Figure shows distribution of silent earthquakes (red), afterslip (orange) and major asperities (blue) of Mw8 class interplate earthquakes. Asperities of the 1923 Kanto and the 1944 Tonankai earthquakes were obtained by seismic waveform inversion by Wald and Somerville (1995) and Kikuchi et al. (2003), respectively. Asperities of the 1946 Nankai earthquake are defined here as the areas which show large slip in all the inversion results obtained from leveling, tsunami and seismic intensity data by preceding studies. Figure shows that the slow-slip-events are separated from the major asperities. The average amount of slip of major asperities of the Mw8 class earthquakes were around 3-5 m, except for the Off-Atsumi smaller asperity of slip of 1-2 m. The average amount of slip for the silent earthquakes were less than 0.2 m, one-order smaller than those of the major asperities.　In other words, the silent earthquakes occurred in the transition zone between the seismogenic zone and the zone where stable sliding is dominant. The depth of the transition zone was suggested to be around 30 km along the Nankai Trough by thermal models and geodetic inversions. Plate-depth contours are after Miyoshi and Ishibashi (2004) for southwest Japan including the Nankai Trough and Ishida (1992) for the Kanto district, including the Sagami Trough. Although a number of silent earthquakes detected are less than 10, we are getting a new view on subduction zone dynamics, which nobody expected 10 years ago.  See for details Kawasaki, I, Silent earthquakes occurring in a stable-unstable transition zone and implications for earthquake prediction, Earth, Planets and Space, 56, 813-821, 2004. Nature news features by David Cyranoski.  Nature, 1032-1034, 10/28, 2004,  A seismic shift in thinking. References Ishida, M.,  Geometry and relative motion of the Philippine Sea plate and Pacific plate beneath the Kanto­Tokai district, Japan,  J. Geophys. Res., 97, 489-515, 1992. Kikuchi, M., M. Nakamura, and K. Yoshikawa,  Source rupture process of the 1944 Tonankai earthquake and the 1945 Mikawa earthquake derived from low­gain seismograms,  Earth, Planets and Space, 55, 159-172, 2003. Kimata, F.,  K  Hirahara and N. Fujii,  Interplate coupling changes in the Tokai region, Japan, estimated from the vertical movements by leveling and tide gauge during 1960-2002,  AGU 2002 Fall Meeting, G61-A­0978, 2002. Miyoshi, T. and K. Ishibashi, Geometry of the seismic Philippine sea slab beneath the region from Ise bay to western Shikoku, southwest Japan, Zisin, 57, 139-152, 2004 in Japanes. Wald, D., J., and  P. G. Somerville,  Variable­slip rupture model of the great 1923 Kanto,Japan,Earthquake: geodetic and body­waveform analysis,  Bull. Seism. Soc. Am., 85, 159-177, 1995.

2006/12/01 Update