HR: 09:35h
AN: S51C-07    [Abstracts]
TI: Upper Mantle Shear Wave Anisotropy for Stations in Mexico and its Relationship to Subduction at the Middle America Trench
AU: Van Benthem, S A
EM: steempien@yahoo.co.uk
AF: Faculty of Earth Sciences, Utrecht University, P. O. Box 80021. Budapestlaan 4., Utrecht, 3508 TA, Netherlands
AU: * Valenzuela, R W
EM: raul@ollin.igeofcu.unam.mx
AF: Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Circ. Investigacion S/N. Ciudad Universitaria., Mexico City, D.F 04510, Mexico
AB: We have calculated the splitting parameters that describe upper mantle shear wave anisotropy under stations in continental Mexico located over the subducting Cocos Plate. SKS and SKKS arrivals recorded on both the radial and transverse horizontal components were used. The splitting parameters which quantify anisotropy are the delay time (δt) and the fast polarization direction (φ). The anisotropy is calculated using the approach by Silver and Chan [1991]. A time segment containing the SKS arrival is selected from both horizontal components. The space of possible solutions is then searched in one-degree intervals with φ ranging between 0 and 180°. Specifically, the coordinate axes are rotated every 1 degree increment and the autocorrelation and crosscorrelation between the components is calculated. For each value of φ, the solution space is also searched in 0.05 s increments. Next the eigenvalues corresponding to each δt and φ 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 our results, we apply a correction in the amount of the measured δt and φ to the original records and then rotate them to make sure that the anisotropy disappears. The shapes and the difference in the arrival times of the fast and slow waves are compared to make sure that the result is robust. As a further check, the polarization of the particle motion for the radial and transverse components before and after correction is plotted. The records used were taken from Mexico's Servicio Sismológico Nacional broadband network [Singh et al., 1997]. The orientation of the fast polarization direction, φ, can be explained by the absolute motion of the North American plate for some of the stations. Most of the stations, however, require a different explanation for the orientation of φ. For example, the orientation of φ for stations Platanillo (PLIG), Yautepec (YAIG), and Popocatépetl (PPIG) is aligned with the direction of absolute motion for the North American plate [Van Benthem, 2005]. The orientation of φ under Cayaco (CAIG) is close to the orientation of the relative plate motion vector between the Cocos and North American plates. The only measurement available under Ciudad Universitaria (CUIG) indicates that the orientation of φ is consistent with the extensional regime in the Trans- Mexican Volcanic Belt, with normal faults running east-west.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 9350 North America
SC: Seismology [S]
MN: 2007 Joint Assembly