Source spectra of S waves were determined using records of eighteen earthquakes ovvurring in the Chia-Yi and Tai-Nan area with local magnitudes of 2.8 ≤ ML ≤ 5.8 as obtained from a rock-site station. In addition to the correction of geometrical spreading, elimination of the anelastic attenuation effect from the observed spectra was carefully examined to measure the high-frequency spectral levels of seismic sources.
As to the source spectra, two types of spectral shapes may be observed. For earthquakes of ML < 5.4, the spectra obey the ω-squared model with a single corner frequency. However, this observation cannot provide an adequate representation for earthquakes of ML ≥ 5.4, since they clearly demonstrate the existence of two corner frequencies on the spectrum. The difference in spectral shapes may reveal that the rupture of larger earthquakes proceeded as a series of multiple events while a single fault patch results in smaller earthquakes. This explanation is supported by both spectral shapes and waveform characteristics, and may disclose the complexity of earthquake sources of larger magnitude.
The seismic moment of M0 measured from spectral level at low frequency range satisfies a relation with lower corner frequency of lo in M0 ∞ f-3. For the set of earthquakes, the average stress drop is 125 bars. Nonetheless, this model is a poor fit to the shapes of source spectra for events of ML ≥ 5.4. The source spectra obtained by the two greater events, the 1991 Chiali (ML = 5. 7) and 1993 Tapu (ML = 5.8) earthquakes, were discussed in this subject. In describing these spectra, a stress drop of about 60 bars was estimated from the spectral level in a lower frequency range, while 600 bars was required to interpret the high-frequency amplitudes. By applying the Sato and Hirasawa (1973) source model, the average scale length of the fault heterogeneities inferred from the higher corner frequency of f0 is about 300 meters, and this is almost identical to the source radius of the Brune (1970, 1971) model for small events with a magnitude of around 3. Based on the seismic moments taken from the Harvard centroid-moment tensor (CMT) solution and this study, for the Tapu earthquake the estimated values of local stress drop obtained using the specific barrier model (Papageorgiou and Aki, 1983) are about 700 and 516 bars. The high stress drop of 600 bars for our result, as observed from high-frequency source spectra, lies in between, and its validity is also confirmed by the agreement of total seismic energy between the results obtained from specific barrier model and those from the Gutenberg-Richter relation (1956).