Fig. 3: Borehole core samples, logging, and DDSS records.
a The four primary units identified from borehole coring: the host rock consists of sediments (conglomerate), the Milun Fault Zone (MFZ, ~490 m −520 m) includes metamorphic rocks (schist and serpentinite) and the fault core comprises fault gouge (~520 m-540 m). b Locations of units in Hole-A and -C cored section (see Fig. 1d), color-coded as in (a). c Borehole logging data with depth, 450 m to 550 m, corresponding to (b) as light and darker shaded gray zones within the dashed lines correspond to the MFZ, and fault core, respectively, indicating fault zone features: resistivity (red), gamma ray counts (green, Hole-A; dark green, Hole-C), compressional wave velocity (VP from logging in light blue, and smoothing one in solid light blue line), and VP by cross-correlation analysis of DDSS records (dark blue, uncertainties with blue band). d Zooming out to the whole hole depth (0-700 m) of the fault zone highlights the contrast in quantities from (c) between the fault zone and host rock. Gray zones indicate MFZ and fault core, respectively. e Example DDSS record (event 4 of Supplementary Fig. 6, Supplementary Table 1) of first arrivals used for cross-correlation analysis (color-coded, waveforms at select channels with black lines) with notable variations in phase shift and amplitude in the MFZ (within two dashed lines), particularly in the lower portion linked to fault core. f Average amplitude spectrum of events used in cross-correlation analysis, including waveforms from (e) normalized to spectral amplitudes in the deepest borehole section (bottommost 100 m). Elevated amplitudes within the MFZ—especially in the fault core—are present across all frequencies. Resonance effects within and above the MFZ further amplify seismic signals at certain frequencies.
