New evidences of rupture of crust and mantle in the subducted Nazca plate at intermediate-depth

Abstract

Between 33°-36°S, the Nazca plate subducts below South American plate with an angle of ~30°, and it is seismically active until ~200-280km depth. At 33.5°S, the seismicity decreases drastically at 120km depth, just below the volcanic arc. In this paper, we studied a pair of associated earthquakes located in the area where the frequency of seismicity changes. The hypocenters of the Mw=6.4, June 16th, 2000 and Mw=5.7 January 7th, 2003 earthquakes were found nearby, adjacent to the oceanic Moho, closely associated with each other. The slip on the plane of the 2000 event produced Coulomb stress changes on the fault plane of 2003, both westward dipping, with a variation from ~1bar near the hypocenter of the latter to ~0.1bars in the deepest part of the plane. The two earthquakes combined process describes a normal focal mechanism, which cuts through the crust and breaks the mantle, reaching depths of ~40km below the Moho.The composed fault plane of the 2000 and 2003 events corresponds to a west-dipping normal fault with strike and dip consistent with those of the outer ridge faults. Thus, these events could be related to a preexisting fault originated in that environment reactivated at depth.The slip on the composed fault plane is consistent with the bending produced by the slab pull. Dehydration could be associated to these events.