Estimates of stress drop of the Chi-Chi, Taiwan, earthquake of 20 September 1999 from near-field seismograms

The apparent stress and stress drop of the Chi-Chi, Taiwan, earthquake are estimated from near-field seismograms. The estimated apparent stress and st ress drop for the southern part of the fault are about 100 bars lower than those for the northern part. The estimated ratio E-s/M-0 also suggests that there is a higher dynamic stress drop in the northern part than in the sou thern one. This indicates the transformation of a higher percentage of stra in energy into the seismic-wave energy in the northern part than in the sou thern part. Based on a parameter proposed by Ramon Zuniga (1993), we propos e that the stress model of frictional overshoot can interpret the rupture o f the Chelungpu fault, on which the Chi-Chi, Taiwan, earthquake occurred.