11:45 AM - 12:00 PM
[SVC38-34] Seismic activity beneath Kikai Caldera by using Ocean Bottom Observation
In general, volcanoes are strongly inhomogeneous regions compared with ordinary crusts. The seismic waves propagate to stations under the influence of scattering and diffraction due to random inhomogeneities along the seismic ray paths. Actually, the monochromatic tremor observed in the Kikai caldera is detected only at the stations near the place where the hydrothermal activities were observed by the high-performance camera of the remotely operated vehicle (ROV) and multi-beam echosounder mapping, suggesting strong three-dimensional inhomogeneity in the volcanic body. An amplitude envelope method is powerful tool for analyzing seismic waves propagating through such a three-dimensional inhomogeneous medium. In this study, a method based on Takahashi et al. (2007) focusing on peak delay time from direct S-wave onset to the maximum amplitude arrival of its envelope was applied to about 1100 low frequency tremors and about 32 high frequency earthquakes.
For high frequency earthquakes and some low frequency tremors, we picked up the onset times arrival of P- and S-waves recorded clearly to determine the hypocenters. The hypocenters were vertically divided into two parts at a depth of around 10 km, most of which occurred at the shallower parts were laterally expanded within the lava dome. In addition, we measured the peak delay times for S-wave seismograms filtered at four frequency bands of 2-4, 4-8, 8-16 and 16-32 Hz and calculated root mean square envelope for the sum of two components in those bands to investigate a frequency. In this presentation, we will discuss characteristics of the obtained peak delay times.