Scaled Energies of ML > 5.1 After shocks of the 1999 Chi-Chi, Taiwan, Earthquake Measured from Local Seismograms

Abstract

In this study, we measure the seismic radiation energy, E(subscript s), and seismic moment, M(subscript o), of twenty-two larger-sized after shocks with 5.1≤M(subscript L)≤6.5 of the 1999 M(subscript s) 7.6 Chi-Chi, Tai wan, earth quake from high-quality digital strong-motion data recorded at stations with epicentral distances of less than 50 km through a method pro posed by Andrews (1986). We also eliminate the effects on the measures of E(subscript s) and M(subscript o) due to site amplification and finite frequency band width limitation. Comparison of the values of M(subscript o) obtained in this study and those listed in the Harvard CMT catalogue shows that Andrews' method to mea sure M(subscript o) from lo cal seismograms is acceptable. The measured values are E(subscript s)=2.0×10^18-8.9×10^21 g cm^2 sec^(-2) and M(subscript o)=1.3×10^23-1.4×10^26 g cm sec^(-2) cm^(-1), which give the scaled energy to be E(subscript s)/M(subscript o)=7.4×10^(-6)-2.6×10^(-4). The scaled energies of the 22 events are de pendent upon earth quake magnitude, M(subscript s), when both E(subscript s) and M(subscript o) are evaluated from lo cal seismograms; yet, in de pendent of M(subscript s) when M(subscript o) is estimated from teleseismic data. Scaled energy slightly depends on the depth, h (in km), through the following form: E(subscript s)/M(subscript o)=1.92×10^(-5)e(superscript 0.09h). In addition, the corner frequency, f(subscript c), is also measured. Its value ranges from 0.15 to 1.34. The scaling law between M(subscript o) and f(subscript c) is: M(subscript o) ~ f(subscript c)^(-3.65).