- 學位論文
台灣南部震源機制及應力分析
Earthquake Focal Mechanisms and Stress Environment in Southern Taiwan
摘要
2010年3月4日的甲仙地震是1999年集集地震 (Mw7.6) 之後發生於台灣島內最大的地震。甲仙地震震矩規模為5.7，震源深度為18公里，發生於台灣南部地震活動度相對較低的地區。在本研究中，我們系統化確定甲仙震源地區的地震震源機制解以檢驗甲仙地震周圍的區域應力環境。地震波形來自台灣寬頻地震網 (Broadband Array in Taiwan for Seismology, BATS) 的測站記錄，採用的地震事件發生於1997年至2010年，與甲仙地震距離50公里範圍內且芮氏規模ML 3.5以上的淺部地震，總共有694筆地震。我們利用Zhao and Helmberger (1994) 和 Zhu and Helmberger (1996) 提出的 Cut And Paste (CAP) 方法得到318筆具可信度的地震震源參數，包括震源機制解、震源深度和震矩規模，這些震源參數是透過擬合體波及表面波各個三分量的波形所決定，P波濾波頻段為0.05至0.3 Hz，S波和表面波濾波頻段為0.02至0.1 Hz。 初步的震源機制解結果顯示研究區域內依照斷層型態可以分為四個區域：西部的逆衝斷層型態；東北部的走向滑移斷層型態；中央山脈南段的正斷層型態；和東部縱谷南段的逆衝斷層型態及走向滑移斷層型態。而這些震源機制解的P軸方向與台灣南部地表變形的模式幾近一致：壓縮方向大致呈現西北-東南向，主要為菲律賓海板塊與歐亞大陸板塊聚合所致。為了進一步調查主應力軸的空間分布和震源機制解之間的關係，我們採用這些地震的震源機制解並利用線性應力張量逆推法 (Hardebeck and Michael, 2006) 反演甲仙震源地區的應力張量。應力逆推結果分布為：在西部的觸口斷層附近，最大壓應力方向為西北西，往南到潮州斷層北段則旋轉至東西向；在雪山山脈的南段，位於西部麓山帶、雪山山脈和中央山脈的交界處，水平最大拉張和壓縮軸分別為東北-西南向和西北-東南向；在中央山脈東部為一拉張區域，拉張方向為東北-西南向；而在東部縱谷南段呈現擠壓應力環境，壓縮方向為東南-西北向。而這些震源機制解和主應力軸的方向變化有助於瞭解區域構造的特性及區域地震和地殼變形之間的關連性。
並列摘要
The largest on-land earthquake in Taiwan since the 1999 Chi-Chi earthquake (Mw7.6) occurred on March 4, 2010, near the town of Jiashian in southern Taiwan. The earthquake (Mw5.7) has a hypocentral depth of 18 km and is located in an area of relatively low seismicity. To examine the tectonic environment surrounding the Jiashian earthquake, we conduct a systematic determination of the focal mechanisms of earthquakes in its source region. Waveform records at stations of Broadband Array in Taiwan for Seismology (BATS) are obtained for earthquakes of magnitudes 3.5 ≤ ML ≤ 6.4 from 1997 to 2010 within a 50-km radius from the Jiashian epicenter. A total of 694 earthquakes are collected, and the source parameters of 318 earthquakes, including focal mechanisms, focal depths and moment magnitudes, are determined with good confidence by fitting three-component waveforms in the frequency bands of 0.05-0.3 Hz for P wave and 0.02-0.1 Hz for S and surface waves using the Cut And Paste (CAP) method of Zhao and Helmberger (1994) and Zhu and Helmberger (1996). A first look at these focal mechanism solutions suggests that they can be grouped into events of similar fault types in four subareas in our study region: thrust faults in the west; strike-slip faults in the northeast; normal faults in the southern Central Range (CR); and mixed thrust and strike-slip faults in the southern Longitudinal Valley (LV). Horizontal projections of the P-axes of these focal mechanisms show patterns consistent with that of the surface deformation in southern Taiwan: an overall SE-NW compression resulted from the convergence of the Philippine Sea Plate and the Eurasian Plate. To further investigate the spatial pattern of the principal stress axes as well as their relationship with the focal mechanisms of earthquakes, we conduct linear stress tensor inversions (Hardebeck and Michael, 2006) using our focal mechanism solutions for the earthquakes around the Jiashian source area. The stress inversion results show distinct spatial patterns: a compressive regime in southern CR with the maximum compression axis in WNW direction around the Chukou Fault and in EW near the northern tip of the Chaochou Fault; alignments of the horizontal maximum extension and maximum compression axes in NE-SW and NW-SE directions, respectively, near the southern tip of the Hsueshan Range (HR) around the triple junction of the Western Foothills, HR and CR; an extensional zone in the CR west of southern LV with a NE-SW maximum extension axis; and a compressive region in southern LV with a SE-NW maximum compression axis. These results from focal mechanisms and variations in principal stress axes have important implications for understanding the characteristics of regional tectonics and their relationship with regional earthquakes and deformation.