本研究結合格點搜尋與地震矩張量逆推技術，透過寬頻地震網的連續記錄資料，針對台灣東北部宜蘭、花蓮外海地區做地震活動的即時監測與分析，可於地震發生後迅速得到完整震源參數資訊，包括震源位置、地震規模以及震源機制解。傳統上，台灣地區例行性推求震源機制解的機構主要為中央研究院地球所BATS(Broadband Array in Taiwan for Seismology)，然而其解必須先等中央氣象局(CWB, Central Weather Bureau)的震源位置發佈後才能開始求解震源機制。因此，求得機制解與震央位置之間，通常存在不可忽略的時間差問題。結合連續格點搜尋與地震矩張量逆推法，震源位置以及機制解可在地震發生之後兩分鐘內同時求得。此系統的所有震源參數均為獨立獲得，因此可提供一套完整的地震參數速報系統與現有的系統相互檢驗。本研究採用Frequency-Wavenumber (FK)的方法計算一維格林函數，並運用格林函數資料庫的概念將格林函數存在記憶體內以進行快速網格搜尋與逆推分析。研究範圍涵蓋東經121.5度至123度，北緯23.5度至25度，深度達136公里；平均每個格點大小為10 x 10 x 10 km3，總格點數為3584，涵蓋整個三維研究區域。為評估此系統的準確性，我們對此系統的結果與BATS CMT於2006年至2010年間已發生的地震事件之震源參數進行比較。從整體統計結果來看，此系統可以有效地透過地震連續記錄監測台灣東北外海的地震活動，未來我們將進一步採用三維格林函數，並將監測區域擴展至全台灣。 We use a grid-based moment tensor (GridMT) inversion technique and broadband continuous recordings to real-time monitor the earthquakes occurred in the offshore northeast Taiwan. The moment tensor inversion technique and a grid search scheme are applied to obtain the information of source parameters, including the hypocenter, moment magnitude, and focal mechanism. In Taiwan, the routine moment tensor solutions are reported by CWB(Central Weather Bureau) and IES which both require some lag time for the information on event time and location before performing the CMT(Centroid Moment Tensor) analysis. By using the Grid-based moment tensor inversion technique, the event location and focal mechanism could be obtained simultaneously within about two minutes after the occurrence of the earthquake. This inversion procedure is based on a 1-D Green’s functions database calculated with the frequency-wavenumber (FK) method. The offshore of northeast Taiwan has been taken into account as our first test area which covers the region of 121.5°E to 123°E, 23.5°N to 25°N, and the depth to 136 km. A 3-D grid system is set in this study area with average grid size of 10 x 10 x 10 km3. We compare our results with the BATS CMT solution obtained for the past earthquakes occurred from 2006 to 2010. We also compare the occurred time detected by GridMT with the CWB earthquake reports. The results indicate that the grid-based moment tensor inversion system is efficient and realizable that can be applied to real-time monitor the local seismic activity. Our long-term goal is to launch the GridMT technique with fully 3-D Green’s functions for the whole Taiwan in the future.
