利用經驗格林函數法探討板塊介面及內部地震對台灣北部地區強地動之影響

The assessment of strong-ground motion impact to the north Taiwan for inter- and intra-plate earthquakes using Empirical Green’s Function Method

楊世緯(Shih-wei Yang)

指導教授 ： 馬國鳳

國立中央大學/地球科學學院/地球物理研究所/碩士(2012年)

摘要

從實際觀測最大加速度強地動值(PGA)資料中顯示，在隱沒帶系統中，板塊內部(Intra-plate)地震與板塊介面(Inter-plate)地震有明顯差異，故本研究利用經驗格林函數法(Empirical Green’s Function method)，探討板塊內部地震與板塊介面其地震強地動產生區域面積(SMGA)及應力降之差異。於此，我們選用由中央氣象局強地動觀測計畫(TSMIP)所收錄的地震資料，計算震源位置相近的大地震與小地震之震源平均頻譜比。我們採用與大地震震源距離相近的小地震當做經驗格林函數，且使用均接收到大地震及小地震波形之測站，以除去場址效應及路徑效應。在假設震源遵守 模型的特性下，利用震源頻譜比擬合法，我們可以對觀測資料震源平均頻譜比進行擬合，計算大地震與小地震之拐角頻率，進一步推估大地震相對於小地震的尺度比值(N)和應力降比值(C)。利用這些參數，可以進一步地進行強地動波形模擬及求得大地震之SMGA及應力降。
結果顯示，板塊內部地震的強地動產生區域面積較小，因此具有較大之應力降，其應力降約為70(bar)至120(bar)之間，而板塊介面地震的應力降約為35(bar)至45(bar)。再者，將模擬最大加速度強地動值(PGA)及實際觀測PGA值與強地動衰減式進行比較，應力降較小的板塊介面地震，實際觀測值與模擬值均落在衰減式平均值範圍內，而應力降較大的板塊內部地震，部分實際觀測值與模擬值則偏離衰減式過大。可見強地動衰減式對於應力降較大之地震的PGA值有低估之現象，此分析結果告訴我們，在隱沒帶系統中，板塊內部地震在地震防災中應予以重視。

關鍵字

經驗格林函數

並列摘要

Observations of peak ground acceleration (PGA) for subduction zone earthquakes show significant diversity in PGA intensity between intra-plate earthquakes and inter-plate earthquakes. In this study, we estimated the strong motion generation area (SMGA) and stress drop for inter- and intra-plate earthquakes using empirical Green’s function method to depict their characteristics. .We used acceleration seismograms of Taiwan Strong Motion Instrumentation Program, and sorted data of main events with smaller events nearby. The ratio of source spectrum mean for a large event to a small event thus excludes the path and site effect. Under assumptions of -square source model the spectral ratio is fitted by non-linear square spectrum fitting to get corner frequency of main event (fcm) and the one of small event (fca). The ratio of fault dimension(N) and stress drop(C) for main event to smaller one was also estimated. With these parameters, we simulated full waveforms, virtual SMGA and stress drop for the large event. Our results show that the intra-plate earthquakes have smaller SMGA and larger stress drop than inter-plate earthquakes. The stress drop of intra-plate earthquakes and inter-plate earthquakes is 70-120 bar and 35-15 bar, respectively. The synthetic PGA for both inter-plate earthquakes and intra-plate earthquakes are similar to the observation ones. Comparing to the statistic PGA attenuation curve, the standard deviations of observation and synthetic PGA for inter-plate earthquakes are in one sigma, but the ones for intra-plate earthquake are larger than one sigma. It suggests that the statistic attenuation curve has a underestimation to large stress drop of ntra-plate earthquakes, which should be paid more attenuation in seismic hazard assessment.

並列關鍵字

EGF ； SMGA

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