INITIAL INVESTIGATION OF SITE AND TOPOGRAPHIC EFFECTS AT ROBINWOOD RIDGE, CALIFORNIA

Following the 1989 Loma Prieta earthquake, a dense array of seven digi tally recorded, three-component seismograph stations was deployed on R obinwood Ridge 7.3 km northwest of the epicenter. The purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest. Aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest. The data present an extremely compli cated pattern of ground motion that demonstrates the importance of the three-dimensionality of the problem. Slowness analysis of P wave trai ns show initial arrivals propagating away from the source with small a ngles of incidence and large apparent velocities, consistent with dire ct arrivals. After 0.5 sec, propagation azimuths become more random an d apparent velocities drop, indicating nearly horizontal wave propagat ion and multiply reflected and diffracted phases within the ridge. Slo wness analysis and particle motion diagrams of horizontal components o f motion show dramatic variations in ground motion with changes in azi muth of the source and a complicated interaction between body waves an d Rayleigh and Love waves. Results suggest that the larger amplitude, more coherent arrivals at the array stations favor a propagation direc tion parallel to the ridge axis. An amplification factor of from 1.5 t o 4.5 is seen for frequencies from 1.0 to 3.0 Hz with wavelengths comp arable to the base of the ridge, part of which may be caused by local site effects and part by topographic amplification. In addition, ampli fications of up to a factor of 5 are seen at higher frequencies and ar e attributed to local site effects. These effects are most notable fro m 4 to 8 Hz on the vertical components, and from 6 to 9 Hz on the hori zontal components. The entire Robinwood Ridge area may also have been situated in a region of heightened mainshock ground motion due to sour ce directivity and radiation pattern effects.