Fluid-rock interaction within the active Milun Fault: In the case of the Milun Fault Drilling and All-inclusive Sensing project (MiDAS)

The 2018 Hualien earthquake (Mw 6.4) resulted in the Milun fault rupture and caused hundreds of casualties. The last rupture of the Milun Fault occurred in 1951, implying a short recurrence interval for the Milun Fault. The outcropped Milun Fault has not been recognized in the field, and its fault architecture and the relevant processes triggered during the seismic cycle remain unknown. This represents a significant limitation in our understanding of fault mechanics and seismic hazard assessment.

The Milun Fault Drilling and All-inclusive Sensing project (MiDAS) was conducted in 2020, and the drilling borehole cores showed the presence of the Milun Fault. The Milun Fault Zone exhibits an asymmetric fault structure, displaying altered spotted schist and non-cohesive serpentinite as the damage zone, and foliated grey and black gouges as the fault core. The damage zone of the Milun Fault has been described as a product of fluid-rock interaction, although direct evidence remains limited.

Here, we conduct synchrotron X-ray diffraction (XRD) on altered spotted schist and non-cohesive serpentinite to investigate fluid-rock interaction during the inter-seismic period. Previous data on the outcropping spotted schist showed that the mineral assemblages are mainly composed of muscovite, feldspar, and quartz. For the outcropping cohesive serpentinite, the major minerals are antigorite and magnetite. Our XRD data show that the altered spotted schist mainly contains quartz, feldspar, and clay minerals such as illite, chlorite, and kaolinite. Non-cohesive serpentinite is composed of chrysotile, talc, chlorite, and actinolite. The altered spotted schist exhibits an anastomosing occurrence, suggesting the presence of fluid-relevant interaction along the fractures and resulting in the observed clay-rich mineral assemblages. The non-cohesive serpentinite shows the reactions of antigorite to chrysotile with some residual antigorite fragments, suggesting the process of severe alteration by low temperature (< 200°C) fluid. To further explore fluid-rock interaction processes, we will conduct X-ray Fluorescence (XRF) analysis to detect changes in chemical elements within the Milun Fault zone in this month. Our findings will help identify the source and composition of the fluids involved and provide insights into the structure and evolutionary history of the Milun Fault.