Effect of structure geometry on strong ground motions: The Duwamish River Valley, Seattle, Washington

A three-dimensional ray tracing algorithm is used to compute the high frequency response of an SH plane wave incident under several models of the sediment filled with Duwamish River Valley of central Seattle. Models are based on valley geometry previously determined using borehole logs. The SH-wave response is considered in an effort to simulate the S-wave radiation from the 1965 magnitude 6.5 Puget Sound earthquake. Although the structure models considered are two-dimensional, three-dimensional ray tracing is needed to treat incident SH waves of various incidence angles and azimuths of approach appropriate for the USCGS and ISC hypocenters of the 1965 event. Generally, amplification of the direct SH wave due to the curved basin geometry is comparable to an equivalent elastic layer over a half-space model. However, for points near the center of the valley, multiple S rays become focused after undergoing several reflections from the curved lower boundary of the valley fill. This produces an order of magnitude increase in effective amplification. A detailed study of these rays shows that significant amplitudes occur over narrow distance ranges (∼200 m) at the surface and that they are sensitive to moderate changes in incident wave direction. Focusing effects of this type were probably an important factor in damage caused by the 1965 earthquake, especially over the thicker portions of the Duwamish River Valley. Effects of “randomly” focused rays also give an explanation for the apparently capricious nature of strong ground shaking in the Puget Sound area.