Estimates of source parameters for the 1999 Chi-Chi, Taiwan, earthquake based on Brune's source model

The general features of the rupture of the 1999 Chi-Chi, Taiwan, earthquake (M-s 7.6) can be explained by the displacement waveforms derived from the accelerograms recorded at short distances from the fault traces. Applying B rune's model, we have determined important source parameters, such as rise time, stress drop, offset, and particle velocity. Generally, the earthquake is characterized as having had two distinct fault segments. The southern s egment, dominated by thrust motion, started from the focus on a fault plane raking at 78 degrees and extended about 30 km to the north. The northern s egment, dominated by thrust with significant strike-slip motion, began next to the end of the southern segment on a fault plane raking at 53 degrees a nd extended northward for 25 km. Slips in the southern segment were followe d by a small dislocation (similar to1 m), while those in the northern segme nt were followed by a much larger dislocation (similar to9 m). The average slip velocity was distributed at 34-49 cm/sec, along the southern segment, and an unusual slip velocity exceeding 2 m/sec was observed along the north ern segment. Furthermore, the southern segment experienced a rise time of 1 .8 sec and a stress drop of 65 bars, in contrast to a rise time longer than 4 sec and a stress drop larger than 300 bars registered to the north. Our results also indicate that, along the southern segment, the rupture propaga ted northward at an average velocity of 2.84 km/sec, but along the northern segment, the rate declined to less than 2 km/sec. The difference in the so urce parameters between these two segments suggests that the rupturing asso ciated with the Chi-Chi earthquake may have encountered a resistive patch a nd changed course in the middle part of the fault. After crushing that resi stance, the long rise time and high stress drop probably caused substantial ly slower motion and larger slip along the northern segment.