Tilt data at the IWTH25 station. (a) Tilt change data observed by the tiltmeter installed at the bottom of the borehole at the IWTH25 station. The full scale of the recording system was about ±1.6 × 10−3 radians (the same as the full scale plotted). The static tilt changes were observed after the large amplitude oscillation ended. (b) Velocity seismogram of the surface accelerometer at the IWTH25 station. The initial offset in the original accelerogram was removed before numerical integration. The gradient of the velocity observed after the main waveforms corresponds to the acceleration step caused by the tilt change.