Seismic attenuation and peak ground acceleration in Taiwan

We have taken advantage of the great collection of ground-motion accelerati on records from the Central Weather Bureau of Taiwan to derive new attenuat ion relations corresponding to different geological settings: the shallow c rustal earthquakes in the active tectonic region and the subduction zone. T he new equations use a shape of magnitude dependence modulated by depth eff ect. For shallow crustal earthquakes, the focal depth also plays an adjusti ng factor for the geometrical spreading. A two-step stratified regression i s used to decouple the evaluations of the distance dependence of data from that of magnitude and focal depth. The resultant attenuation relations are as follows. Using the data from shallow crustal earthquakes, we obtained: In A = 2.8096 + 0.8993M - 0.4381 ln D-p - (1.0954 - 0.0079D(p)) ln D-e, sig ma = 0.60. Using the data from the subducting-plate earthquakes, we obtained: In A = 4.7141 + 0.8468M - 0.1745 1n D-p - 1.2972 ln D-h, sigma = 0.56, where A is the ground-motion acceleration in gaI (cm/sec(2)), D-e and D-h a re the epicentral and the hypocentral distances (km), respectively, Dp is t he focal depth (km), M is the moment magnitude, and sigma the standard devi ation. Several comparisons between our resulting attenuation relations and the oth ers are presented in this study. Compared with other empirical attenuation models of Taiwan, our results show a better fit for the Chi-Chi earthquake. These new attenuation laws predict a stronger ground motion in the far dis tance for the shallow crustal earthquakes as compared with previous studies . Our results also show lower ground motions than the other countries for s ubduction zone events. The residual ground-motion maps show a high consiste ncy with the local geology of Taiwan.