Earth Planets Space, Vol. 56 (No. 12), pp. 1185-1189, 2004
Shiro Ohmi, Issei Hirose, and James J. Mori
Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611-0011, Japan
(Received April 30, 2004; Revised October 20, 2004; Accepted October 20, 2004)
Deep low-frequency (DLF) events have often been discussed in association with fluid (magma) activity around volcanoes, in this paper, however, we will show another example of DLF activity beneath active faults. On October 6, 2000, a Mw=6.7 crustal earthquake occurred in the western Tottori, southwest Japan. Beneath the focal region of the earthquake, DLF earthquakes had been observed prior to the mainshock and the activity increased after the mainshock. The events are distributed around 30 km depth and seem to be located around the downward extension of the seismogenic fault. Three types of DLF events have been observed and their seismological features indicate fluid activity around the focal region of the DLF events. If the downward extension of the seismogenic fault in the lower crust exists, fluid activity indicated by the DLF events possibly affect the aseismic slip process of the deep portion of the fault and likely control the occurrence of the earthquakes on the seismogenic portion of the fault.
Key words: Deep low-frequency earthquake, seismogenic faults, fluid.