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, Qas, along the well is 9. Across the fault zone the Qas 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.