[S04-1] Historical and macroseismic studies of earthquakes I

[S04-1-01] Revision of the world's best-known recurrence pattern of historical subduction earthquakes along the Nankai trough off southwest Japan and their relationship with large inland earthquakes

^○Katsuhiko Ishibashi (Professor Emeritus, Kobe University, Kobe, Japan)

invited

Nine series of M 8-class interplate earthquakes, whose recurrence is a nationwide concern, have been recognized along the Nankai trough off SW Japan; in 684, 887, 1096/99, 1361, 1498, 1605, 1707, 1854 and 1944/46. For deeper understanding of their recurrence nature I reevaluated their space-time pattern in a careful historiographical manner. Some examples of discussions are as follows. On the 684-887 interval, a historian supposed an earthquake in 794 based on an ancient document. But, by critical reading of the document I strongly suggest that this event was a thunderstorm. As for the 1096/99 series it has been considered that the 1096 event occurred in the east and the 1099 one followed in the west. The latter, however, has no evidence of a great earthquake except for an inferred coseismic subsidence in Shikoku, while the former has various evidence of a great earthquake. I examined the only material referring to the subsidence in Shikoku, and clarified that it had originally been written about 80 years after the earthquake and probably duplicated around 100 years later, suggesting unreliability of the 1099 subsidence. I interpret that the 1099 event was an inland earthquake and the 1096 event was an “entire-Nankai trough" earthquake. Regarding the 1605 event, though I had once proposed an idea that it had been a great tsunami quake along the Nankai trough, which has been widely accepted, I reexamined historical documents in the early 17th century thoroughly and propose a new view that the 1605 event was a great earthquake along the Izu-Bonin trench, referring to the 2010 off-Bonin earthquake, and that the Nankai trough earthquake of this series was the 1614 event so far commonly regarded as an inland earthquake. I also discuss remaining problems and the relationship between Nankai trough earthquakes and large inland earthquakes, and propose a seismotectonic model attaching importance to the Amur plate motion in addition to the Philippine Sea plate subduction.