[S-SS10] Fault Rheology and Earthquake Physics

[SSS10-P13] Mapping Fault Zone Attenuation Using MiDAS Downhole Optical Fiber and Borehole Seismic Arrays

*Lu Hsiao¹, Kuo-Fong Ma^1,2, Yen-Yu Lin¹ (1.Institute of Geophysics, National Central University, 2.Institute of Earth Sciences, Academia Sinica)

Keywords:Milun Fault, Seismic attenuation, Fault-zone attenuation, Milun Fault Drilling and All-inclusive Sensing Project (MiDAS), Distributed Acoustic Sensing (DAS)

Fault zone architecture offers critical insights into earthquake dynamics. The structural components of a fault zone, including the fault core, damage zones, and surrounding host rocks, play pivotal roles in seismic behavior. The quality factor (Q), representing seismic attenuation (Q^-1), is inversely related to the degree of fracturing within these zones; higher fracture densities typically correspond to lower Q values, indicating greater energy dissipation. Moreover, the permeability of these fractured zones facilitates fluid migration, which can alter fault strength and affect earthquake nucleation processes. 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 been responsible for multiple major earthquakes, including the 1951 Longitudinal Valley Fault Earthquake and the 2018 Hualien Earthquake. The Milun Fault Drilling and All-inclusive Sensing Project (MiDAS) initiated a scientific drilling with two boreholes operation in 2021 integrated with optical fiber sensing and borehole seismometers in 2022. The MiDAS Hole-A intersects the recent slipped fault zone at 520-540m providing the unique opportunity to map fault zone attenuation architecture using high spatial resolution of distributed acoustic sensing (DAS) and borehole seismometer arrays. To characterize the detail attenuation structure, we estimate the attenuation factor Q along MiDAS Hole-A using DAS records, with borehole seismometer data for validation. Our analysis focuses on seismic events approximately 20 km from the borehole site between June and December 2023. Before exploring the high-resolution attenuation mapping within the fault zone using DAS (4m spatial resolution), we first examine two depth station pairs: borehole seismometers MDSA1 and MDSA5 (positioned at 94 m and 638 m, respectively) and optical fiber nodes 1257 and 1390 at corresponding depths. The datasets are in acceleration and strain-rate, which are subsequently integrated into velocity and strain, respectively. We applied the STA/LTA method to identify the seismic arrivals and use polarization analysis to select events with incidence angles below 45°. Arrival time and effective shaking duration (ESD) are used to define P-, S-wave, and noise windows for frequency-domain analysis. We compute the spectral ratios between stations to estimate the attenuation parameters,Q_p and Q_s, from Δt_p^* and Δt_s^* by applying least-squares fitting. Preliminary results indicate consistently low Q (with 10s of values) across the fault zone, with comparable estimates from DAS and borehole seismometers at depths of 94 m to 639 m. Further analysis will refine the attenuation character within fault zone and explore the potential of fiber-optic data for achieving high-resolution attenuation mapping along the fault zone.