1997 年 50 巻 3 号 p. 291-302
The 1782 August 23 Ten'mei Odawara earthquake, magnitude 7.2-7.3, is one of the three M7-class earthquakes that occurred beneath the Odawara area on the northwestern coast of Sagami Bay, the Pacific coast of central Japan, since the 17th century. Two other events, the 1633 Kan'ei and 1853 Ka'ei Odawara earthquakes, were definitely accompanied by tsunamis and their rupture zones are estimated to have lain just beneath Odawara, a seashore city in the area, extending both inland and offshore directions. This estimation is in agreement with the fact that the 1633 and 1853 earthquake ground motions at Odawara were very strong. On the other hand, the 1782 Ten'mei Odawara earthquake had been considered to have been non-tsunamigenic after critical readings of historical documents by a few investigators, and its source region had been inferred to be situated inland north of Odawara city. TSUJI (1986), however, claimed that the 1782 earthquake generated a tsunami and estimated that the tsunami height was 4 m at a fishing village, Ajiro, based on the examination of two newly found historical documents. He estimated a nearly 30 km-long tsunami source region south off the Odawara coast in addition to the inland rupture zone. Tsuji's interpretation yields a north-south extent of faulting too long to be consistent with an M7-class earthquake. It also conflicts with the fact that the 1782 earthquake ground motion was not the heaviest at Odawara, which strongly suggests that Odawara was not just above the rupture zone. TSUJI (1986) reported at the same time that Atami, a seaside town between Ajiro and Odawara, was not struck by a tsunami in 1782, which seems unreasonable from the viewpoint of tsunami behavior; actually, at the time of the 1633 earthquake the estimated tsunami height at Atami was 4-5 m, whereas that at Ajiro was 3-4 m. Whether the 1782 Ten'mei Odawara earthquake generated a tsunami or not is very important for not only the estimation of its rupture zone, but also the seismotectonics of the series of Odawara earthquakes. TSUJI (1986) drew his conclusion by very intricate interpretation of two historical documents which don't give any explicit description of a tsunami at Ajiro in 1782 at all. In this paper I reexamine the two documents more carefully and address the difficulties in Tsuji's conclusion. By referring to various materials describing the history of Ajiro village from the 17th to the mid-19th centuries, I clarify that TSUJI (1986) misread vague, rather general, descriptions of huge waves due to storms as a tsunami. Thus, I reject the suggestion that the 1782 Ten'mei Odawara earthquake generated a tsunami, and I conclude that its source region is inland north of Odawara city.