17:15 〜 19:15
[STT42-P09] Thermal Characteristics of the Milun Fault Using Borehole Optical Fiber Sensing
キーワード：Optical Fiber, Subsurface Temperature Anomalies, Distributed Temperature Sensing
Understanding the thermal characteristics of fault zones is crucial for gaining insights into earthquake dynamics, fluid transport processes, and fault mechanics. Temperature anomalies within fault zones are generally attributed to frictional heating during seismic slip, fluid migration, or lithological variations. The Milun Fault, an active east-dipping, left-lateral reverse fault, is located in the northern section of Taiwan’s Longitudinal Valley, near the boundary between the Eurasian Plate and the Philippine Sea Plate. This fault has generated multiple major earthquakes, including the 1951 Longitudinal Valley Fault Earthquake and the 2018 Hualien Earthquake. Due to its frequent slip behavior and potential seismic hazard, the Milun Fault Drilling and All-inclusive Sensing Project (MiDAS) initiated a scientific drilling operation in 2021 to monitor the fault’s thermal and mechanical properties. The MiDAS Hole-A observation site is situated on the northwestern edge of the Milun Terrace and directly intersects the fault zone at a depth of 520-540m. The borehole is equipped with a Distributed Temperature Sensing (DTS) system, enabling high-resolution, continuous thermal monitoring along its depth.
Since December 2022, DTS measurements have been conducted to monitor long-term temperature profile along MiDAS Hole-A, from the surface to a depth of ~700m. These measurements capture variations in the geothermal gradient in the subsurface, which might be influenced by environmental changes. In addition to that, a notable temperature anomaly (~0.04oC) has been consistently observed at depths of 540-580 m, located below the fault zone, throughout the year. The absence of significant temperature changes within the fault zone suggests that this anomaly is unlikely to result from frictional heating associated with coseismic slip in 2018. Instead, this deviation in the regional temperature profile may be related to compositional variations in the fault or fluid movement.
Notably, a significant Mw 7.4 earthquake on April 3, 2024, occurred near the MiDAS site. Following the earthquake, DTS data revealed a subtle temperature variation in the previously identified anomaly at 540–580 m, located below the fault zone. The cause of this anomaly remains unclear. To further investigate this phenomenon, we continue to conduct a comprehensive analysis of long-term DTS data to assess the potential influence of external factors, such as fluid migration or seismic activity. These investigations will contribute to a better understanding of the mechanisms driving temperature variations near active fault zones and enhance our ability to interpret thermal signals in seismically active regions.
Since December 2022, DTS measurements have been conducted to monitor long-term temperature profile along MiDAS Hole-A, from the surface to a depth of ~700m. These measurements capture variations in the geothermal gradient in the subsurface, which might be influenced by environmental changes. In addition to that, a notable temperature anomaly (~0.04oC) has been consistently observed at depths of 540-580 m, located below the fault zone, throughout the year. The absence of significant temperature changes within the fault zone suggests that this anomaly is unlikely to result from frictional heating associated with coseismic slip in 2018. Instead, this deviation in the regional temperature profile may be related to compositional variations in the fault or fluid movement.
Notably, a significant Mw 7.4 earthquake on April 3, 2024, occurred near the MiDAS site. Following the earthquake, DTS data revealed a subtle temperature variation in the previously identified anomaly at 540–580 m, located below the fault zone. The cause of this anomaly remains unclear. To further investigate this phenomenon, we continue to conduct a comprehensive analysis of long-term DTS data to assess the potential influence of external factors, such as fluid migration or seismic activity. These investigations will contribute to a better understanding of the mechanisms driving temperature variations near active fault zones and enhance our ability to interpret thermal signals in seismically active regions.