Fig. 1: Geological context and fiber array configuration.

a depicts Taiwan’s tectonic setting with topography, bathymetry, major trenches (purple lines), active faults (black lines) according to the Taiwan Earthquake Model (TEM²⁰), and the experiment site in Hualien (red box). Major historical earthquakes in the Hualien region (gray), the 2018 and 2024 Hualien earthquakes (black and red) and two earthquake sequences in 2022 used in this study (June in blue and March in orange, respectively) are shown with magnitudes, and focal mechanisms. The earthquakes (yellow) used in the cross-correlation analysis of P-wave velocity profile, and the local M_L 5.8 earthquake (magenta) used for the reliability analysis of borehole DDSS records are also shown. The inset in a shows locations (stars) of eleven teleseismic earthquakes used in this study with color-coded magnitudes. b Zoom-in on the Hualien region, showing geological units and the Milun fault (red line), along with the fiber array ___location (blue box). c Zoom-in to the fiber array ___location, detailing its outline (purple line) and boreholes Hole-A, -B, and -C, and geophone array locations (blue triangles). Fiber was installed in Hole-A and Hole-B on either side of the Milun fault (red line), which experienced surface ruptures during the 2018 Hualien earthquake (red stars). d Cross-section along the fiber array parallel to (c)’s horizontal axis, showing the fiber ___location (red line) in Hole-A and Hole-B (black lines) and the locations of the surface station (broadband and force-balance accelerometer, blue diamond) and borehole seismometers with different instrument types (fullband, orange square; acceleration, circle; short period, triangles).The Milun fault’s ___location (dashed black line) was identified from a seismic reflection profile (gray shaded image in the background, Fig. S1). Borehole coring took place in parts of Hole-A and Hole-C (yellow boxes).