HR: 09:30h
AN: S41A-07    [PDF]
TI: Top-to-Northwest Shear of Nazca Subducted Slab Beneath Andean Region
AU: * Berrocal, J
EM: berrocal@iag.usp.br
AF: Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Rua do Matao, 1226 Cidade Universitaria, Sao Paulo, SP 05508-090 Brazil
AU: Fernandes, C
EM: celia@iag.usp.br
AF: Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Rua do Matao, 1226 Cidade Universitaria, Sao Paulo, SP 05508-090 Brazil
AB: The peculiar morphology of Wadati-Benioff Zone (WBZ) beneath Andean region and the focal mechanism of deep (h$>$500 km) South American earthquakes, present controversial seismotectonic characteristics of the subducting process of Nazca plate beneath South American plate. WBZ beneath Andean region presents almost flat behaviour under Central-Northern Peru region, and beneath Central Chile/NW Argentina region, intercalated with steeply portions of the slab in Southern Peru and Northern Chile. The almost completely aseismic portion between 300 and 500 km of depth is also an irregular characteristic of Nazca slab. Besides the polemic morphology of that WBZ, focal mechanism of deep South American earthquakes is another controversial subject. There are several hypotheses trying to explain a suitable origin for deep earthquakes - deeper than 500 km - which occur in depths with high temperature and pressure conditions, being improbable brittle failure occurs. We propose in this work - based mainly in the spatial distribution of a new set of hypocentral data, and in the joint interpretation with recent published results related to seismotectonic aspects of Andean region - a top-to-northwest shear of the portion of Nazca subducting plate between 24$^o$S and 01$^o$S, in such amount its deepest corresponding extremes, around 600 km of depth, seem to be, at present, under latitudes of 29$^o$S and 06$^o$S, respectively. The northwestern displacement of South American plate may be provoking that shear process of Nazca slab, which is larger at shallower depths of the slab, and limited to those latitudes in Central Andean region. The NW displacement of Nazca slab could explain the flat subduction beneath Central-Northern Peru region as a consequence of a probable northwards migration of the buoyant Nazca ridge after subduction under South American plate in around 15$^o$S. This hypothesis permits to infer an almost horizontal focal mechanism for the very deep South American earthquakes, similar to the mechanism on a sub-horizontal nodal plane determined for the M$_W$ 8.3 deep Bolivian event of June 1994
DE: 1527 Paleomagnetism applied to geologic processes
DE: 7212 Earthquake ground motions and engineering
DE: 7230 Seismicity and seismotectonics
DE: 8155 Plate motions--general
DE: 8180 Tomography
SC: Seismology [S]
MN: 2003 Fall Meeting