1. Ueno H., et al., Newslett. Seismol. Soc. Jpn 14, 3 (2002).
2. Long-period volcano seismicity: its source and use in eruption forecasting
3. We used the Sompi method (22) to determine the complex frequencies (frequency and quality factor Q ) of the decaying harmonic oscillations in the tail of the VLP waveform.
4. The source-time functions of six moment-tensor components were simultaneously determined by the inversion method of (16) in which we applied a slight procedural modification as given by (23). We used three-component displacement seismograms from five stations (Fig. 1C) which were bandpassed between 5 and 15 s for our inversion assuming a point source. We found that a one-cycle cosine function with the characteristic period of 2.0 s was appropriate as an elementary source time function for this inversion. We conducted a grid search to find the best-fit point-source location which is determined at a depth of 5 km below the northern flank of Mount Hachijo Fuji (Fig. 1C). The amplitude ratios for three principal axes of the best-fit moment-tensor solution in Fig. 2 are roughly represented by 1:1:2 where the axis with the maximum amplitude is slightly rotated (roughly 15°) counterclockwise from the east-west direction in the horizontal plane. This feature can be interpreted as a vertical crack if we assume a Poisson ratio of 0.33 in the source region (16). The solution is robust although the directions of the principal axes depend slightly on the source location. We also performed an inversion assuming six moment-tensor and three single-force components and found that the force components contribute less than 10% of the waveform amplitudes. Numerical tests indicated that this result is roughly the same as that from poorly resolved force components caused by a lack of the capability to decouple a moment-tensor component from a single-force component with the same direction because of an insufficient receiver coverage. Therefore a significant single force is not resolved in these signals.
5. Dynamics of a fluid-driven crack in three dimensions by the finite difference method