Fig. 8: Finite-difference simulation for DDSS records.

a The 2-D finite-difference model with a P-wave propagating vertically upward (green arrows) in a simplified fault geometry at Hole-A for varying material properties (phase velocities, densities, Supplementary Fig. 14) in a two-layered fault zone (gray and dark gray, 490-540 m indicated by inserted Figure) and the background medium. The layer thicknesses are derived from the fault core and corrected for geometric effects. Under the assumption of 70° fault dipping, the gouge layer has a thickness of 6.8 m, and the spotted schist layer has a thickness of 10.3 m. The origin of the x-coordinate coincides with the ___location of the borehole. b Simulated strain-rate amplification depth profile. The median of the amplifications and their range (green band) are calculated similar to those for P waves and Rayleigh waves (Figs. 4 and 5) and is relative to the average amplitude between 600 m to 700 m. The dotted red lines show the amplification of the strain rate from local earthquakes (Fig. 2b) for reference. Highest variability occurs within the depth range of the fault core. c Simulated strain-rate wavefield (first 0.45 s) with color-coded amplitudes every 2 m along the borehole. The dashed black lines enclose the portion of the wave field in which the peak absolute amplitude shown in (b) is picked, and the fault zone is between the green dashed lines, with observable amplified strain rates.