Prediction of strong ground motion in the Puget Sound region: The 1965 Seattle earthquake

Spatial variations in strong ground motion in the 1965 Seattle, Washington, earthquake (M_S = 6.5) can be accurately reproduced using three-dimensional ray tracing techniques. Synthetic accelerograms calculated at nearly 1600 receivers spread over the Puget Sound region indicate that peak ground accelerations are significantly affected by subsurface structure as well as by near-receiver soil properties. The large synthetic accelerations computed for the Harbor Island/downtown Seattle area appear to be due to subsurface focusing of rays within a deep sedimentary basin and are consistent with the observed isoseismal distribution. In extreme circumstances, such as a bedrock site adjacent to a site situated upon a soft sedimentary lens, peak accelerations may vary by nearly an order of magnitude. Predicted values of peak ground acceleration as a function of distance reproduce both the trend and scatter in data from subduction zone earthquakes of comparable depth in Japan.