HR: 15:45h
AN: S43C-06    [Abstracts]
TI: Peculiar seismotectonic characteristics of Nazca's subducted slab, in the Andean region: Why do they exist?
AU: * Berrocal, J
EM: berrocal@iag.usp.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, São Paulo, SP 05508-090, Brazil
AU: Fernandes, C
EM: celia@iag.usp.br
AF: University of Sao Paulo, Rua do Matao, 1226 Geophysics Departament, São Paulo, SP 05508-090, Brazil
AB: The peculiar morphology of Wadati-Benioff Zone (WBZ) beneath Andean region presents controversial seismotectonic characteristics of the subducting Nazca plate beneath South American plate: WBZ with an almost flat behaviour under Central-Northern Peru region, and beneath Central Chile region, intercalated with steeply portion of the slab; the almost completely aseismic portion between 300 and 500 km of depth; the existence of deep earthquakes in South American and their controversial focal mechanism. There are several hypotheses trying to explain a suitable origin for those deep earthquakes that occur in depths between 500 and almost 700 km, where the occurrence of brittle failure is improbable to exist due to the presence of high temperature and pressure conditions at those depths. We propose in this work - based mainly in the spatial distribution of relocated hypocentres, and in the joint interpretation with recent published results related to seismotectonic aspects of Andean region - a top-to- northwest shear of the portion of Nazca subducting plate between 24°S and 01°S, in such an amount that its deepest corresponding extremes, at around 600 km of depth, seem to be presently, under latitudes between 29°S and 06°S, respectively. The proposed northwestern displacement of South American plate may be provoking that shear process of Nazca slab, which should be larger at shallower depths of the slab, and limited to those latitudes below the Andean region. The NW displacement of Nazca slab could explain the existence of flat subduction beneath Central-Northern Peru region as a consequence of a probable northwards migration of the buoyant Nazca ridge after subduction under South American plate in around 15°S. Similar explanation could be used for the flat WBZ beneath Central Chile and the Juan Fernandez ridge. This hypothesis permits to infer for some very deep South American earthquakes shear, planar mechanisms at high pressure, some times as almost horizontal focal mechanism, similar to the mechanism on a sub-horizontal nodal plane determined for the MW 8.3 deep Bolivian event of June 1994.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Joint Assembly