1. Long-period volcano seismicity: its source and use in eruption forecasting
2. Volcanic tremor: Nonlinear excitation by fluid flow
3. NIED Hi-net is a newly established seismic network (10). Each station consists of a three-component velocity seismometer with a natural frequency of 1 Hz installed at the bottom of a borehole with a depth of 100 to 200 m. The data are digitized at each station with a sampling frequency of 100 Hz and then the data packets attached with the absolute time information from a Global Positioning System clock are transmitted to the data center.
4. Vertical-component waveforms for a pair of stations are converted to envelopes with a frequency range of higher than 4 Hz and with a smoothing time of 10 s and they are resampled with a sampling interval of 1 s. A pair of envelope seismograms with a length of 2 min is used for calculation of the cross-correlation coefficient by moving a trace with the time lag of every 1 s to another fixed reference trace. The time lag which gives the maximum correlation coefficient should be the difference of the arrival time for a coherent seismic signal in the selected 2 min observed in the two stations. If the maximum correlation coefficient is less than 0.9 the time lag is not applied for the further process because there is no coherent signal. Such a correlation process is carried out for all pairs of stations in the target area. The measured time lags with a good correlation are averaged spatially to calculate the distribution of the relative arrival time like the net adjustment which is used in the geodetic survey. The cross-correlation analysis is carried out with the moving time window of 1 min; therefore we can calculate the location of tremors once every 1 min continuously.
5. A. Katsumata N. Kamaya Seismol. Soc. Jpn. Fall Meeting B16 (2001).