Tsunami and shaking simulations of potential large subduction zone earthquake along the southernmost Ryukyu Trench

台灣位處於環太平洋地震帶上，其附近的琉球海溝及馬尼拉海溝存在引發大型隱沒帶地震的構造地質條件，其中台灣東北外海的琉球海溝最南段，透過地球物理及地質資料分析指出，其於未來具有發生大型地震之可能性。前人以花蓮-蘇澳沿海的間震期CGPS資料分析此區域的應力累積狀況，推估最大地震規模可達Mw 8.7，其所產生之地震波將會對台灣東北部及台北都會區造成嚴重威脅。另外，藉由歷史地震與海嘯紀錄顯示，除了地震本身所造成之強地動之外，亦可能引發大規模海嘯侵襲台灣本島及鄰近海域。本研究以琉球海溝南段為研究目標，透過數值模擬分析此區大型地震發生時可能產生的強地動與海嘯傳播情境。研究中採用強地震預估準則建構四種不同震源破裂模型，同時亦利用間震期CGPS資料進行震源逆推，透過實際觀測資料求得未來地震發生時斷層破裂面上之錯動量分布。進一步透過譜元素法(Spectral-Element Method, SEM)進行三維地震波傳模擬，分析此五種隱沒帶地震的破裂情境對台灣各地的強地動過程，了解各地區地表最大加速度(Peak ground acceleration, PGA)分佈。同時利用彈性半空間錯位模型(Elastic half-space dislocation model)計算各破裂情境之海底同震變形，將之作為海嘯初始波高模型，並以Dispersive海嘯傳播模式來模擬與分析所有破裂情境所產生的海嘯傳播至台灣各沿海地區之情形。由數值模擬結果得知，儘管地震發生於台灣東北外海，由於破裂方向加成效應(Directivity effect)，台灣島上部分區域將可能產生超過2g的強烈震動，其主要位於近震源區之台灣東部宜蘭、花蓮一帶。台北都會區因所處之台北盆地產生的放大效應，儘管其距離震源區達150公里，產生之PGA量值仍可超過1g。所有情境之海嘯傳播模擬結果於台灣東岸皆產生顯著的海水抬升量(> 5m)，如宜蘭、花蓮及台東沿海。若考量斷層錯動所造成的同震地殼下陷，宜蘭地區將可能遭受到的海嘯波高甚至達20公尺以上。透過本研究之數值模擬結果，可評估此段隱沒帶未來發生大型地震時可能的影響程度與範圍，包括地震波的震動與海嘯的襲擊，同時，這些情境地震之模擬結果，亦可作為將來研擬地震防災對策的參考。During the last century, many big earthquakes with magnitude larger than 9 had occurred along the subduction zones. For example, the 1960 Chile earthquake (M9.5), the 2004 Sumatra-Andaman earthquake (M9.3), and the 2011 Tohoku earthquake (M9.0). In this study, we focus on the southernmost Ryukyu Trench which is extremely close to northern Taiwan. Interseismic GPS data in northeast Taiwan shows a pattern of strain accumulation suggests that the maximum likely magnitude of a potential future large earthquake in this area is probably about moment magnitude 8.7. In order to evaluate the influence of the potential megathrust event, we consider a 3-D fault plane along this portion of subduction zone at depths shallower than 50 km. We apply the interseismic GPS data to invert the source slip pattern on the subducting fault plane. In addition, several source rupture scenarios with different characterized slip patterns are considered to simulate the ground shaking based on 3-D spectral-element method. We analyze ShakeMap and ShakeMovie from the simulation results to evaluate the influence over the island between different source models. A dispersive tsunami propagation simulation is also carried out to evaluate the maximum tsunami wave height along the coastal areas of Taiwan due to coseismic seafloor deformation of different source models. From the results of all rupture scenarios, the peak ground acceleration larger than 1g can be observed in many areas even though the rupture occurs off northeastern coast of Taiwan. The tsunami simulation results show that the sea level raised significantly along the eastern coast, especially in the offshore area of Ilan where tsunami high can over 20 meters. The results of this numerical simulation study can provide a physically based information of megathrust earthquake scenario for the emergency response agency to take the appropriate action before the really big one happens