We have recorded and modeled the amplitudes of regional seismic waves produced by the Nevada Test Site Non Proliferation Experiment explosion along a 400-km profile across the southern Sierra Nevada of California. 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 attenuation given by Q≂138f0.76 (1≤f≤8 Hz). This observation implies that the crust of the Sierras is not more attenuative to P and S waves than the crusts of the adjacent Great Valley and Basin and Range. The only amplitude anomaly related to the crustal structure is a strong attenuation of Lg 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 eastern edge of the Great Valley. Then, some 30 km west of the exposed Sierran batholith, the amplitude of Pn suddenly decreases by a factor of 10. On the basis of a new refraction model produced by the Southern Sierra Nevada Continental Dynamics study and on numerical simulations of the Pn/Pg energy ratio, these changes in Pn amplitude are shown to be due to a local increase in crustal thickness, from roughly 35 to 45 km, centered under the western Sierra. The size and location of the amplitude anomaly can be explained with a gently sloping two-layer crust-mantle model with P velocities of 6.0 km/s and 8.0 km/s. The small offset crustal root focuses Pn waves toward the westernmost Sierra Nevada and the easternmost Great Valley, significantly increasing the Pn energy in these regions. The focusing effects diminish rapidly toward the west as the Moho is again horizontal. ¿ American Geophysical Union 1996