米崙斷層在過去一百年內多次被花蓮地區的地震誘發導致破裂，因此被視為極為活躍的活動斷層。然而，米崙斷層在過去並沒有野外的露頭，缺乏對於斷層帶之觀察。因此米崙斷層鑽井研究計畫(簡稱MiDAS)於2020年成立，旨在透過科學鑽井鑽穿米崙斷層帶並且建立長期的監測系統。本研究利用偏光顯微鏡、熱場發射掃描式電子顯微鏡、穿透式電子顯微鏡、同步輻射X光繞射、拉曼光譜儀以及雷射粒徑分析儀分析MiDAS岩芯並定義米崙斷層的滑動帶。其中，MiDAS井A岩芯中522公尺處有約12公分的黑色層狀斷層泥。結果顯示，斷層帶的礦物組成相似，其中黑色泥的黏土礦物含量較高且含有碳質物以及非晶質物質。此外，我們也在黑色泥中觀察到粒徑減小以及不透光的注入脈，推測黑色泥中可能有應變集中、水-岩反應以及摩擦熱的作用。另外，本研究觀察到廣泛分布(約7公尺)的奈米尺度之纖維狀礦物。此結果暗示著斷層帶中具有熱液活動，而且可能與斷層之強化和癒合相關。本研究推論黑色層狀斷層泥為米崙斷層之滑動帶。同時也暗示米崙斷層為具有熱液活動之斷層帶。本研究所提供的斷層帶特性，說明米崙斷層在過去極為活躍之可能原因，並可供地震物理相關參數之參考。;The Milun fault, Taiwan, was triggered by multiple earthquakes in the Hualien region in the past one hundred years and is therefore considered to be an active fault. However, the fault-zone characteristics of the Milun fault remain unknown due to the lack of an exposed fault zone. The Milun fault Drilling and All-inclusive Sensing (MiDAS) project was designed to penetrate the active fault zone of the Milun fault and to deploy multiple monitoring systems. Here, we characterize the fault zone from the MiDAS borehole cores using microanalytical methods, including optical microscopy, field emission scanning electron microscopy, transmission electron microscopy, synchrotron X-ray diffraction, Raman spectroscopy and particle size analysis. In particular, a 12-cm black gouge layer is observed from MiDAS borehole cores from Hole-A at 522 m. The mineral assemblages of all the samples are similar but with an increase in clay mineral abundance and presence of carbonaceous material and amorphous material within the black gouge. The black gouge also contains reduced-size grains and injection vein. These observations imply strain localization, fluid-rock interaction and potential frictional heating within the black gouge. Furthermore, nanoscale fibrous structures were observed within a wide range of ~7 meters, implying the presence of hydrothermal fluid-rock interaction within the fault zone as well as possible fault strengthening and healing. The principal slip zone of the Milun fault is potentially identified within the black gouge layer. Hydrothermal activities within the fault zone were also implied and its features may explain the short recurrence interval of the Milun fault and the underlying earthquake physics.
