2006年屏東雙地震前後恆春半島之地表變形與恆春斷層活動性

Abstract

臺灣位於菲律賓海板塊及歐亞板塊之間，為活躍的弧陸碰撞帶，活動構造引起的災害性地震可能危及許多重大建設。例如位於本研究區域的核能第三發電廠鄰近位於恆春半島的恆春斷層。因此，了解恆春斷層的活動性和孕震潛能即是重要的防災評估議題。前人藉由精密水準測量的研究中發現，恆春半島的垂直抬升速率在2006/12/26屏東外海地震之後有明顯的改變，跨斷層速率增加至原本的2-3倍，且改變的抬升形貌持續約3-6年。為確認2006屏東外海地震和地表抬升速率改變之間存在觸發關係的假設，本研究利用庫侖靜態應力轉移模型，得出恆春斷層位於靜態應力增加的區域，增加值約為0.2-0.6 bar，驗證觸發關係的假設。本研究進一步使用Okada (1985)彈性半無限空間錯位模型，以正演方式搜尋得不同時段，斷層參數及斷層面上的滑移量，求取地表模擬變形與觀測值的最佳擬合。另外，為補足精密水準與GPS觀測在研究區空間上分布之不足，本研究使用時間段分別為1996-2000年ERS (C波段)和2006-2010年ALOS(L波段)衛載的SAR影像，以PS-InSAR技術分析兩時段內的恆春區域地殼變形形貌，並沿恆春斷層走向由北至南切出六個視衛星方向變形剖面，分析此變形剖面與已知活動構造的關聯性，發現恆春半島的地表變形除了恆春斷層的活動與恆春西台地持續西傾的影響之外，在恆春西台地與恆春谷地之間有一圓穹狀突起，與地質調查結果相比較，可以發現正好對應於泥灌入體分布的範圍，推測為泥灌入體所致。因此PS-InSAR所呈現的結果，提供泥岩貫入作用亦在恆春半島地表變形扮演重要的角色。

Taiwan is situated in the active arc–continent collision region between the Philippine Sea plate and Eurasia plate. The destructive earthquakes induced from active structures could poses threats on infrastructures such as the 3rd Nuclear Power plant, which is seated close to the Hengchun fault in Hengchun Peninsula, southernmost of Taiwan. Thus, understanding the activity of the Hengchun fault and seismogenic potential is a crucial topic on assessment of the seismic hazards. On the decadal time scale geodetic observation, previous studies suggested that the surface uplift rate on Hengchun peninsula based on leveling measurements was changed significantly after the 26th December 2006 Pingtung offshore earthquake doublet.The uplift rate across the Hengchun fault can be 2-3 times than the original rate. Furthermore, the changed slip pattern lasted for 3-6 years. In order to verify the hypothesis of triggering relationship between 2006 Pingtung earthquake doublet and the influence on crustal deformation in the surrounding fault systems, we adopt Coulomb static stress change model to characterize the deformation occurred on the Hengchun fault. The results indicate Hengchun fault fall in the stress increasing zone, providing a plausible evidence for the hypothesis of static stress transfer. Furthermore, the elastic half-space dislocation model is used to forward slip distribution on the Hengchun fault and find the best-fit fault parameter as well as the slip distribution on the fault patches associated with the surface deformation. Moreover, in this study, we attempt to apply ERS (C-band) and ALOS (L-band) satellite-based Persistent Scatterers SAR Interferometry (PS-InSAR), in the duration of the 1996-2000 and 2006-2010 respectively, to detect regional crustal deformation patterns. Six profiles perpendicular to the Hengchun fault are made to analyze the relationship between deformation and active structures. A dome-shaped deformation pattern between western Hengchun fault and Hengchun Valley is coincide with the mud diaper inferred from geological survey and seismic profile. The results suggest that the mud diapirism should play an important role on the deformation on Hengchun Peninsula.