Preliminary results from vertical seismic profiling of Oroville microearthquake S‐waves

Seismograms recorded along a 0.5 km‐deep borehole in Oroville, CA., show that ground‐level s‐wave velocity spectra at this location are a product of site effects. In the 6.25 to 50 hz range studied, both the Cleveland Hill fault, through which the borehole was drilled, and the rock above it contribute to a substantial loss of s‐wave energy. In first‐order approximation, the average apparent s‐wave quality factor, Q s a , along the well is 9. Across the fault zone the Q s a drops to a low of 3. Downhole, the seismograms can readily be separated into longitudinal and transverse components. Particle motion diagrams show that the s‐arrival is made up of two differently polarized waves. This splitting is possibly an effect of wave propagation in the highly cracked and probably anisotropic rock of the region.