Crustal and upper mantle velocity gradients in the vicinity of the East Kazakh test site

The Lg wave is simulated three dimensionally by ray methods, in homogeneous models of the crust with smooth variations in thickness. Variations typical of mountainous regions are utilized, and the effect of these variations is examined in detail. Ray modeling shows that Lg is strongly attenuated by propagation across the thickened crustal root found beneath mountain ranges. While variations in the width of the mountain root are found to be unimportant, variations in the width of transition between thick and thin crust strongly influence attenuation. Propagation paths across transition regions 100 km wide result in weak Lg while 200 km transitions cause very little Lg attenuation. Lg is found to propagate very efficiently within a mountain root, with amplitude increased slightly by lateral reflections. Propagation across mountain belts at angles from 90 to 20 degrees (relative to strike) is shown to cause strong attenuation at all angles, with no identifiable optimal path. The technique is applied to a rough thickness model of the Central Asian crust and although attenuation is predicted, close comparison of the synthetics with earthquake recordings reveals discrepancies that can not be entirely explained by ray methods in a homogeneous crust. This suggests that a more accurate model of the crust is required. Including variations in crustal Q, surface sedimentary layers in the basin regions and the large scale 3-D heterogeneities associated with the uplifted regions.