Fault materials and creep characteristics in mudstone areas: A case study of Chegualin Fault in southwestern Taiwan

The active Chegualin Fault in southwestern Taiwan's mudstones exhibits high annual horizontal and vertical displacement rates, severely damaging the viaduct section of National Freeway No. 3 and endangering driving safety. The Chegualin Fault displays interseismic creep characteristics, which have always been a topic of concern. This study discusses the factors that affect fault behavior, including the mineral phases, microstructure, water content, water-rock reactions, and water sources in the Chegualin Fault zone. We proposed a fault creep model and monitoring indicators for the fault creep mechanism. The experimental results revealed that the deformation and weakening mechanisms of the Chegualin Fault are primarily associated with mechanical deformation, mineral composition, water, and chemical reactions. Optical microscopy demonstrated the development of microstructures in the fault rock influenced by stress, while scanning electron microscopy showed feldspar alteration, sulfate mineral precipitation, and carbonate mineral deposition, indicating fluid involvement and water-rock reactions. The high content of smectite clay minerals in the fault core influenced fault slip by affecting water content, friction coefficients, and healing rates. Water, a crucial factor influencing fault rock deformation and movement mechanisms, originates from three primary sources: meteoric water, formation water, and clay-mineral dehydration. Thermogravimetric and differential thermal analyses, as well as Fourier-transform infrared spectroscopy, showed a higher water content in the fault core and damage zone, predominantly in the form of adsorbed and interlayer water. The proposed Chegualin Fault creep model consists of six stages: I. Increase in water supply; II. Weakening of the fault; III. Stress release leading to fault slip; IV. Fault-slip caused rock fragmentation and microstructural development; V. Diffusion and adsorption to reduce water content; and VI. Fault restoration to stability and initiation of healing. This model suggests that during periods of increasing water supply, the fault underwent a cycle of slow-slip events and exhibited periodic creep behavior.