本研究利用衛星雷達干涉法，量測彰化縣員林地區的地表變形行為；目的除了希望有效偵測該地區快速的地層下陷，同時也希望藉由密集的觀察，尋找地層下陷速率與人為作用以及自然因素之間的關係。研究中我們採用十一幅不同時段擷取的衛星影像，時間從1995到2002年之間，組配成七組差分干涉圖。初步研究成果顯示，員林地區自1995年以來就持續有地層下陷發生，平均下陷速率約為每年1公分；另外，在921地震前後，下陷速率則發生明顯的變化。大約在大地震發生前數個月，地層下陷的速率就有增加的情況，速度比平常大約增加兩倍；而大地震發生後的差分干涉結果更顯示，在地震後僅僅大約一個月的時間內，員林地區地表至少下陷了0.5~1公分。直到地震發生後大約一年，地層下陷速率才逐漸趨緩。本研究顯示，歷史性的地表變形，依然可以藉由調閱衛星影像，用差分干涉法測量出來。至於員林地區地層的下陷，根據初步結果可以推論，它不但受到人為活動的影響(如地下水抽取)，自然界的力量應該也扮演重要的角色(如構造活動或大地應力的改變)。 The behavior of land subsidence in Yuanlin area, Changhua County, Taiwan has been monitored by the two-pass method of Differential Interferometric Synthetic Aperture Radar (DInSAR) during the period from 1995 to 2002. Our interferometric result has shown that the subsidence behavior is unusual right before and after the Chi-Chi earthquake. Two-month before the earthquake, the pre-seismic differential interferogram detects a substantial increase in land subsidence with a prominent U-shaped pattern of groundwater level change. Two days after the devastating earthquake, our one-month image-pair shows a five-fold increase in land subsidence and an apparent shift of subsidence center. In this study, we suggest mechanisms that contribute to land subsidence in pre-seismic, co-seismic and post-seismic. We tend to believe that the circular/elongated pattern shown in our interferograms are caused by a point-source deformation. Besides, strain also plays a very important role in accelerating land subsidence shown in the post-seismic differential interferogram. It causes a very sudden, step-like surge in groundwater. The shaking of the earthquake as well as the increase of groundwater trigger the occurrence of soil liquefaction, in return, accelerating land subsidence. We propose there are two center of land subsidence right after the Chi-Chi earthquake though only one subsidence center can be observed in our differential interferogram.
