Earth Planets Space, Vol. 60 (No. 8), pp. 871-875, 2008LETTER
Youichi Asano1, Kazushige Obara1, and Yoshihiro Ito2
1National Research Institute for Earth Science and Disaster Prevention, Ten-nodai 3-1, Tsukuba 305-0006, Japan
2Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Aramaki-aza-aoba, Aoba-ku, Sendai 980-8578, Japan
(Received March 12, 2008; Revised April 24, 2008; Accepted April 29, 2008; Online published September 8, 2008)
We have developed a signal-processing method for array data recorded at densely and widely distributed tiltmeter stations in Japan and analyzed the data recorded during the last 5 years. Filtered seismograms were analyzed to detect coherent seismic waves and estimate the epicenters. In order to identify very-low frequency earthquakes (VLFEs) with only low-frequency energies only, we selected only events not listed in the earthquake catalogue provided by the Japan Meteorological Agency from all of the detected events. The epicenters of the obtained VLFEs were estimated to be distributed not only along the Nankai Trough, where the events occurred on the reverse faults in the well-developed accretionary prism, but also in Tokachi-oki without accretion. This result suggests that even in such a region, VLFEs can occur on a shallower portion of the plate boundary than the seismogenic region of ordinary earthquakes and/or in a deformed area of the overriding plate along the subduction zone without accretion.
Key words: Very-low frequency earthquake, array signal processing, subduction, Tokachi-oki.