| 摘要: | 本文之研究目的在藉助大地電磁法(MT)多站多頻阻抗分析來還原真實的地下電性構造。經由三維模型二維逆推的模擬分析得到結果： MT對高導電反應極靈敏而對高電阻較不靈敏。此外，多站多頻張量分解可以提取出區域二維構造的信號，捨棄局部三維異常體所造成扭曲的分量，對於還原區域信號有著非常優異的功效。 應用於台灣中部地區與金門地區大地電磁資料，以多站多頻分析後從其一維與二維逆推後的電性構造獲致結果如下： 第一、集集孕震帶在震源區之東西兩側分別存在低、高電阻異常。根據集集地震所推論出的應力分布來看，壓縮體積應力減少了孔隙率而增加了震源附近的電阻率，主震發生時震源兩旁的擴張應力使孔隙水壓迅速下降，但緊接者觀測到孔隙壓力隨後上升，原因是震源壓縮區超壓將地殼流體向擴張區排放，將有助於降低震源兩旁的地層強度並進而導致地層破壞。集集餘震發生區與預期擴張區提供了重要的空間相關證明，擁有深部地殼流體的破裂岩層能增強孔隙間的流通性，因此減低了高導區的電阻率值，大量位於擴張區的餘震群也反映出受到地殼流體的影響。 第二、台灣中部的兩條MT剖面cc-1及cc-2。cc-1逆推結果將電阻率約248 ohm-m視為莫荷面處，可發現莫荷面深度自測線最西端的35km，逐漸上升至測站ccm-4的30km，然後維持相同的深度直到測站ccm-12，之後開始呈現向東漸深的態勢，一直到測線最東端，莫荷面下降至約50km的深度。cc-2逆推結果將電阻率約167 ohm-m視為莫荷面處，可發現莫荷面深度自測線最西端的31km，開始往東漸深；一直到測線最東端，莫荷面下降至約43km。 第三、金門有別於台灣為典型大陸地殼且處於弧後張裂階段，金門地區MT二維剖面的逆推結果中莫荷面深度約為32km上下，其莫荷面電阻率為129~219 ohm-m，莫荷面深度南北向沒有太大變化，但往西漸深。 Application of multi-site and multi-frequency impedance tensor decomposition to resolve the crustal structures of the MT data in central Taiwan and Kinmen area Sung-Ching Chi Abstract The purpose of this study was to strip the shallow 3D resistivity anomalies by using the multi-site and multi-frenquency impedance tensor decomposition (MSMFITD) during the inversion of the MT 2D profile. Computer simulations of the MSMFITD technique, after using the synthetic data coming from the MT 3D forward modeling, indicated that: 1) MT data sensitively responses to high conductors; 2) the MSMFITD is quite powerful to strip the MT data pertubated by the local 3D inhomogenities, and perfectly recovered the true 2D structures. The application of the MSMFITD technique to the MT data collected from the central Taiwan and the Kinmen area obtained three important results: 1) Two resistivity anomalies were dectected around the hypocenter of the Chi-Chi earthquake, the high resistivity anomaly on the west and the low resistivity anomaly on the east. This resistivity pattern mostly indicates that deep-crustal fluids may participate in the rupture process of the Chi-Chi earthquake. A striking spatial correlation between the crustal conductor and occurrence of aftershocks beneath the Chelungpu fault suggests a postseismic pore pressure adjustment ongoing after the mainshock. Additionally, the hypocenter exhibits an electrical resistive zone, consistent very well with a predicted compact zone from a crustal deformation and transient fluid flow modeling. 2) There were two MT profiles in the central Taiwan, cc-1 and cc-2. The resistivity structure of the northen MT profile cc-1 indicated that the depth of the Moho, about 248 ohm-m of resistivity, is at about 35km to the west, uplifting to 30km between MT soundings ccm-4 to ccm-12, and then deepening to 50km to the eastern end of the profile. As to the southen profile cc-2, the depth of Moho, about 167 ohm-m of resistivity , is at about 31km to the west and then gradually deepen to about 43km to the east. 3) The Kinmen islet, which is quite different from the Taiwan island geologically, belongs to the continental crust in the stage of the backarc spreading. Based on the resistivity profiles, in Kinmen area the depth of Moho, 129~219 ohm-m of resistivity, is around 32km with little variation in NS direction, but a little bit deepen to the west. |
