S-Local-Wave Seismic Anisotropy in the Forearc Above the Subducted Nazca Plate Between 33A degrees S and 34.5A degrees S

摘要

S-wave splitting from local earthquakes within the Nazca plate that are deeper than the interplate seismogenic zone enabled the determination of the fast velocity direction, I broken vertical bar, and the lag time, delta t, in the forearc of the overriding plate. Data were collected from 20 seismic stations, most of which were temporary, deployed between +/- 33.5A degrees S and +/- 34.5A degrees S and included part of the normal subduction section to the south and part of the transitional section to flat subduction to the north. The fast velocity direction has a complex pattern with three predominant directions northwest-southeast, north-south and northeast-southwest and relatively high delta t. A quality evaluation of the highest measurements enabled us to identify possible cycle skipping in some of the measurements, which could be responsible for the large observed lag time. We consider that most of the anisotropy that was observed in the forearc is probably located in the mantle wedge, and a minor part is located in the crust. The complex pattern of splitting parameters when the anisotropy is associated at the mantle wedge could be the result of three-dimensional variations in the subducting Nazca plate at these latitudes. Also, similarities between the splitting parameters and the principal compressional stress direction from Pliocene and Quaternary rocks suggest that the anisotropy in the crust could originate by tectonic local stress.