Spatiotemporal monitoring of a frequently-slip fault zone using downhole distributed acoustic sensing at the MiDAS Project
The occurrence of seismic or aseismic slips on a fault is primarily controlled by fault zone structures and their properties. However, the buried subsurface locations or strongly weathered outcrops of active faults often pose a challenge for conducting high-resolution in-situ observations. The Milun fault ruptured both to the surface during the Hualien earthquakes of 1951 and 2018 in eastern Taiwan with a relatively well-known geometry, offering a unique venue to investigate the active fault zone using a cutting-edge distributed acoustic sensing (DAS) technique. DAS utilizes the interaction of photons with intrinsic defects of fiber to translate the phase shift of scattering echoes into longitudinal dynamic strain every few meters along the fiber, enabling continuous and high-resolution monitoring across the fault zone. The Milun Fault Drilling and All-inclusive Sensing project (MiDAS), launched in late 2021, drilled two holes in the hanging wall (Hole A) and footwall (Hole B) of the Milun fault and reached the fault zone at a depth of approximately 500 meters in Hole A. A 3-D fiber array including surface segments connecting two downhole fiber segments was deployed sequentially and completed in June 2022. The DAS strain rate data shows good agreement with the acceleration data of a nearby geophone in terms of both spectrum and amplitude. The high-density sampling of downhole fibers allows us to identify a 20-meter-thick major fault zone and several hidden faults at different depths in Hole A. An amplitude-based method is also proposed to employ the unique properties of the strain data to directly map the subsurface velocity structures over time. The derived velocity profiles are consistent with the logging velocity profile. With high spatial resolution from DAS, the method may offer a new means for detailed temporal structural monitoring for wide applications such as energy exploitation, groundwater management, and geohazards monitoring.