The Miocene strata widely distributed in the Ou Backbone Range on the east of the Kakunodate Town are mainly composed of volcanic rocks, whose geochronological data are very few. In this study, we conducted U–Pb and FT double dating of zircons obtained from the lower and upper part of the Miocene Mahirugawa Formation. The lower Mahirugawa Formation is dated at 13.2 ± 0.6 Ma and the upper is dated at 13.7 ± 0.3 Ma. The result shows that the boundary age of the Mahirugawa Formation and the younger Uchizawagawa Formation is ca. 14–13 Ma. Judging from their age and lithofacies, they seem to be correlated with the Nishikurosawa Formation in the standard stratigraphic succession in the Akita area.
Abnormal phenomena observed on the coast of the Kii Peninsula prior to the 1946 Nankai earthquake were collected and examined. Most of the information was acquired from literature and some testimonies were obtained by interview with residents. Water level lowering and/or turbidity of well water was witnessed two or three days before the main shock. Land subsidence had been observed on the eastern coast of the Kii Peninsula. The subsidence is thought to be the post-seismic deformation of the 1944 Tonankai earthquake. Oscillatory sea level changes which were witnessed in some areas suggests that a tsunami might have occurred. Testimonies by some residents that they had heard roaring sound and/or felt tremor prior to the main shock were also recorded.