Development of volcano monitoring technique using repeating earthquakes observed by the Volcano Observation Network of NIED

After the Grate East Japan Earthquake (M9.0) on March 11, 2011, the M6.4 earthquake occurred beneath Mt. Fuji on March 15, 2011. Although the hypocenter seemed to be very close to an assumed magma chamber of Fuji volcano, no anomalies in volcanic activity have been observed until August 2012. As an example, after the M6.1 earthquake occurred in 1998 at southwest of Iwate volcano, a change of seismic velocity structure (e.g. Nishimura et al., 2000) was observed as well as active seismicity and crustal deformation. It had affected waveforms of repeating earthquakes occurring at a plate subduction zone, that is, the waveform similarities were reduced just after the earthquake due to upwelling of magma. In this study, first we analyzed for Mt. Fuji where such changes are expected by the occurrence of the earthquake to try to develop a tool for monitoring active volcanoes using the Volcano Observation network (V-net) data. We used seismic waveform data of repeating earthquakes observed by short period seismometers of V-net and the High Sensitivity Seismograph Network Japan (Hi-net) stations near Fuji volcano after 2007. The seismic data were recorded with a sampling rate of 100 Hz, and we applied 4-8 Hz band pass filter to reduce noise. The repeating earthquakes occurred at the plate subduction zone and their catalog is compiled by Hi-net data (Kimura et al., 2006). We extracted repeating earthquake groups that include earthquakes before and after the M6.4 earthquake on March 15, 2011. A waveform of the first event of the group and waveforms of the other events are compared and calculated cross-correlation coefficients. We adjusted P wave arrivals of each event and calculate the coefficients and lag times of the latter part of the seismic waves with the time window of 1.25 s. We searched the best fit maximizing the cross-correlation coefficients with 0.1 s shift time at each time window. As a result we found three remarkable points at this time. [1] Comparing lag times of (a) a pair that both earthquakes are before March 15, 2011, and (b) a pair that before and after March 15, 2011, (b) has bigger time delay at several stations than (a). [2] The delays for several pairs of earthquakes are obtained at NIED V-net stations established just around Mt. Fuji. On the other hand Hi-net stations which are far from the edifice have the smaller delays. [3] Some stations, FJNV, FJHV, FJYV, located on the west part of the edifice have bigger delays than the others. An obvious delay comes out at the S wave coda part, and does not in the P wave coda part or S wave. As one of the possibilities to create the delays, it is considered that, for example, seismic velocity structure beneath Mt. Fuji had changed since the M6.4 earthquake. In the case of the unrest of Iwate volcano in 1998, waves passing through the magma chamber had got delayed (e.g. Yamawaki et al., 2004). It is important to install borehole type seismometers just around a volcano, like V-net stations, to get high quality seismograms for precise study like this analysis and to get some changes of volcano activities.