| 摘要: | 地震波可視為震源、波傳路徑與測站﹙場址效應﹚等三大參數所組成。其中震源參數包括地震規模、震源時間函數、震源機制與地震矩等項目，這些都是瞭解地震破裂物理行為的重要依據。傳統上，評估斷層整體的破裂行為，主要利用不同方位角測站所觀測的震源歷時，藉由破裂方向性分析斷層的破裂長度、破裂速度、平均震源歷時等；而震源機制普遍採用地震矩張量逆推法推算，但其基本假設須將各測站的震源時間函數視為同一定值，忽略破裂方向的影響。在本研究中，我們依據遠場震波理論發展出一套新方法，可同步計算震源破裂方向性及震源機制。其過程可分成下列步驟：第一步驟透過簡易的逆推技巧搜尋可能的震源時間函數，即可單獨求得每個測站的最佳震源時間函數與擬輻射模式﹙pseudo radiation pattern﹚；第二步驟利用蒙地卡羅法搜尋斷層的走向、傾角及滑移角度，以最小平方法為基礎，比較各測站的擬輻射模式與理論輻射模式求算最佳震源機制與地震矩；第三步驟根據有限震源理論﹙finite source theory﹚，由不同方位角之震源時間函數變化推算震源破裂方向性。將本法應用於4個全球與1個台灣地區之中大規模地震﹙1992 Landers、1994 Northridge、2004 Iran、2005 Miyagi-Oki、1999嘉義地震﹚，分析結果與前人研究相當吻合，顯示本法確可有效同步計算各項地震斷層參數。歸納主要結論如下：第一、利用訊噪比可有效篩選測站，以輔助人工辨識初達 波之不確定性；第二、震源機制為準確且相對穩定的因子，利用單一 波波相即可求得穩定信賴的結果；第三、本研究可明確決定屬於走向斷層的1992年Landers地震的最佳破裂面，其餘4個逆衝斷層則較難決定；第四、所決定的斷層長度及平均震源歷時一般隨地震矩增加而增加，透過本研究可有效決定中規模地震的斷層參數，對決定震源尺度律﹙source scaling law﹚有重大的幫助，可藉此了解中、大規模地震間物理行為的異同。未來本研究努力的方向，將著重於發展半自動系統，以便應用於主要地震斷層參數的快速求算，並作為各種震源研究、地動預估及地震災害評估的分析基礎，若能更進一步搭配適當的區域速度模型與即時傳輸的強地動資料，相信將可為台灣地區的地震預防與減災工作提供參考。 For systemically estimation of the fault parameters and rupture directivity, we developed a new algorithm for the simultaneous determination of the earthquake source parameters, including seismic moment, focal mechanism, rupture length and rupture velocity, using far-field waveforms. The observed waveforms are corrected for instrument response, geometrical spreading and surface effects. The focal mechanism of an earthquake was determined through a grid search, while the pseudo radiation patterns of -wave were obtained for each station. The best solution for the focal mechanism was determined by comparing the pseudo and theoretical radiation patterns for every step on grid search. The source time functions obtained from the stations are, furthermore, to be used to estimate the rupture length and rupture velocity accounted for rupture directivity. We applied this method to four moderate to large global earthquakes for Mw5.8-7.3. A moderate Taiwan earthquake is also analyzed to demonstrate the possibility of rapidly simultaneous determination on the finite fault source parameters for local earthquake. The estimated fault parameters and rupture directivity for the five earthquakes are quite consistent with those obtained by previous studies. For the 1992 Landers earthquake, which had a strike-slip mechanism, the rupture plane was clearly determined from the developed algorithm. For other events with thrust mechanism, it is difficult to determine the rupture plane, but other fault parameters were determined robustly, especially on focal mechanism. The rupture lengths and source durations of the five earthquakes have positive correlation to the seismic moment. In this study, a promising approach to determine the focal mechanism, seismic moment and rupture length of an earthquake was presented. Future efforts related to this work will focus on developing a semi-automatic data process based on the method to quickly estimate the source parameters. Further studies on this work can also provide important information on scaling of the earthquakes for better understanding of earthquake physics on rupture characteristics. Moreover, on purpose of seismic hazard assessment, this method will be efficiently applied to regional earthquakes on account of proper local velocity structure model and real-time data transmission. |
