Basin waves on a seafloor recording of the 1990 Upland, California, earthquake: Implications for ground motions from a larger earthquake

The velocity and displacement time series from a recording on the seafloor at 74 km from the 1990 Upland earthquake (M = 5.6) are dominated by late-ar riving waves with periods of 6 to 7 sec. These waves are probably surface w aves traveling across the Los Angeles basin. Response spectra for the recor ding are in agreement with predictions from empirical regression equations and theoretical models for periods less than about 1 sec but are significan tly larger than those predictions for longer periods. The longer-period spe ctral amplitudes are controlled by the late-arriving waves, which are not i ncluded in the theoretical models and are underrepresented in the data used in the empirical analyses. When the motions are scaled to larger magnitude , the results are in general agreement with simulations of wave propagation in the Los Angeles basin by Graves (1998).