The modified reflectivity method is used here to interpret seismic refraction data from the west side of the Great Valley, California, in terms of a one-dimensional compressional wave velocity model of the crust. In addition, the apparent attenuation, Qp has been estimated for the 6-km-thick sedimentary section. The reflectivity method was used to calculate the prominent free-surface multiples that propagate in this sedimentary section. Modeling of the normalized amplitudes of these multiples yielded average Qp values of 100 from 0 to 2.83 km depth and 200 from 2.93 to 6.20 km depth. The velocity structure in the crust is complicated. It is characterized by positive velocity gradients within the sediment to a depth of 4.8 km and by a 1.4-km-thick, low-velocity zone below this depth. The sediment overlies a basement with a velocity of 5.7 km/sec at its top, which increases to 6.3 km/sec at 12 km depth. Ambiguity of the data in critical regions leads to two equally valid interpretations of the lower crust at depths of 16 to 27 km: simple step increases or a more complex structure consisting of alternating high- and low-velocity zones.

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