Fig. 13From: Analyzing the continuous volcanic tremors detected during the 2015 phreatic eruption of the Hakone volcanoComparison of amplitude spectra for the vertical ground velocity waveforms at OWD station. The blue and red lines show the amplitude spectrogram of a seismogram during the acoustic quiet period (indicated by blue line in Fig. 11a) and immediately before the infrasonic onset (indicated by red line in Fig. 12)Back to article page