Abstract

Results obtained from the distribution of local seismicity recorded with a temporarily expanded network in Central Chile during two months in 1986 are presented. Data from the Bulletin of Regional Seismicity for South America (SISRA), between 1965 and 1981 and for depths over 50 km, are added to extend the spatial covering. All of this information evidences the geometry of the subducted plate in ten E-W-oriented cross-sections, 33 km wide. The passage from subhorizontal subduction north of 33-degrees-S to normal subduction is well established as a continuous transition. The geometry of the subducted lithosphere beneath the Chilean territory remains unchanged throughout with a dip of 25-degrees. A difference between segments may be seen to the east of the high Andes: an almost horizontal seismic zone, 300 km wide in the northern part, narrows gradually as we move south and disappears completely near 33-degrees-S. This correlates very well with the beginning of active volcanism. It is shown that the vanishing of the horizontal part of the subducted lithosphere represents the transition from subhorizontal to normal subduction. No clear activity is observed deeper than 150 km.