Abstract

Although outer rise seismicity is less common than interplate seismicity in subduction zones, a significant level of seismicity has occurred between the Nazca trench and Juan Fernandez Ridge, in central Chile, during the past 20 years. We first study the 9 April 2001 (Mw = 7.0) event and determine its focal mechanism, depth, and source time function by body-waveform inversion from teleseismic broadband data. The results indicate tensional faulting in the upper part of the mechanical lithosphere. Its strike (41°) is similar to those observed in events down dip of the slab at about 100 km depth, which could indicate that these earthquakes occur in preexisting structures formed at the trench. Compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 Mw 7.8 Valparaíso Earthquake. To understand their relation with the state of stress of the lithosphere, we construct yield stress envelopes of the oceanic lithosphere, including static and dynamic stresses. Dynamic stresses are due either to slab pull, ridge push, resistive, and drag forces. We explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the Valparaíso interplate event. The yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after. Copyright 2007 by the American Geophysical Union.