Seismicity quiescence and activation in western Japan associated with the 1944 and 1946 great earthquakes near the Nankai trough

Significant changes in seismicity (quiescence and activation) took place during the period of 1944–1946 in some regions in western Japan. They are well explained by the corresponding changes in Coulomb failure stress caused by the 1944 Tonankai earthquake of M7.9 and the 1946 Nankai earthquake of M8.0, both of which occurred in the Nankai trough. From the seismicity changes I try to identify precursory anomalies such as the quiescence in several regions before the Tonankai event and the enhanced activity in southern Kii peninsula before the Nankai event. These quiescence and enhanced activity may be due to precursory aseismic slips in an area on the plate interface downdip of the Tonankai rupture and slips transferring from the Tonankai to Nankai rupture zone. The records of felt shocks at the Wakayama Observatory during 1900–1995 provide support for the quiescence in the Wakayama District during 1944–1946, as well as a scenario of seismicity cycle till the next great event(s) in the Nankai trough to occur sometime this century.