Source spectrum and source time function of volcanic tremor determined with a dense seismic network near the summit crater of Izu‐Oshima Volcano, Japan

Abstract

Digital seismic records of episodic volcanic tremor, obtained with a dense seismic network near the summit crater of Izu‐Oshima volcano, were analyzed to determine source spectrum and source time function. Source spectrum and transfer function could be separated because the seismic records showed a systematic change with distance from the source. The source spectrum of velocity amplitude had a different frequency, ƒ, dependence above and below a corner frequency of 8 to 10 Hz. At high ranges, the spectrum was proportional to ƒ−2, while at low ranges, it was proportional to ƒ2. Inversion of this frequency‐dependent source spectrum yields a source time function that can be represented by an impulse that attenuates in about 0.1 s. Repeated impulses could explain observed volcanic tremor that persists for many minutes or longer and that have complicated phase spectra. The source spectrum gives an energy release rate of about 5.2×102 J/s, so that the total energy released is about 1.0×105 J during a tremor episode of about 3 min at Izu‐Oshima. Such energy release is comparable to the seismic energy released by an earthquake of magnitude 0.1.