[S21-2] Seismic images of the upper mantle

[S21-2-04] Shear Wave Splitting and Upper Mantle Flow in Mexico

^○Raul W. Valenzuela^{1, 2, 3}, Gerardo Leon Soto^{4, 5, 6} (1.Instituto de Geofisica. Universidad Nacional Autonoma de Mexico, 2.Mexico City, Mexico, [e-mail], 4.Instituto de Investigaciones en Ciencias de la Tierra, 5.Universidad Michoacana de San Nicolas de Hidalgo, 6.Morelia, Michoacan, Mexico)

The results of shear wave splitting studies of the upper mantle in Mexico are presented. Upon entering an anisotropic medium a shear wave becomes split, meaning that a fast and a slow wave are produced. Two parameters are used to quantify anisotropy. These are the fast polarization direction and the delay time between the fast and the slow wave. The covariance method of Silver and Chan [1991] was used. Data from both permanent and temporary networks were used. Measurements were made mostly from core-transmitted phases such as SKS, but also S waves from local intraslab events were used. A motivation for studying seismic anisotropy is to determine the nature of upper mantle flow and its relationship to tectonic processes. Mexico has many diverse tectonic environments, some of which are currently active, or were formerly active, and have left their imprint on seismic anisotropy. This has resulted in a wide variety of mechanisms for driving mantle flow. The following regions have been studied: the Middle America Trench, the Baja California peninsula, the Western Mexican Basin and Range, northern and northeastern Mexico, and the Yucatan peninsula. Depending on the unique characteristics encountered within each region, some of the various explanations proposed for the anisotropy are (1) entrained subslab mantle flow, (2) corner flow in the mantle wedge, (3) slab rollback, (4) mantle flow driven by the extinct Farallon slab, (5) vertical upwelling at a former ridge, (6) early episodes of extension, (7) mantle flow around the edge of the North American craton, and (8) absolute motion of the North American plate.