利用週期1~8秒的表面波頻散資料研究臺灣的西南部上部地殼速度構造，與影響S波側向變化的因素，一共採用16個地震130筆地震資料來研究，最後的研究結果顯示出表面波的頻散曲線的值有由東北向西南遞減現象，且西部濱海平原較容易觀測到表面波的頻散。以0.20°×0.20°的網格做分區，利用純路徑法求出每個區塊的區域化群速，並繪製臺灣西南部的群速分佈圖，再分區逆推出地下的S波速度。無論是群速分佈或S波分佈都有一個共同的結果：相對速度低區，主要分佈於彰化以南的西部濱海平原以及阿里山以南的西部麓山帶；而相對速度高區分佈於臺中附近與阿里山以北的西部麓山帶以及中央山脈西翼地質分區。因此推斷影響速度側向變化的主要因素：若以南北向來比較的話，臺灣西部的岩性由北而南分佈大致是砂岩、頁岩以及泥岩所組成，因此靠南部的地區的組成物質相對於北邊鬆、軟，所以震波的傳遞速度就會比較慢；若以東西方向來比較的話，因受到造山運動的影響，變質度由東而西逐漸變低，因此在東邊的高變質度的物質就較西邊的堅硬，所以就有較高的傳遞速度。從逆推的結果顯示在深度10公里之下的S波速度就沒有太大的側向變化，整體速度約3.6km/sec左右，推測這深度已經達到上部地殼的深度。 Shallow-Depth Velocity Structure in Southwestern Taiwan from Short-Period Surface-Wave Dispersion Data Hung-Cheng Chang ABSTRACT Using fundamental-mode Rayleigh- and Love-wave with periods between 1 and 8 seconds, we inferred the shallow-depth velocity structure in southwestern Taiwan. In this study, group velocities were measured by applying the multiple-filter technique. In addition, the phase-matched filter was also employed to refine these dispersion curves. For constructing a 3-D S-wave structure under southwestern Taiwan, we first divided the study region into 28 subregions. Each subregion has a size of 0.2°×0.2°. Second, by a tomographic inversion, images of Rayleigh- and Love-wave group velocities in southwestern Taiwan can be expressed. Subsequently, the 1-D S-wave velocity structure of each subregion was obtained through the simultaneous inversion of both Rayleigh- and Love-wave. Finally, these 1-D structures were combined to construct a 3-D structure in the study region. Results show that the higher velocity distributes in the northeast of study region, and the lower velocity distributes in the southwest one. On the other hand, the depth of upper crust is estimated to be about 9 to 12 km according to the distribution of S-wave velocity. Additionally, several anomalous velocities were also pointed out in the study region, such as Peikang high, Taichung area and Gutingkeng mudstone of south Taiwan. Perhaps, this is related with Matemorphic grad and lithology.