B-type and explosion earthquakes recorded at Asama Volcano, central Ja
pan, are analyzed. Waveforms in B-type events are gradational from a h
igh-frequency B-type (BH-type) to a low-frequency B-type (BL-type). Me
dium-frequency B-type events also exist. The focal depths of BH-type e
vents are deeper than those of BM-type and BL-type events. The low-fre
quency contents (1-4 Hz) which predominate in explosion events and typ
ical BL-type events are probably Rayleigh waves. The high-frequency co
ntents (5-9 Hz) of BH-type events can be interpreted to be body waves.
Medium-frequency B-type events exhibit the wave characteristics of bo
th BH-type and BL-type events. We present a simple P-wave source model
for B-type and explosion earthquakes. The P-wave source model can sat
isfactorily explain the distribution of initial motions and difference
in the wave characteristics of these events. The waveform difference
between BH-type and BL-type events is attributable to the different so
urce depths. We conclude that B-type and explosion events are due to g
as explosion or gas emission.