| Summary: | 博士 === 國立中央大學 === 地球物理研究所 === 87 === The method of spectral analysis was first to estimate the source parameters of 18 earthquakes ( ), occurred from 1991 to 1996, then, the near source displacement waveforms were simulated for two larger events to obtain their rupture processes. Finally, the characteristics of earthquake source model in CN was discussed accordingly.
S-wave source spectra were estimated from the strong motion records of a rock-site station. In addition to the correction of geometrical spreading, the anelastic attenuation effect was also carefully examined to accurate recover the high-frequency spectral characteristics. The results show two types of source spectra. For earthquakes of , their spectra obey the -squared model with single corner frequency. However, for earthquakes of the existence of two corner frequencies in source spectra is clear. The difference in spectral shapes may reveal that the rupture of larger earthquakes proceed with an inhomogeneous faulting processes while a single fault patch is for smaller earthquakes. This explanation is supported by both spectral shapes and waveform characteristics, and may disclose the complexity of earthquake source with larger magnitude.
The seismic moment ( ) measured from spectral level at low frequency range satisfies a relation, with lower corner frequency ( ), of . This relationship can be nearly explained in terms of constant stress drop irrespective of seismic moment. The average stress drop is 125 bars for the set of earthquakes. Nonetheless, this model is a poor fit to the shapes of source spectra for events of . The source spectra of two larger events of 1991 Chiali ( ) and 1993 Tapu ( ) earthquakes were discussed in this subject and a stress drop of about 60 bars was estimated from the lower frequency spectral level while 600 bars was required to interpret the high-frequency amplitudes for both events. Based on the seismic moment taken from CMT solution and this study and the specific barrier model (Papageorgiou and Aki, 1983a), an estimated local stress drop is about 700 and 516 bars for the Tapu earthquake. The high stress drop of 600 bars observed from high-frequency source spectra of our result lies in between. Its validity was also confirmed by the agreement of total seismic energy obtained from specific barrier model and that from Gutenberg-Richter relation (1956). Applying Sato and Hirasawa’s (1973) source model, the average scale length of fault heterogeneity inferred from the higher corner frequency is about 600 meters. This is almost identical to the source radius of small events with magnitude around 3.
The Tapu and Rueyli earthquakes can be both described by a circular fault with radius 2 km, based on waveform simulations. The Tapu earthquake was related to a westward dipping thrust fault with strike of and rake of . Such a pattern is consistent with its aftershock distribution of the Tapu earthquake (Chang and Shin, 1993). The Rueyli earthquake was realted to a eastward dipping thrust fault with strike of and rake of . This suggests that the Rueyli earthquake might be associated with the Ta-Chien-Shan fault system.The other source parameters are: seismic moment, dyne-cm, and average dislocation, 74 cm, average stress drop 153 bars for the Tapu earthquake; and seismic moment, dyne-cm, average dislocation, 140 cm, and average stress drop, 240 bars for the Rueyli earthquake. We also isolated the rise time from rupture velocity for both events. The rise time and rupture velocity is 0.5 sec and 2.3 km/sec for the Tapu earthquake; and 0.4 sec and 2.3 km/sec for the Rueyli earthquake.
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