Fig. 4From: Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor amplitudea Fourier spectra for the vertical ground velocity at KSW (Red: for 500 s from 09:58:19; Green: for 100 s from 10:14:09; Gray: for 500 s from 09:50:00). Pink color corresponds to the frequency band of 5–20 Hz, which is focused on our ASL analysis. b Spectrum ratios to background signalsBack to article page