Crustal structure in the California-Nevada region

A case is made for the combined use of three methods for the study of the earth's crust in a given region: seismic refractions, surface wave phase velocity, and gravity. Only by this approach can the fine details of crustal structure be revealed. The standard phase-velocity curves are revised to take into account recent refraction results in South Africa and the Gutenberg low-velocity zone of the upper mantle. Restrictions on the use of the phase-velocity method alone are discussed. In the California-Nevada region seismic refractions reveal the following structure below the sediments: 23 km of granitic rock with velocities of 6.11 km/sec and 3.49 km/sec for compressional and shear waves; 26 km of gabbroic-ultramafic rock with a compressional velocity of 7.66 km/sec underlain by a zone of ultramafic rock with a compressional velocity of 8.11 km/sec. When this structure is used in computing theoretical Rayleigh wave phase velocities and gravity anomaly, discrepancies are found with the observed values which can be resolved by reducing the mean shear velocity and the density in the crust. It is probable that this reduction is limited to the intermediate crustal layer, and several modifications consistent with all three exploration methods are discussed.