1999年9月20日（格林威治時間）臺灣地區發生了二十世紀以來規模最大的地震（Mw 7.6），在集集地震發生的同時，臺灣中西部濱海平原地區激發出長週期的表面波，而中央氣象局TSMIP強震觀測網對這些表面波資料有著相當好的紀錄。雖然並沒有任何直接的災情報告是與這些表面波有關，但是台中港區圓柱型儲密槽的損毀可能是與此表面波有關。此外表面波對興建中的高速鐵路高架軌道系統與長跨距橋樑將是可能的潛在破壞因子。在本研究中，我們以找出在台灣中部平原上激發出表面波的條件為主要研究課題。使用的數值計算方法是虛擬頻譜法（pseudo-spectral method）。模擬過程中，我們針對各種可能影響表面波生成的參數如破裂速度、斷層傾角及斷層破裂是否到達地表均加以測試。 研究結果獲得三項主要結論。一是只要在台灣中部麓山帶發生中大型地震，而且破裂面有達到表層低速沈積物或地表，則會產生長週期表面波。二是破裂速度會控制表面波持續時間的長短，較慢的破裂速度會產生較長的表面波訊號。這可引申當有一地震以慢速度破裂，並破裂至地表則激發出之表面波會有相當大的振幅並且有相當長的持續時間。三是地震破裂面之上部近地表部分能量主要控制表面波之生成，而深部破裂能量則控制體波訊號。 The 20 September 1999 Chi-Chi, Taiwan, Earthquake (Mw 7.6) was the largest earthquake to strike Taiwan in the twentieth century. At the mean time, long-period surface waves were excited in the western coastal plain of central Taiwan. The seismic signals were well recorded by the Central Weather Bureau’s strong motion seismometers. The surface wave period was between 2 and 10 seconds. Although there was no report on direct damage caused by surface waves, the breakage of several syrup tanks, at Taichung harbor, is suspected to be the result of surface waves. The long period surface wave is a potential source of damage to long period bridges and elevated railroad systems, such as high-speed bullet trains. In this study, we focus on simulating the factors that control the generation of long period surface waves in the western plain of central Taiwan. The numerical method used is the pseudo-spectral method. We tested several parameters to see the effects on generating surface wave signals. The tested parameters are the rupture velocity, fault dip angle, and whether the fault rupture arrives at the ground surface or not. In this study, we reach three conclusions. First of all, if a medium-sized earthquake occurs in the foothills of central Taiwan, long-period surface waves can be generated in the western coastal plain only if the fault rupture has reached the top low velocity sediments or the ground surface. Second, rupture velocity will strongly control the duration of surface waves. A slower rupture velocity will generate longer surface waves. Third, the excitation of surface waves is mostly controlled by the near surface fault rupture; the deeper part of the rupture will only affect body wave signals.
