Surface wave studies of the Sierra Madre Occidental of northern Mexico

Abstract Seismograms originating from earthquakes in the Gulf of California and recorded in West Texas (TXAR) were used to construct Rayleigh wave group velocity dispersion curves for propagation paths in northern Mexico. These paths were approximately orthogonal to boundaries of several different tectonic provinces including the Gulf of California, the Sonoran Basin and Range, the Central Plateau of Mexico, and the Sierra Madre Occidental (SMO); thus, the effects of horizontal refraction on the Rayleigh waves were neglected. Prior surface wave studies have determined velocity structures for the crust and upper mantle for all of these provinces except the SMO, and the results of these studies were used to generate Rayleigh wave dispersion curves for each segment of the path. The group delays associated with each province were calculated and subtracted from the observed dispersion curve for the entire path, thus isolating the effects of the SMO on group velocity dispersion between periods of 12 and 40 sec. Linearized inversions showed that the most realistic geophysical model that fits the SMO dispersion data consisted of a 55 km thick crust. The crustal velocity structure for the SMO consists of three layers: a thin (5 km), low-velocity (2.8 km sec 1 l ) upper layer, a middle layer (approximately 20 km) with a mean shear velocity near 3.6 km sec 1 l , and a third, lower crustal layer (30 km) with shear velocity of about 4.0 km sec 1 l . The mantle has shear velocities between 4.55 and 4.7 km sec 1 l , but the thickness of the mantle lid is poorly resolved.