On-going Slow Slip Event at the Tokai Region, Central Japan

On-going slow slip event started from March, 2001 or earlier is detected at the Tokai region by a dense GPS network, GEONET operated by Geographycal Survey Institute (GSI), Japan. By subtracting a linear trend and annual variations from the GEONET data, a thrust type slip movement is occurring for at least half a year at deeper part of the anticipated fault zone of the "Tokai earthquake (M>=8)", although the model suggests that the slip region could be extending into the locked zone. As the maximum slip at the plate interface zone becomes a few centimeter and the estimated area extends from the eastern border of the 1944 Tonankai Earthquake fault zone to the middle of the "Tokai earthquake" fault zone. As the area becomes about 40km x 150km, the moment magnitude of this slow slip events becomes ~6.5, which is similar to those previously found at the Bungo Channel (western edge of the Nankai subduction zone) event [Hirose, et al., 1999; Ozawa, et al., 2001] and the Cascadia event [Dragert et al., 2001]. The Tokai region, central Japan, is located at the eastern end of the Nankai trough (continued to the Suruga trough), where the Philippine Sea plate is subducting beneath the southwest part of Honshu in the NW direction at average rate of about 3 - 5 cm/yr. Repeated occurrence of the temporal changes of the interplate coupling in the Tokai region are also suggested (see Kimata, et al., in this meeting) during 25 years using a back-slip inversion technique similar to Yabuki and MatsuOura [1992]. The data used are horizontal length changes by the EDM ranging at two lines and level changes between Hamanako and Shizuoka, where are located just above the lower part of the locked zone. Results seem to indicate at least two slow slip events during the periods of 1978-1983 and 1987-1991 when both of the subducting rate and direction changes from the average plate motion. A variety of time scale of slow slip event could closely related with the fluctuation of plate motions and/or changes in friction characteristics at the plate. Slow slip processes at deeper part of the locked zone of the plate interface could obviously play a key role in the stress accumulation and nature of repeated occurrence of the plate interface earthquakes.