Monitoring System of Non-volcanic Low-frequency Tremors in Southwest Japan using Vertical Seismic Array

Along the Philippine Sea plate boundary in southwest Japan, non-volcanic low frequency tremors (LFTs) and episodic short-term slow slip events are observed (e.g. Obara and Hirose, 2006). Because these phenomena are closely related to the occurrence of a megathrust earthquake, a detailed monitoring is necessary. Geological Survey of Japan, AIST has recently started an integrated borehole observation in southwest Japan for forecasting the Tonankai and Nankai earthquakes. In 2007, we have started monitoring of water levels, strains, tilts, water temperatures and seismic waves at two stations. Each site has three boreholes of different depth levels (about 30m, 200m and 600m) and we installed high-sensitivity seismometers at the bottom of each borehole. At present, other ten stations are also in operation. In order to investigate properties of the LFTs, we calculated a semblance using this vertical seismic array. Results of the semblance analysis suggest that seismic signals of the LFTs are clearly detected even if they have a very low signal-to-noise ratio and an unclear wave phase. The apparent velocity deduced from the semblance analysis is consistent with the S-wave velocity estimated by sonic log, when the tremor activity occurred just below the station. We also found an empirical relation between the apparent velocity and an epicentral distance, suggesting that we can roughly estimate a source location of the LFTs. Based on these observations, we developed a monitoring system of the LFTs by incorporating the semblance analysis. We applied this monitoring system to some tremor activities and confirmed that this system makes it possible to detect and locate minor LFTs, which cannot be determined by an envelope correlation method. Together with the location of the LFTs, focal mechanisms are a key to understand geodynamic processes on the plate interface. Because of the absence of sharp P-wave onsets and low signal-to-noise ratio, general approaches cannot be applied to the LFTs. Therefore we tried to determine the focal mechanism using the S-wave polarization. Although it was difficult to obtain reliable polarization angle of the LFTs, we found that the polarization estimates can be improved by combining information from individual sensors of vertical seismic array. In order to reduce the number of possible focal mechanism solutions, we used the SV/SH ratio for other stations and constrained the range of T-axis direction. A preliminary analysis suggests that the LFTs are characterized by low angle thrust faults, which are consistent with the subduction of the Philippine Sea plate. We are planning to incorporate the focal mechanism determination in our monitoring system in future.