S-WAVE SEISMIC ANISOTROPY IN THE FOREARC ABOVE THE SUBDUCTED NAZCA PLATE BETWEEN 33º S AND 34.5º S

SILVINA NACIF; ENRIQUE TRIEP

LIMA

Congreso; CONGRESO PERUANO DE GEOLOGÍA SEG 2012 CONFERENCE; 2012

Asociación Geológica PERUANA, SEG

S-wave splitting, from local earthquakes within the Nazca plate that are deeper than the interplate seismogenic zone, allowed determinations of fast velocity direction and lag time in the overriding plate forearc. Data are collected from 20 seismic stations, mostly of them temporary ones, deployed above the normal subduction section south of ~33.5ºS and also above the transitional section to flat subduction north of this latitude. Fast velocity direction  shows a complex pattern and lag timeare generally between 0.6 and 1.0 sec. We restrict here our interpretations assuming that the seismic anisotropy is located within the upper mantle wedge of the overriding plate. Interpretations are heavily based on already published vp, vs and vp/vs tomography and possible temperature ranges. We visualize the anisotropy within three distinctive volumes in the north and two in the south, each of these reaching different depth levels. In the north, the most superficial with trench-parallel fast velocity direction  by aligned serpentinite fabric, or flow by lateral pressure gradients as a consequence of plate dip variation; the intermediate with two predominant fast velocity direction  directions might be by overlapping of two different horizontal anisotropic layers, the deepest with probable C type olivine and approximated trench-perpendicular flow. In the south, trench-parallel fast velocity direction  between 95km to 100km depths, might be originated by aligned melt peridotite pockets or bands, or olivine B type flow, and between 80 to 95km trench-perpendicular fast velocity direction  by depleted and frozen peridotite probably left over in a previous flattening plate process.