Fig. 2

Example of waveforms at IGK. a 2–4 Hz band-pass-filtered waveform. Data acquired at 12:00–13:00 on June 1, 2005 (UT). Red arrows show the low-frequency earthquakes (LFEs). Yellow arrows show the ordinary earthquakes. b 0.02–0.05 Hz band-pass-filtered waveform. Time range is the same as a. Blue arrows show the very low frequency earthquakes (VLFEs). c Examples for the event at 12:00:00 enlarged from a and b. Upper and lower figures show the waveforms of LFEs and VLFEs, respectively. Red and blue dotted lines show the root-mean-square (RMS) waveforms of 2.00–4.00 and 0.02–0.05 Hz band-pass-filtered waveforms using 10-s boxcar window. d Same as c but for the event at 12:28:20. e Same as c but for the event at 12:45:00