Variation of P-wave travel-time residuals before and after the

Abstract P-wave velocity variations may provide precursory information before large earthquakes. We present results from a simple study on variations of P-wave travel-time residuals before and after the 1999 Chi-Chi, Taiwan, earthquake. The short-period seismic network deployed in Taiwan by the Central Weather Bureau (CWB) is routinely used for earthquake location. The accuracy of earthquake location is often judged by the travel-time residual of seismic waves. Because the velocity model used by the CWB was one-dimensional, the travel-time residuals will show lateral inhomogeneity of the velocity structures and unmodeled vertical structures. Also, the travel-time residuals may change with time if the velocity structure changes temporally. We used the data of the Central Weather Bureau Seismic Network (CWBSN) from 1991 to 2002 to obtain the difference between observed and calcu-lated P-wave travel times, which is defined as the P-wave travel-time residual. The purpose of this study is to find the mean value of the P-wave travel-time residuals at each station and its possible temporal changes at stations near the Chelungpu fault. Our results show that the mean P-wave travel-time residuals at stations 40 km eas