由於台灣位於菲律賓海板塊與歐亞大陸板塊碰撞的交界帶上，受到板塊間相互作用之影響，產生了許多複雜的構造。本研究利用中央研究院地球科學研究所於2003年2月至8月中旬，在嘉義地區佈置之臨時線形地震陣列，從收到的波形資料中找尋一些特殊的遲達波相(later phases)，對這些波相的波傳走時以波線追跡正演（forward）的方式，建立出台灣中南部可能之二維地下構造，再加上波形上定性的分析以及配合重力資料模擬的結果，共得到以下幾點結論：(1)中央山脈下方深約40~45公里處有一向東傾之速度不連續面的存在，推測與花蓮外海深約35~40公里一向西傾之界面同為莫荷面的位置，顯示地殼在此區因碰撞而有增厚的現象；(2)從波形觀測上發現，似乎受到觸口斷層之影響，地震波穿過斷層帶時有被拉長且頻率減小的現象，使得斷層兩邊測站收到之波形有顯著的不同；(3)觸口斷層西側下方約10公里有一不連續面的存在，角度近乎水平；另外在嘉義市東南方深約12~14公里有一向東傾之不連續面，角度約20~25度左右，這些應是地殼中主要的構造，需更多資料以了解其形成之因素。

Taiwan is located at an ongoing collision boundary between the Eurasian and Philippine Sea Plates. The convergent plates formed a lot of tectonic structures. In this study, we investigated the subsurface structures of the south-central Taiwan by analyzing seismograms of local earthquakes recorded by a linear seismic array in Chiayi area. The linear seismic array was installed by IES (Institute of Earth Sciences, Academia Sinica) between 2003 Feb to middle of Aug. The main method of this study is the ray tracing of some particular later phases pick out from the waveform data of the linear seismic array. The final two-dimensional velocity model was obtained through a detail forward modeling of travel times for these later phases. In addition to ray tracing, this study analyzed waveform data qualitatively and examined the final model by gravity data, and obtained conclusions as follows: (1) The Moho-discontinuity is about 40~45 km deep in the Central Range and dips eastward. But along the coast of Hualien, it is about 35~40 km deep and dips westward. These results reveal that the crustal thickness increases by collision of plates in this region. (2) From the result of analyzing waveform data, we suggest that the wavelength and amplitude have changed between both sides of Chukou fault are caused by the effect of fault zone. (3) There are two velocity discontinuity in Chiayi area. One is located at the west of Chukou fault of depth about 10 km and approximate horizontally dipping, the other one is in the southeast of Chiayi city of depth about 12~14 km and dipping eastward of 20~25 degrees. We suggest these are main structures of crust in this region, and still needs more information to understand the factors which formed them.

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