Exploring Fault Zone Dynamics and Subsurface Motions: Unveiling the PGV-Strain Relationship with Downhole Cross-Fault Optical Fiber Sensing and Borehole Seismometers

By employing Optical Fiber Sensing techniques in boreholes post-drilling, we effectively map and analyze fault behavior, such as the Milun fault, known for frequent slip events and significant earthquakes like the Mw6.4 Hualien earthquakes in February 2018 and the M7.1 event in November 1951. Additionally, our borehole Distributed Acoustic Sensing (DAS) records several moderate to large earthquakes, including the M7.0 Chishang earthquake on September 18, 2022. Traditionally, seismology relies on peak ground velocity (PGV) as a proxy due to the absence of direct strain measurements. However, the high cost and global limitations of traditional borehole strain-meters have prompted exploration into emerging fiber-optic-based strain sensing technology, offering scalable measurements and a deeper understanding of the strain-PGV relationship. The "Milun fault Drilling and All-inclusive Sensing (MiDAS)" project in Taiwan showcases this advancement, deploying state-of-the-art sensors, including downhole optical fiber and a 3-component seismic array, for long-term monitoring and high-resolution data collection during seismic events like the M7 earthquake sequence on September 18, 2022. We examine the PGV/strain relationship as an indication of earthquake dynamic triggering, considering teleseismic events from 2023 ranging from M6.6 to 7.8, local seismic events like the M7.1 sequence on September 18, 2022, and subsequent events from 202302 ranging from M4.0 to M5.6. By correlating peak strain and velocity, linked to shear-wave velocity, we establish the strain-velocity relationship for the first-time using co-site borehole optical fiber and a 3-component seismic array. We reveal the strain field at depth and the strain-velocity relationship using co-site downhole optical fiber and borehole seismic array data. Studies indicate strain amplification in weak zones regardless of wave types and frequency bands, alongside various modes in frequency contents of subsurface structures, and significant strain energy accumulation near lithology boundaries. Deriving the PGV-Strain relationship for earthquake dynamic triggering involves establishing a triggering threshold in strain, with strain emerging as a direct parameter in seismology, evident in consistent features of local and teleseismic events, thus shedding light on fault zone responses during interseismic periods from dynamic strain energy during teleseismic earthquakes.