Raúl W. Valenzuela (Instituto de Geofísica; Universidad Nacional Autónoma de México; 04510. México, D. F., Mexico; ph. (52) (5) 622-4138; e-mail: raul@ollin.igeofcu.unam.mx)

I have calculated the splitting parameters that describe upper mantle anisotropy under stations in Mexico using records of SKS and SKKS arrivals from both the radial and transverse horizontal components. The splitting parameters used to quantify anisotropy are the delay time (d t) and the fast polarization direction (f ). Previous studies have shown that the most recent tectonic episode in a particular region will result in upper mantle anisotropy and will thus determine the orientation of f and the magnitude of d t. If no tectonic events have taken place in the region, then the anisotropy will be controlled by the direction of the absolute plate motion. Therefore, measurements of anisotropy should help us improve our understanding of Mexico's tectonic evolution.

The anisotropy is calculated using the approach by Silver and Chan [1991]. A time segment containing the SnKS arrival is selected from both horizontal components. The space of possible solutions is then searched in one-degree intervals with f ranging between 0 and 180°. Specifically, the coordinate axes are rotated every 1° increment and the autocorrelation and crosscorrelation between the components is calculated. For each value of f , the delay time solution space is also searched in 0.05 s increments. Next the eigenvalues corresponding to each d t and f combination are calculated. In the presence of noise, the desired solution will be given by the matrix which is most nearly singular. In order to check my results, I apply a correction in the amount of the measured d t and f to the original records and then rotate them to make sure that the anisotropy disappears.

I present the results of the May 16, 1998 event south of the Fiji Islands. The records were taken from Mexico's Servicio Sismológico Nacional broadband network. Given the current station distribution, I will focus the study on Mexico's Southern Range, the Transmexican Volcanic Belt, the Central Plateau and the Jalisco Block.