THE M(S)=8 TENSIONAL EARTHQUAKE OF 9 DECEMBER 1950 OF NORTHERN CHILE AND ITS RELATION TO THE SEISMIC POTENTIAL OF THE REGION

The only known great (M(s) = 8) intermediate depth earthquake localized downdip of the main thrust zone of the Chilean subduction zone occurred landward of Antofagasta on 9 December 1950. In this paper we determine the source parameters and rupture process of this shock by modeling long-period body waves. The source mechanism corresponds to a downdip tensional intraplate event rupturing along a nearly vertical plane with a seismic moment of M0 = 1 X 10(28) dyn cm, of strike 350-degrees, dip 88-degrees, slip 270-degrees, M(w) = 7.9 and a stress drop of about 100 bar. The source time function consists of two subevents, the second being responsible for 70% of the total moment release. The unusually large magnitude (M(s) = 8) of this intermediate depth event suggests a rupture through the entire lithosphere. The spatial and temporal stress regime in this region is discussed. The simplest interpretation suggests that a large thrust earthquake should follow the 1950 tensional shock. Considering that the historical record of the region does not show large earthquakes, a 'slow' earthquake can be postulated as an alternative mechanism to unload the thrust zone. A weakly coupled subduction zone-within an otherwise strongly coupled region as evidenced by great earthquakes to the north and south-or the existence of creep are not consistent with the occurrence of a large tensional earthquake in the subducting lithosphere downdip of the thrust zone. The study of focal mechanisms of the outer rise earthquakes would add more information which would help us to infer the present state of stress in the thrust region.