Corner flow in the Isthmus of Tehuantepec, Mexico inferred from anisotropy measurements using local intraslab earthquakes

Abstract

Shear-wave splitting measurements were made using S waves from local earthquakes recorded by stations of the Veracruz-Oaxaca array, which was deployed across the Isthmus of Tehuantepec. In this segment of the Middle America Trench, the oceanic Cocos Plate subducts under the continental North American Plate. Intraplate earthquakes within the Cocos slab reach depths of 185 km and thus make it possible to sample the mantle wedge. The results can be divided into two regions which are separated by the 100 km isodepth contour of the subducting plate. Northeast of the contour the measurements show a well-organized pattern of trenchperpendicular fast axes with a delay time of about 0.35 s and sample the core of the mantle wedge. Physical conditions throughout the wedge core are such that the development of A-type olivine fabric is expected. Therefore, the existence of 2-D corner flow, driven by the downdip motion of the Cocos slab, is consistent with the trench-perpendicular fast polarizations. In the region southwest of the 100 km isodepth contour, the fast axes orientations do not show a consistent pattern and the delay times are on the order of 0.30 s. This is the shallow part of the slab and the source-to-station path segments have roughly the same lengths through the continental crust, the mantle wedge, and the subducted slab. Only a handful of splitting measurements in the continental crust were possible but these indicate an average delay time of 0.17 s, suggesting that the crust plays a major role in the anisotropy of this region. The observation of trench-perpendicular fast polarizations away from the trench is consistent with the pattern found in many other subduction zones. The local data from this study and teleseismic measurements from earlier work indicate that the fast axes are trench-perpendicular both above and below the Cocos slab. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.