本研究以地震震源分佈為主探討在南部地區台灣造山運動初期西邊邊緣岩石圈連續變形情形，以了解台灣造山之初期機制。我們發現菲律賓海板塊有一個南北走向的邊緣。此邊緣先向西俯衝，在121.5ﾟE附近其上部開始撓翹而仰衝到南海岩石圈上方，成為弧前地條(forearcsliver)。菲律賓海板塊前緣之上部構造有兩個各約為20公里厚的副層，上副層地震密集，主要為地殼，下副層地震明顯少很多，可能是地函上部。此弧前地條撓翹到南海岩石圈之上方的情形可由此兩副層的態勢明顯看出，撓翹的程度由南至北變大。由此菲律賓海板塊前緣撓翹的弧前地條可以看出，中央山脈是由此弧前地條向西向上推舉馬尼拉海溝前方之增積岩體所造成的。由海底地形中央山脈的前身可以看出，此造山運動在菲律賓海板塊與歐亞大陸板塊接觸前即早已開始。

In order to understand the incipient orogeny of the Taiwan island, in this paper we studied the lithospheric structure in the southern Taiwan area based on the hypocentral distribution of earthquakes recorded by the seismic network on Taiwan. We found that the behavior of the lithospheric forearc sliver of the Philippine Sea plate in northern Manila subduction zone plays an important role in the growing of the Hengchun accretionary prism and in the incipient Taiwan orogeny. The sliver consists of two layers about 20km thick each, with the upper layer having high seismicity, which is the crust, and the lower layer low seismicity, which may correspond to the uppermost mantle. The whole lithosphere of he Philippine Sea plate underthrusts westward beneath the Luzon arc, and then the forearc sliver warps up and separates from the underlying lower part of the lithosphere, which may still underthrust further west and then sinks together with the eastward-subducting South China Sea lithosphere beneath Taiwan. The forearc sliver then overthrusts along the upside of the South China Sea lithosphere, and pushes up the accretion prism on the plate boundary. This situation develops gradually northward from near latitude 20oN to enlarge the Hengchun accretionary prism up to southern Taiwan, and then intensifies to build the backborne of the Taiwan island. Key words: Incipient Taiwan orogeny, Lithospheric behavior, Southern Taiwan area, Hypocentral profiles.