| 摘要: | 臺灣位於歐亞大陸板塊及菲律賓海板塊碰撞帶，屬於太平洋地震帶的一部分，淺層強震紀錄中約五成分佈於東部外海，且根據歷史紀錄，自西元1771年起，臺灣東部曾有十四起海嘯事件的記載，因此臺灣東部海嘯的危害度分析，有其重要性及應用的價值。本研究在北緯21-26度，東經121-125度之區域內，每0.5度劃分為一個海嘯源發生區，共有70個海嘯源發生區，每次模擬在一個0.5 × 0.5度的中心，放置一個8 × 8公里，高10公尺的波高，以線性淺水波方程為波傳之控制方程，波傳範圍為北緯21-27度，東經119-125度，模擬時間共計120分鐘，海底地形精度為一分，並在臺灣東部設置8個虛擬驗潮站，模擬出理論到時與最大波高。由於海嘯波有線性疊加的特性，在此海嘯源區的設置，可視為虛擬地震中，地震斷層區域錯動的分量，對於真實地震海嘯的模擬，可依此海嘯源區做結合，依不同錯動分量的尺度加以線性結合。但地震破裂的方向性在此並無列入考量。藉由此模型之結果分析，歸納出由路徑效應引起的波線轉折現象：振幅集中於琉球島弧區域並沿著琉球島弧由東向西朝宜蘭平原彎曲、與那國島與綠島、蘭嶼之繞島效應、加瓜海脊之能量反射，其中琉球島弧及與那國島的雙重效應使得與那國島東方與八重山群島的其他島嶼之間的區域成為海嘯潛勢最高的區域。而海嘯的到時與最大振幅之分析中，顯示海嘯潛勢較高區域為琉球島弧及近岸地區海域，根據海嘯到時的計算，臺灣東部區域的海嘯傳遞，依據其海底地形可劃分為三區，北緯24.5度以北、琉球島弧區域及其以南區域，其平均波傳速率分別為8.1公里/分, 10.2公里/分及13.9公里/分。此平均波傳速率及振幅衰減式，可應用於未來氣象局建立之海嘯預警系統之初步判斷。本研究並藉由淺水波方程模型與布氏方程模型，驗證臺灣東部之海嘯模擬可忽略頻散現象，即線性淺水波模型在臺灣東部外海可做為海嘯估計之下限。 Historically, literature had shown that eastern Taiwan had been attacked by tsunamis at least 14 times since 1771. For the location of Taiwan as in the Circum-Pacific Seismic Belt, many large earthquakes occurred in and offshore Taiwan, especially in eastern Taiwan, with frequent events for the magnitude of 6 or larger. To examine the tsunami threat in eastern Taiwan, through numerical simulating, we assess the tsunami potential to help on the precaution against tsunami calamities. To model the possible tsunami occurred in eastern Taiwan, we divided the eastern offshore Taiwan into 70 different source regions of latitude of 21o to 26 o, and longitude of 121 o to 125 o to examine the tsunami propagation characteristics (path effects) from finite-difference method by solving linear shallow water wave equation in spherical coordinates. The source region has a dimension of 0.5 degree by 0.5 degree. We gave an area of 8km by 8km with 10m uplift for the tsunami source in the region, individually, to examine how the tsunami propagating toward Taiwan from each source region. A grid size of 2 km is utilized in the tsunami simulation with 1 minute high resolution bathymetry. By analyzing the maximum amplitudes and the arrival time for the assumed stations around the coastal of Taiwan, we select special cases those have significant effect toward Taiwan to discuss the path effects and bathymetry affections. We found that the most possible tsunami threat region in eastern Taiwan is Ilan. Ilan plain located close to the end of the Okinawa trough, tsunami waves tend to lead to Ilan by following the trough structure. The Gagua Ridge in eastern offshore Taiwan, actually, diverts the wave to prevent the tsunami threat from long distance. Considering the deep water depth in eastern Taiwan, we also try to discuss about the dispersion effect on tsunami propagation for deeper water events to the south of Ryukyu trench. The results show that the dispersion effect from the deep ocean is not significant for the tsunami modeling in eastern Taiwan. Our simulations also show that the tsunami speed in eastern Taiwan can be divided into three regions, north of latitude 24.5 o, the Ryukyu arc region, and the south region of the Ryukyu arc. The average tsunami speed in these three regions are：8.1km/min, 10.2km/min, and 13.9km/min, respectively. These values and the amplitude attenuation characters can be used for the future tsunami warning. The identified tsunami potential regions in offshore Taiwan could also be examined with the background seismicity and possible marine landslide to further investigate the possibility on the occurrence of the larger events (6 or larger) for the future studies. The overall survey on the tsunami potential in Taiwan region will toward the goal for future tsunami warning, together with recent developing earthquake warning system in Taiwan. |
