2001 年 54 巻 3 号 p. 397-414
We have investigated the waveform characteristics and source mechanisms of deep low-frequency earthquakes (DLFeqs) that occurred near the Osorezan volcano in the Shimokita Peninsula and near the Esan volcano in the Oshima Peninsula, northern Japan. The seismograms were collected by the temporal observation conducted for seven and six months in 1998 and 1999, respectively, using many broadband (with a flat velocity response up to 20 or 30s) and midband (up to 5s) seismometers. DLFeqs in both areas form clusters with focal depths ranging from the lower crust to the uppermost mantle. The most remarkable feature of seismograms is the dominance of low-frequency oscillation around 2Hz. The amplitude of source displacement spectra also shows a prominent peak at the same frequency and rapid decay with increasing frequency beyond the peak. Thus the source spectra of DLFeqs are different from the ordinary ones approximated by the omega-square model. From the broadband spectra we confirmed that the lower bound of significant seismic energy of DLFeqs is above 1Hz. A particle motion analysis exhibits the change in oscillation pattern with time in the major part of seismograms from well-excited S-wave to long-tailed S coda, suggesting the time variation of radiation characteristics of low-frequency waves from the source. The source type of DLFeqs inferred from the amplitude ratio of S- to P-waves is either single force or fault slip in low frequency range (1-5Hz), while the ratio in high frequency range (5-20Hz) shows good agreement with a type of tensile crack or single force. The waveform of initial part of P- and S-waves from one DLFeq can be relatively well explained by a moment tensor solution of a reverse faulting with little non-double-couple component. Though we could not constrain the source mechanism of DLFeqs uniquely from the results of above analyses, we propose the following model under the assumption of a source driven by magma. The pressurized magma in one reservoir extends the preexisting crack, producing high-frequency seismic radiation by a tensile crack. If the crack connects to the other reservoir, magma flow from one to the other excites low-frequency radiation of single force type, sometimes followed by reversed flow to balance the pressure between the reservoirs.