Shear-wave velocity structure near Oroville, California

Average upper crustal shear-wave velocities ( V ¯ s ) near Oroville, California, have been determined from S -wave arrivals on timed, strong-motion accelerograms of eight aftershocks of the Oroville earthquake (August 1, 1975; M L = 5.7) at twelve sites. The value of V ¯ s to depths of 8 to 10 km is approximately 3.3 km/sec along paths to sites located on or near crystalline basement but decreases systematically at sites across the contact with Great Valley sediments. The average S -wave velocity in this sedimentary column, which varies in thickness from 0 to approximately 600 m thickness in the instrumented area, is about 1 km/sec. S waves from two additional shallow aftershocks and an explosion are used to infer a two-layer structure appropriate for the bedrock sites, consisting of a 2-km thick layer with v s = 2.9 km/sec overlying material with v s = 3.65 km/sec. With the use of one or two trigger times as P waves, well-recorded events can be located within 2 to 3 km of the USGS locations with S -wave arrivals taken from the strong-motion accelerograms.