Rupture directivity and source-process time of the September 20, 1999 Chi-Chi, Taiwan, earthquake estimated from Rayleigh-wave phase velocity

Rayleigh-wave phase-delay differences between the main shock and its aftershocks, regarded as source duration varying with azimuth, at period range of 60 ∼ 120 seconds, were used to rapidly determine the rupture directivity and the source-process time of the 1999 Chi-Chi, Taiwan, earthquake. Results show that the earthquake faulting exhibits an obvious directivity with an optimal rupture azimuth of about 42° in the northeast direction. The earthquake has an average source-process time at 40.6 ± 0.5 sec and a rupture length of 77.0 ± 3.6 km during rupture. In addition, periods at nodes of amplitude spectra of the main shock were adopted to estimate the rupture time, to be about 34.0 ± 0.7 sec; the rise time of 6.7 ± 0.6 sec, and a slightly slow rupture velocity, 2.27 ± 0.15 km/sec, were also obtained. According to Savage’s suggestion, the rupture width of the fault was estimated to be 30.5 ± 4.5 km. Relying on these estimated faults parameters, we calculate the average static stress drop of about 56 bars, lying in between the stress drop of interplate earthquakes and intraplate earthquakes. We also infer the average dynamic stress drop of 52 bars based on Brune’s theory. By the comparison of the two stress-drops, an Orowan’s stress model or a stress model of frictional overshot is probably appropriate to describe the rupture behavior of the earthquake. The radiated seismic energy, then, was estimated at about 1.9 × 1023 ergs, about 16 times larger than the value reported by the USGS.