Near-fault spatial variation in strong ground motion due to rupture directivity and hanging wall effects from the Chi-Chi, Taiwan earthquake

Damaging earthquakes feature large variations in the spatial ground motion distribution mainly due to source complexity, radiation pattern, and site conditions. In this study, we tried to capture the hanging wall and rupture directivity effects from the 1999 Chi-Chi, Taiwan earthquake (M-w=7.6), which provides the largest acceleration data set since the strong-motion seismology studies began. First, the event-specific attenuation relations of strong-motion parameters for the near-field, hanging wall and footwall sites were developed. Then an empirical hanging wall model for peak ground acceleration (PGA) was introduced to get the residuals from the obtained attenuation relationships. The new empirical model for the PGA on the hanging wall indicates 46% to 50% higher values than the predicted means over the near-field distance ranges. Finally, a period-dependent empirical spectral amplitude factor model representing the directivity effects for the dip-slip faulting system was developed using the Somerville et al. directivity model. The proposed model shows a larger spectral amplification factor than the result of the previous study. During the Chi-Chi earthquake, the maximum spectral directivity factors have been registered in the up-dip region, located around the surface exposure of the Chelungpu fault, in the range of 1.1 to 1.3 for the structural periods from 0.6 s to 5.0 s. Since the proposed spectral directivity factor was derived from the residual function of the single event without any influence of inter-event variability, the model is magnitude- and distance-independent. Hence the result can be easily implemented into attenuation relations, seismic hazard assessment, and building code revision studies for the regions with a dip-slip faulting. Copyright (C) 2003 John Wiley Sons, Ltd.