Vertical Ground Deformation Detected by the Leveling and the Tidal Observation in Tokai Region, Central Japan in 1980 - 2002

Great earthquakes more than M8 have occurred along the Suruga-Nankai subduction zone of the Philippine Sea Plate (PHS), southeast Japan, every 100 to 150 years. The last events are 1944 M7.9 Tonankai and 1946 M8.0 Nankai earthquakes. Interseismic subsidence is detected along the subduction zone from the precise leveling and tide gauge. Discussion of the occurrence of great earthquake in near feature brought the expanding the measurements and observation of earthquake and ground deformation in Tokai region. Whereas nation-wide dense GPS network (GEONET) makes clear of the ground deformation of contraction in west-northwestward contraction and east-southeastward tilting in Tokai region, no northwestward horizontal displacements are observed since 2001 in the western Tokai region. Slow slip event (SSE) toward trough is discussed (Ozawa et al., 2002). Precise leveling and line length measurements also suggest the episodic changes with time interval of 4-5 years in the late 20 years (Kimata and Yamauchi, 1999; Kimata et al., 2001). Moreover pre-seismic ground tilt is suggested in the same area of the 2001 Tokai SSE from the leveling of the day in 1944 Tonankai Earthquake. Meanwhile pre-seismic slip is estimated in the SSE area from the numerical experiment of rock crusher (Kato, 2003). We discuss the episodic change of the ground deformation and the recurrence of the SSE in Tokai region with more detail from the vertical ground deformation detected by precise leveling and tide gauge in the period from 1980 to 2002. Geographical Survey Institute of Japan (GSI) has repeated the precise leveling in Tokai region every year since 1980. Tide gauge measurements have also been continued at more than 10 sites in Tokai region by GSI, Japan Meteorological Agency (JMA) and Aichi prefecture (AP). Time series of vertical movements at the benchmarks are analyzed by spline function and Annual relative sea levels at tide stations are corrected (Savage and Thatcher, 1992). The 2001 Tokai SSE is characterized by the uplift of 3-4cm around Hamana Lake, central Tokai region, which is detected by GPS measurements, precise leveling, and tide gauge measurement (Ozawa et al., 2002; Oota et al., 2003). The same uplift is detected by precise leveling and tide gauge in late 1988, Episodic changes of tilt movements also detected by bore hall tilt meter north Hamana Lake site in 2001 and 1988 (Yamamoto et al., 2003). It is sure that Tokai SSE should be occurred in the central Tokai in 1988. We have one conclusion that SSE has repeated in Tokai region at least in last 20 years.