為探討三義斷層是否為活動斷層，本研究採用淺層反射震測法，在斷層可能經過之位置佈線施測，除了尋找斷層在地表淺處的正確位置，並推論斷層錯動的形式及機制，以進一步瞭解該斷層之特性。本研究由北而南共安排了十條測線，所獲資料品質尚稱良好（深度達600公尺），可以清楚看出剖面東半邊為向東傾斜之地層，應為斷層上盤之東坑層以上之地層，剖面西半邊則為水平之頭嵙山層。
本研究獲得以下幾點結論：
1.三義斷層上盤為東坑層，地層傾角為20度至25度之間，下盤為頭嵙山層，地層維持水平狀。東坑層沿著其底部之三義斷層逆衝到頭嵙山層之上。
2.上盤東坑層內有一些次生破裂，但規模不大，若干可能延伸到地表（如大甲溪北岸露頭），下盤之頭嵙山層則未受到東坑層逆衝太大影響，故在頭嵙山層內並沒有發現任何破裂痕跡。
3.三義斷層之位態及運動行為適用於「薄皮逆衝理論」，此斷層面沿著東坑層底部滑動，逆衝到頭嵙山層上。在大安溪以北，逆衝爬昇至銅鑼向斜及出磺坑背斜南端之上，在大安溪以南，則擠壓上衝至厚層之頭嵙山層之上。
4.由地物資料及鑽井資料可知東坑層底部近地表處已受到侵蝕移除，並由台地堆積層或現代沖積層所取代，此長期之侵蝕沈積，代表此斷層形成年代已經很久。
5.由地形上可發現近地表之斷層線受到河流的侵蝕沈積後，已向東移動了數百公尺，明顯指出三義斷層已經有一段很長的時間沒有活動，故現今活動可能性應屬不高。

The purpose of this study is to re-examine the Sani fault and to determine the degree of its activity. The method of the shallow seismic reflection is used for the fault detection. Besides finding accurate surface positions of the fault, we also carefully examine the patterns of faulting in order to understand its characteristics. More than 10 seismic lines are deployed form the north to the south across the fault. The quality of the obtained seismic profiles is considerably good. We can easily identify the structures dipping to the east with the Tungkeng Formation thrusting over the young Toukoshan Formation. The following conclusions can be drawn:
1.The hanging wall of the Sani fault is the Tungkeng formation which dips to the east by 20o-25o. The footwall is the Toukoshan formation which is relatively flat.
2.There exist several secondary fractures in the Tungkeng formation, which sometimes are large and may extend to the surface. However, the thrusting dose not affect the Toukoshan formation much. No obvious fractures in the Toukoshan formation are found.
3.The behaviors of the Sani fault can be explained by the theory of thin-skinned thrust. The sliding fault plane is along the bottom of the Tungkeng formation.
4.According to the drilling data and the geophysical results, we find that some front parts of the Tungkeng formation have been eroded and replaced by terrace deposits. This strongly implies that the fault may have certain ages and could not be active at the present time.
5.The fault line is found to move to the east by several hundred meters on some deep deposited river bank. It also indicates that the SanI fault has no activity for a long time. The possibility of fault’s reactivity could be quite low.

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