(a) The ΔCFF associated with three large earthquakes that occurred in central Japan (the 1961 Kita-Mino, 1969 Gifu-Ken Chubu, and 1984 Nagano-Ken Seibu earthquakes) and recent seismicity. The assumed receiver fault mechanism is the mainshock of the 1984 Nagano-Ken Seibu earthquake. (b) Distributions of F-net solutions from October 1997 to May 2010. The ΔCFF assuming a thrust-type earthquake as a receiver fault mechanism and a low apparent coefficient of friction is also indicated. The other symbols are the same as in Fig. 2.