Abstract

We study the seismicity and stress transfer in the Coquimbo region of central Chile, where an exceptional series of more than 12 earthquakes of magnitudes from 6 to 7.6 has occurred since July 1997. In this area, the oceanic Nazca plate is subducted under the continental lithosphere of South America. Below 50 km, the downgoing slab slips aseismically with respect to the South American plate at a rate close to 6.5 cm/yr. The Coquimbo region was the site of major earthquakes of M > 8 in 1880 and 1943. After many years of quiescence, the seismic activity of the plate interface suddenly increased in mid-1997 and continued at least until 2004. The first group of events occurred in July 1997 in the middle of the locked plate interface. In October 1997, the activity moved inland to the Punitaqui-Ovalle area, just above the transition from the seismogenic zone to that of aseismic slip. The main event of the series was the M = 7.6 15 October 1997 Punitaqui earthquake. This is an intraslab compressional earthquake that occurred at ∼60 km depth, on a subvertical plane located very close to the downdip edge of the seismogenic coupled interface. We performed simulations of Coulomb stress transfer for earthquakes near the bottom of the seismogenic zone. We found that a simple model of stress transfer from the aseismic slip at depths greater than 50 km can explain the triggering not only of the Punitaqui earthquake but also of the July 1997 sequence. Additional simulations show that the seismicity that followed the 1997 event for almost 7 years can also be simply explained as a result of increased Coulomb stresses on the seismogenic plate interface as a result of the 1997 event and aseismic slip. Copyright 2006 by the American Geophysical Union.