127 DEVELOPMENT OF AN ENGINEERING MODEL OF THE AMPLITUDE AND DURATION EFFECTS OF BASIN GENERATED SURFACE WAVES Paul G. Somerville, Nancy Collins, Robert Graves, and Arben Pitarka URS Group, Inc., Pasadena, CA Abstract Basin waves are polarized predominantly in the directions parallel to and normal to the edge of the basin, consistent with the assumption made by Joyner (2000). There are significant differences between the amplitudes of the horizontal component parallel to the basin edge, which is predominantly Love waves, and the horizontal component perpendicular to the basin edge, which is predominantly Rayleigh waves, although these amplitudes will be affected by the relative strength of the incoming waves, which depends on focal mechanism and other factors. This is also consistent with the assumption made by Joyner (2000). There is a significant dependence of the ground motion amplitude and duration on basin depth. Basin depth dependence was not included in the Joyner (2000) model. When depth increases away from the basin edge (i.e. when basin depth and basin edge distance are correlated), this can cause ground motion amplitudes and durations to increase away from the basin edge. The Husid plot derived from the velocity waveform provides an appropriate duration measure that is independent of the absolute amplitude level. Introduction Joyner (2000) developed a procedure for modifying standard spectral attenuation relations to account for the amplitude effects of surface waves in deep sedimentary basins. The objective of this project is to extend his work so that it is more broadly applicable in earthquake engineering. We are extending the model to include duration in addition to spectral amplitudes. We are extending the model to include basins other than the Los Angeles basin. In particular, we are including data from shallower basins, such as the San Bernardino, San Fernando, Santa Clara, and Eel River basins, in which the basin effects are expected to extend to shorter periods. We are extending the lower bound of the period range covered by Joyner from 3 seconds to 1 second, which will make the model relevant to a much larger number of structures. The result of this study will be a model, suitable for earthquake engineering application, that modifies standard ground motion models to account for the amplitude and duration effects of basin generated surface waves. This paper describes results that have been obtained to date. Mode of Generation of Basin Waves The mode of generation of basin-trapped waves is illustrated schematically in Figure 1. If a seismic wave enters a basin through its edge, it can become trapped within the basin if post- critical incidence angles develop. The resulting total internal reflection at the base of the layer is SMIP02 Seminar Proceedings