利用地震尾波估算震源頻譜

Abstract

本研究利用地震尾波震源頻譜法(coda-derived source spectrum method)估算台灣地震的震源頻譜和規模。我們選用由台灣寬頻地震網(Broadband Array in Taiwan for Seismology, BATS)所記錄到的台灣西南部地震中的76筆地震，其發生時間為1996到2010年，震矩規模(moment magnitude, Mw)介於3.2至6.21之間。我們將地震波的兩個水平分量處理成12個頻段(0.05-8.0赫茲)的尾波波包，並建立相對應的合成尾波波包。以合成尾波擬合實際的尾波可估算出各頻段的無因次的尾波振幅值(Dimensionless coda amplitude)，並經由路徑效應、場址效應以及震源尾波轉換函數(Source-to-Coda transfer function)的修正後，即可得到尾波震源頻譜並藉此計算出地震規模。本研究所導出的尾波地震規模(coda-derived moment magnitude, MC)和震矩張量逆推法得到的地震矩規模(Mw)吻合度相當高，兩者之間的標準差為0.2817。顯示本研究所導出之各種經驗修正式具有相當的可靠性，也代表尾波震源頻譜法可適切地應用於台灣地區之地震規模計算。MC和中央氣象局發佈的芮氏規模(Local magnitude , ML)的標準差是0.3170，此結果小於Mw和ML的標準差(0.48840)。這指出本方法所採用的寬頻訊息可緩和在台灣的高頻規模ML與低頻規模Mw之間的高度差異，但其更深入的原因仍有待進一步探討。另外，我們以各地震的2.5Hz尾波震源頻譜值和ML做線性迴歸，發現由此求得的尾波芮氏規模(coda-derived local magnitude, MLcoda)和氣象局的芮氏規模之間的標準差相當小(0.1588)。這顯示高頻的尾波仍與震源能量有穩定的相關性，可見本方法延伸至小型地震的可能性極高。

We apply the coda-derived source spectrum method to estimate earthquake magnitudes in Taiwan. 76 events occurred during the time period from 1996 to 2010 around west-south Taiwan recorded by BATS (Broadband Array in Taiwan for Seismology) with magnitudes between Mw 3.2 and 6.21 are used in this study. From two horizontal components of waveform data, we extract the coda envelopes at 12 frequency bands between 0.05 and 8.0 Hz. We construct the synthetic coda-envelope by using various empirical distance-frequency-dependent corrections. The derived synthetic coda envelops are compared to the observed coda-envelope to evaluate the dimensionless coda amplitudes. With further considerations on path effects, site effects and source-to-coda transfer functions, the dimensionless coda amplitudes at each frequency band are used to construct the source spectra, which is then used to determine the earthquake magnitudes. The 76 derived coda-magnitudes (MC) are consistent with corresponding moment magnitudes (Mw). Our result also shows the overall deviations between MC and local magnitude (ML) reported from Central Weather Bureau are less than those between Mw and ML. While the implications need further investigation, the results indicate that the broad-band coda information does reconcile, to some extent, the long-lasting discrepancy between the low frequency magnitude (Mw) and the high frequency magnitude (ML) in Taiwan. Moreover, we found out that there is a very stable linear relationship between coda-derived source amplitudes at 2.5 Hz and the ML scale, implying that the source information is well maintained by the high-frequency coda, and this property shows it is potentially very feasible to extend this method to smaller earthquakes in Taiwan.