AMPLITUDES OF REGIONAL SEISMIC PHASES IN RELATION TO CRUSTAL STRUCTURE OF THE SIERRA-NEVADA, CALIFORNIA

We have recorded and modeled the amplitudes of regional seismic waves produced by the Nevada Test Site Non Proliferation Experiment explosio n along a 400-km profile across the southern Sierra Nevada of Californ ia. Along this profile the amplitudes of the crustal phases Pg and Lg decrease monotonically with distance because of the cumulative effects of geometrical spreading and a frequency-dependent crustal attenuatio n given by Q similar or equal to 138f(0.76) (1 less than or equal to f less than or equal to 8 HZ). This observation implies that the crust of the Sierras is not more attenuative to P and S waves than the crust s of the adjacent Great Valley and Basin and Range. The only amplitude anomaly related to the crustal structure is a strong attenuation of L g on the vertical component in the Great Valley due to the refraction of rays in the low-velocity sediments of the basin. In contrast to Pg and Lg, Pn amplitude increases across the Sierra Nevada and the easter n edge of the Great Valley. Then, some 30 km west of the exposed Sierr an batholith, the amplitude of Pn suddenly decreases by a factor of 10 . On the basis of a new refraction model produced by the Southern Sier ra Nevada Continental Dynamics study and on numerical simulations of t he Pn/Pg energy ratio, these changes in Pn amplitude are shown to be d ue to a local increase in crustal thickness, from roughly 35 to 45 km, centered under the western Sierra. The size and location of the ampli tude anomaly can be explained with a gently sloping two-layer crust-ma ntle model with P velocities of 6.0 km/s and 8.0 km/s. The small offse t crustal root focuses Pn waves toward the westernmost Sierra Nevada a nd the easternmost Great Valley, significantly increasing the Pn energ y in these regions. The focusing effects diminish rapidly toward the w est as the Moho is again horizontal.