台灣地區BATS地震矩張量震源解的品質評估及其在地震地體構造上的應用

Abstract

本研究挑選1130個1995年七月∼2005年九月，由中央研究院地球科學所使用台灣寬頻地震網(BATS)所接收波形資料逆推而得的震源解。藉由BATS CMT地震參數的統計，得到其完整規模(Mc)為Mw＝4.0，b值為0.921。本研究使用Frohlich(1992)對震源機制解斷層破裂型態的方法進行分類，其中485個(42.92％)為逆衝斷層型，主要分布於東北部的板塊隱沒帶、東部碰撞帶、西部麓山帶與蘭嶼地區；51個(4.51％)為正向斷層型，集中於弧後擴張區與中央山脈南北段；109個(9.65％)為走向滑移斷層型，集中發生於宜蘭外海地區、縱谷東側與中央山脈南北側。 為了評估BATS CMT震源解品質的優劣，本研究將其與GCMT(Global Centroid Moment Tensor)震源參數比較，結果顯示在BATS波形擬合誤差值低於0.5且GCMT地震矩相對誤差值在0.2以下時，兩者的震源解88％以上Kagan角(兩個震源機制解在三度空間中旋轉以達到相同的最小角度)小於34°。BATS Mw平均小於GCMT Mw∼0.17，這很有可能是因為GCMT求震源解過程，小地震無法有效激發足夠的長週期波形，而被長週期雜訊干擾，降低GCMT對小地震的解析能力。比較過程也發現，BATS CMT震源參數解的品質，容易受到測站覆蓋率、複雜的地下構造與地震定位誤差影響，本研究提出未來能夠針對這三方面進行改善的建議。 本研究以震源解得到台灣地區應力與應變場，據以描述區域的變形狀態與應力環境，應變率在1E-11~1E-4/yr之間，與GPS所推估的間震期地表應變比較，明顯小一個數量級。接著使用震源解以Michael(1984)的方法進行應力張量逆推，得到台灣全區與十個孕震構造區的主應力狀態。最後，我們藉由震源解斷層型態、應變與應力軸，進一步解釋各區的地震地體構造，以期作為將來評估地震危害的依據。

This study collected 1130 regional centroid moment tensor (CMT) solutions, which were obtained by the Institute of Earth Sciences, Academia Sinica for earthquakes occurred in the Taiwan region in 1995-2005 by inverting the BATS broadband waveforms, to characterize their statistic properties and tectonic implications. The moment magnitude ranges from 3.3 to 7.1 in this catalog with a complete magnitude (Mc) of ~4.0 and the b-value of 0.921. Predominant thrusting events(42.92％) are common in the colliding boundary, fold-and-thrusting belt, and subducting interface, whereas the normal faulting(4.51％) presents mainly in the Okinawa Trough, northern Hoping Basin, and the eastern Central Range.

To evaluate the robustness of BATS CMT solutions, this study compares 96 CMT solutions in common between BATS and GCMT catalogs. Results show that the BATS solutions are well constrained for events with misfit < 0.5, which are roughly comparable to solutions of GCMT with moment error < 0.2. The BATS Mw is generally less than GCMT Mw by an average of ~0.17. This is very likely due to the insufficient quality of solutions in GCMT catalog for small earthquakes. For obtaining more robust solutions in the future, the uncertain of epicenter location, velocity model and station coverage must be improved by new techniques such like location searching, 3-D Green’s function.

Horizontal seismic strain directions, which are shown in terms of well constrained BATS CMT dataset, agree well with the surface geodetic GPS observations. Seismic strain rate along the Costal Range is ~1E-7/yr, which is about one order of magnitude smaller than the one derived from GPS measurements. This study applies Michael’s method (1984) to invert the stress tensor for obtaining the seismic stress field in the Taiwan region. Finally, we take advantage of these results to delineate the seismogenic structures associated with tectonic processes around Taiwan. Our results provide seismological constraints on the mode of deformation and stress field in the Taiwan region.