Abstract
B-type and explosion earthquakes recorded at Asama Volcano, central Japan, are analyzed. Waveforms in B-type events are gradational from a high-frequency B-type (BH-type) to a low-frequency B-type (BL-type). Medium-frequency B-type events also exist. The focal depths of BH-type events are deeper than those of BM-type and BL-type events. The low-frequency contents (1–4 Hz) which predominate in explosion events and typical BL-type events are probably Rayleigh waves. The high-frequency contents (5–9 Hz) of BH-type events can be interpreted to be body waves. Medium-frequency B-type events exhibit the wave characteristics of both 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 satisfactorily 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 source depths. We conclude that B-type and explosion events are due to gas explosion or gas emission.
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