米崙斷層經歷了多次破壞性的地震(如1951年規模7.3的縱谷地震、2018年規模6.4的花蓮地震)，說明其具有高度活躍性。為進一步探討米崙斷層之活動特性，米崙斷層鑽井研究計畫(簡稱 MiDAS)於2020年啟動，引進光纖的地震觀測技術，結合傳統井下地震儀與斷層帶水氣監測，建立跨斷層地震觀測系統。此計畫旨在監測米崙斷層的活動，提供周圍孕震構造及斷層力學數據。先前研究指出，米崙斷層之滑動帶的特徵為層狀分布的黃色和黑色斷層泥，其中黑色泥大多以密集條帶反覆出現，暗示可能為多次滑移事件的應變集中現象。因此，本研究以米崙斷層帶之黃色斷層泥作為實驗材料，利用岩石旋剪試驗，在排水與不排水兩種條件下進行多次快速滑移之摩擦試驗，以探討數次同震滑移時主要滑動帶的摩擦行為和形成機制。結果顯示，在不排水條件下，視摩擦係數(apparent friction coefficient)上升至峰值(peak)約0.5後降至穩態(steady-state)約0.1。而在排水條件下的摩擦行為則較為複雜: 視摩擦係數先上升至峰值約0.55，接著下降至約0.37，隨後再次上升至0.65，並持續震盪(摩擦震幅為視摩擦係數0.25-0.45之間)至滑動結束。隨著滑移脈衝次數增加，後續滑移中的摩擦峰值形狀變得更加尖銳，耗散之破裂功(breakdown work)也隨之減少。實驗後斷層泥的微觀構造顯示，在旋轉端的黃色斷層泥轉變為黑色剪切帶(厚度約為0.2-1.3毫米)，並伴隨著顆粒粒徑減小和非晶質物質的產生。綜合上述結果顯示，自然界中層狀分布的黃色、黑色斷層泥，可能是在排水條件下，經歷多次重複地震滑移事件所形成。此外，破裂功在第一次滑移後大幅降低，顯示當主要滑動帶形成後，後續滑移可能更傾向於沿既有滑動面發生，並以較高的效率、較少能量耗散進行。本研究的結果可能解釋米崙斷層條帶斷層泥的形成機制。;Several destructive earthquakes (e.g.， the 1951 magnitude 7.3 Earthquake and the 2018 MW 6.4 Hualien Earthquake) have been recorded in Hualien City， eastern Taiwan. Seismological observations indicate that these events triggered the rupture of the Milun Fault， suggesting that this fault is highly active. To understand the faulting process of the Milun Fault， the Milun fault Drilling and All-inclusive Sensing project (MiDAS) was launched in 2020 to retrieve borehole cores and to monitor seismic signals and fluid anomalies. Previous studies of borehole cores identified a slip zone within the Milun Fault as a foliated yellow-black gouge layer characterized by grain-size reduction. These observations may imply repeated strain localization during multiple coseismic events. However， the frictional behavior of the Milun Fault and the mechanism responsible for the formation of the foliated gouge remain poorly constrained. In this study， we conducted high-velocity rotary shear experiments on yellow gouge samples retrieved from the Milun Fault， mainly composed of quartz and clay minerals (illite， chlorite， and kaolinite)， and subjected to multiple identical seismic slip pulses (1 m/s) under both drained and undrained conditions. Under undrained conditions， the apparent friction coefficient (μapp = shear stress/normal stress) initially increased to a peak μp ~ 0.5， then significantly decreased to a steady-state value of μss ~ 0.1. Under drained conditions， friction evolved in a more complex behavior: μapp increased to a peak value of μp ~ 0.55， gradually decreased to μapp ~ 0.37， and then increased again to 0.65， followed by an oscillatory manner to the end of the slip. The peak friction becomes sharper with increasing number of pulses， indicating an energy reduction dissipated as breakdown work. Microstructural observations of deformed gouge reveal that part of the yellow gouge was transformed into a fine-grained black shear zone， accompanied by grain-size reduction and the presence of amorphous materials. These observations suggest the foliated yellow-black gouge observed in the natural slip zone may have formed through repeated coseismic slip events under fluid-drained conditions. Furthermore， the reduction in breakdown work suggests the development of a mechanically efficient and localized slip zone that facilitates subsequent slip along the pre-existing PSZ. Our results provide a possible explanation for the formation of foliated gouge within the active fault zone of the Milun Fault.
