Scaled Energies of ML ≥ 5.1 Aftershocks of the 1999 Chi-Chi, Taiwan, Earthquake Measured from Local Seismograms


In this study, we measure the seismic radiation energy, Es, and seismic moment, Mo, of twenty-two larger-sized after shocks with 5.1 ≤ ML ≤ 6.5 of the 1999 Ms 7.6 Chi-Chi, Taiwan, earthquake from high-quality digital strong-motion data recorded at stations with epicentral distances of less than 50 km through a method proposed by Andrews (1986). We also eliminate the effects on the measures of Es and Mo due to site amplification and finite frequency band width limitation. Comparison of the values of Mo obtained in this study and those listed in the Harvard CMT catalogue shows that Andrews' method to measure Mo from local seismograms is acceptable. The measured values are Es = 2.0 × 1018 − 8.9 × 1021 g cm2 sec-2 and Mo = 1.3 × 1023 − 1.4 × 1026 g cm sec-2 cm-1, which give the scaled energy to be Es/Mo = 7.4 × 10-6 − 2.6 × 10-4. The scaled energies of the 22 events are dependent upon earthquake magnitude, Ms, when both Es and Mo are evaluated from local seismograms; yet, independent of Ms when Mo is estimated from teleseismic data. Scaled energy slightly depends on the depth, h (in km), through the following form: Es/Mo = 1.92 × 10-5e0.09h. In addition, the corner frequency, fc, is also measured. Its value ranges from 0.15 to 1.34. The scaling law between Mo and fc is: Mofc-3.65.

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