台灣地區Pd衰減關係式之分析及於地震預警之運用

Abstract

台灣位在環太平洋地震帶上，地震活動頻繁，其中不乏破壞性的大地震。為了降低地震所帶來的災害，發展地震預警系統是一個快速有效的減災方法之一。所謂的地震預警，是在大地震發生後數秒內，利用地震波初期的特徵值，初步推斷地震規模大小及可能的震動強度，並在強震波來襲前發出警告，以降低地震所帶來的損害。由過去的研究(Wu and Zhao, 2006)顯示，由P波到達後3秒垂直分量之最大位移振幅(Pd)可運用於地震預警系統中，由於台灣地區尚未建立適合的Pd衰減關係式。因此，本研究利用台灣寬頻地震網紀錄來建立臺灣地區的Pd衰減關係式。本研究資料選取範圍自2002年至2007年，以深度40公里為分界，對於規模大於4.5，震央距離小於120公里之紀錄，分別探討淺層(深度40公里以內)和深層地震(深度40~100公里)。採用台灣寬頻地震網速度型紀錄，經一次積分成位移紀錄，並利用0.075 Hz高通濾波器去除低頻雜訊後量取Pd值。並參考Wu and Zhao (2006)發表的南加州經驗關係式，求出台灣地區的經驗式。由此經驗衰減式和P波資訊即可於地震發生時快速估計地震規模，達到地震預警之功效。由分析結果，我們得到淺層(1)與深層(2)各自Pd衰減式： log(Pd)＝-2.616+0.774M-1.599log(R) S.D.V.＝0.358 (1) log(Pd)＝-2.173+0.854M-1.946log(R) S.D.V.＝0.355 (2) 從Pd所得規模分析得知，僅利用離震央最近的6個測站，即可得到良好的規模預估值，其標準偏差分別為0.25和0.14針對淺源及深部地震。因此，本研究認為即便測站密度相較其他研究低，利用Pd決定規模依舊可以發揮良好的效益在台灣的地震預警系統中。

Taiwan is located in the circum-Pacific seismic zone where earthquakes occur frequently and large one may induce disaster. Earthquake early warning (EEW) system is one of methods to reduce the seismic hazard. The EEW system is operated after a large earthquake occurrence. The characteristics of initial P waves are used to estimate the possible earthquake magnitude and intensity for the early warning purpose. Before strong motion coming, an EEW system would give an alert to a distant metropolitan region for real-time hazard mitigation. According to the previous studies, the peak displacement of the initial first three seconds after P arrival from the vertical component, Pd is a good parameter for the EEW purpose. Currently, the empirical attenuation relationship of Pd in Taiwan region is not yet be determined. Thus, the broadband and strong motion records from Central Weather Bureau and Academia Sinica, Taiwan were used to determine the Pd attenuation relationship. Totally, 163 and 41 for shallow (focal depth less than 40 km) and deeper events (focal depth large than 40 km) were used, respectively. The Pd attenuation relationships for the shallow (1) and deeper (2) earthquakes are determined as follows: log(Pd)＝-2.616+0.774M-1.599log(R) S.D.V.＝0.358 (1) log(Pd)＝-2.173+0.854M-1.946log(R) S.D.V.＝0.355 (2) According to these relationships, after the hypocenter is determined the earthquake magnitude (MPd) could be estimated using Pd. Using the closest six stations to determine the magnitude the standard deviation of the shallow and deep earthquakes are 0.25 and 0.14, respectively. The uncertainties are reasonable small enough for EEW purpose.