Mechanism and spatial distribution of Chilean earthquakes with relation to subduction of the oceanic plate

Abstract

The focal mechanisms for 61 earthquakes occurring in northern and central Chile during the years 1962–1970 indicate underthrusting of the oceanic plate for earthquakes with focal depth 30–60 km. The axis of tension for intermediate-depth earthquakes is parallel to the direction of dip of the plate. For deep-focus earthquakes the axis of compression is parallel to the axis of the plate. Together with the seismicity of the region, the focal mechanisms indicate that subduction of the oceanic plate under this part of the coast of South America takes place in discrete and localized episodes and that the lithospheric slab itself is broken into a series of tongues that are absorbed independently and quite differently from one latitude zone to the next or even at one depth as opposed to another. Near the Chile-Peru corner, 18°S–25°S, the more principal present day activity is at intermediate focal depths. The motion of the plate is in the azimuth N85°E at shallow depths, veering to N65°E at intermediate depths. The zone 25°S–27°S is at present a silent zone at intermediate depths. The zone 27°S–34°S corresponds to underthrust of the oceanic plate but such that at depths of about 120 km this segment of the plate moves horizontally under the continent. The deep-focus zone, 19°S–28°S, overlaps the three zones just mentioned and is discontinuous with them. It more probably corresponds to an independent and earlier epoch of plate absorption. Under central Chile the plate motion appears to correspond to a current episode of subduction of relatively recent initiation. The motion is in the azimuth N80°E.