Deep low-frequency earthquakes beneath the focal region of the Mw 6.7 2000 Western Tottori earthquake

On October 6, 2000, an Mw=6.7 crustal earthquake occurred in western Tottori prefecture, southwest Japan. Beneath the focal region of the earthquake, deep low-frequency (DLF) earthquakes were observed at depths of around 30 km. Five DLF earthquakes were detected within 3 years before the mainshock and more than 60 DLF earthquakes were observed during the 13 months after the mainshock. We investigated the focal mechanism of the DLF earthquake that occurred 9 hours before the mainshock, using amplitude ratios of the S-waves to the P-waves and polarization patterns of the S-waves. Both analyses indicated that a single-force source mechanism is more preferable than a double-couple source mechanism, which suggests the transport of fluid such as water or magma. This event is probably another example of DLF earthquakes that occur beneath active fault zones.