Rupture of the Valparaiso (Chile) Gap from 1971 to 1985

We discuss the seismicity near Valparaiso (Chile) from 1965 to 1985. This area was the site of a large M = 8.2 earthquake on August 16, 1906, and had been identified as a gap by Kelleher. We performed a detailed study of all the body waves and some surface waves for all the shocks of M≥5.8 which occurred in the gap since 1965. This analysis was necessary to separate the different populations of earthquakes that included several large (up to M = 7.5) events of intraplate origin and a few events on the outer rise of the Nazca plate. From these studies a plausible scenario for the rupture of the Valparaiso gap is proposed. Rupture started with a large earthquake (M0 = 5.6 1020 Nm) on July 9, 1971 and continued with several large aftershocks and a medium sized (M0 1019 = 2.810 Nm) earthquake in October 5, 1973. The latter earthquake occurred at the southern end of the aftershock zone of the 1971 event. A large barrier appears to have impeded the propagation of rupture to the south of Valparaiso. We propose that the failure of this barrier initiated the March 1985 earthquake which was a relatively simple large event with a small foreshock of about 10% of the total moment release. The main event started 10 s after the foreshock with a very sharp onset and then propagated rapidly toward the south. From the aftershock area it appears that the earthquake propagated also toward the less stressed region to the north, rupturing almost halfway into the source area of the July 9, 1971 earthquake.