Static stress drop inferred from near-fault parameters for the 1999 Chi-Chi earthquake in Taiwan

摘要

In 2001, a special issue of the Bulletin of the Seismological Society of America (BSSA) featured seismolog-ical research for the 1999 Chi-Chi Taiwan earthquake. This study uses source parameters suggested by the first author in this special issue to estimate static stress drop associated with the Chi-Chi earthquake. The waveform simulation method was used to carefully examine these source parameters. The simulation results indicate that source parameters, inferred from near-fault observations, are well determined. According to the rupture area and slip, the static stress drops (Δσs) obtained were distributed between a small value of 47 bars near the epicentral region and a much larger value (>200 bars) to the north. Similar trends in dynamic stress drop (Δσs) were also recognized by the first author in his paper published in 2001 BSSA special issue. Comparing the Δσs with Δσs, satisfies the relation Δσs/Δσd≈1. This relation suggests that fault motion is mostly spent releasing seismic wave energy during the rupture process of the Chi-Chi earthquake. The consistency between static and dynamic stress drops thus provides a measure of energy-moment (E_s/M_0) ratios, which range from 9.0 × 10(-5) to 6.5 × 10(-4). The average E_S/M_0 ratio estimated for the northern portions of the fault is 3.4 × 10~(-4), which is about 3 times that of the south. Such a high E_S/M_0 ratio can be interpreted as having low strength in the rupture for the northern portions of the fault, where the fault would release less energy per unit rupture surface to create the new rupture.