events, at three different segments defined by the rupture zones of great thrust earthquakes. Between latitudes 26–33°S the slab flattens at intermediate depths and moves upwards probably related to the subduction of buoyant lithosphere associated to the Juan Fernandez Ridge (JFR). To the south the subduction is steeper and the slab penetrates into the mantle with a constant dip of 27° implying a sharp dip change around 33°S. The slab seismicity reaches a maximum depth of 200 km south of 29°S while to the north deep seismicity is observed at depths of 570–630 km, with a gap between 200 km and these depths. A remarkable along-strike variation in the seismic slab length is observed from 1100 km north of 29°S, to 800 km between 30 and 32°S and 350 km south of 33°S. The seismicity suggests a thick oceanic crust related to the JFR subduction, furthermore this intermediate depth seismicity is more intense and more seismic moment is released than in the adjacent zones. The stress tensors show differences between the steep subduction and the flat slab zones, being more important at intermediate depths where the slab flattens. At shallow depths the stress is compressive due to plate convergence. At intermediate depths the slab is under tensional stress regime, related to the slab pull in the steep subduction zone and also to buoyant forces in the flat slab zone. The maximum depth of interplate coupling between Nazca and South American Plates is 60 km, similar to other places along the Chilean subduction zone.