LOCAL SITE EFFECTS ON WEAK AND STRONG GROUND MOTION

This is a review of the current state of the art in characterizing effects of local geology on ground motion. A new horizon is clear in this aspect of strong motion studies. Non-linear amplification at sediment sites appears to be more pervasive than seismologists used to think. Several recent observations about the weak motion and the strong motion suggest that the non-linear amplification at sediment sites may be very common. First, on average, the amplification is always greater at the younger sediment sites for all frequencies up to 12 Hz, in the case of weak motion; while the relation is reversed for frequencies higher than 5 Hz, in the case of strong motion. Secondly, the application of the amplification factor determined from weak motion overestimates significantly the strong motion at sediment sites observed during the Loma Prieta earthquake within the epicentral distance of about 50 km. Thirdly, the variance of peak ground acceleration around the mean curve decreases with the increasing earthquake magnitude. Finally, the above non-linear effects are expected from geotechnical studies both in the magnitude of departure from the linear prediction and in the threshold acceleration level beyond which the non-linearity begins.