(a) DC: A-type
(b) AC: B-1 type
(c) AC: B-2 type
(d) AC: B-3 type
(e) AC: C-type
Figure 7: Waveform of the pulse-like particular signals. Every event has large strength compared with noise level with the result of none data filtering. (a) DC band (type A). Type A has a duration of several tens of seconds to several tens of minutes and a height of about 2-3 mV. The form was first identified by the temporally rapid recording at the time of volcanic eruption activities in 1990 at Izu-Oshima [23, 25]. Afterward, we detected similar signals at almost cases of nearby volcanic eruption and seismic swarms [25]. (b) AC band (type B-1). Type B-1 waveform of the pulse-like signals in the AC band. The form is very similar to type A except that it is of smaller duration (several tens of ms) and shows several steps in the process of relaxation. There are events both with positive and negative polarities. But the majority events are positive. (c) AC band (type B-2). Type B-2 waveform of the pulse-like signals in the AC band similar with a wave-packet. There is no coda phase, in contrast to seismic waves. A similar pattern was detected in the case of wetted granite specimens with a dominant frequency of 500 kHz [12, 13]. The S-P time was about 30 ms, suggesting an epicentral distance of 270 m. The absence or very slight appearance of a P phase indicates an epicentral distance larger than a few km. (d) AC band (type B-3). Type B-3 waveform in the AC band with a waveform compounded by type B-1 and type B-2. There are different degree of compounding. (e) AC band (type C). Type C waveform in the AC band. The waveform is the same as type B-2, but frequency is about 5 kHz and amplitude is very large of 20–30 mV, some 10 times larger than those of types A and B types.