東海・南海道沖における大津波の波源

―1944年東南海, 1946年南海道津波波源の再検討と宝永・安政大津波の規模と波源域の推定―

抄録

Source areas of the 1944 Tonankai and the 1946 Nankaido tsunamis are reexamined on the basis of tide gauge records and visual observations. The height distribution of three large tsunamis accompanying the giant earthquakes in 1707 and 1854 (two tsunamis on 23 and 24 December) are summarized from old documents and the source areas of these tsunamis are estimated. These tsunamis are very large tsunamis (Imamura and Iida's magnitude m=2.5-4) generated along the Nankai trough since 1700.
For the 1946 Nankaido tsunami, the subsidence of the sea-bottom may have occurred in the shoreward area and the tsunami source in the west side may be effectively uplifted, judging from the initial motion of tide records and geodetic data. The uplifted area of tsunami source is almost the same as Omote's result (1947). The elongated source area of the 1944 Tonankai tsunami along the Kumanonada agrees approximately with the aftershock area. The shoreward sea-bottom in the tsunami source is subsided. The inundation height and seismic intensity distributions at the Tokaido district for the 1854 (on Dec. 23) tsunami are larger than those of the 1944 tsunami, suggesting the difference of the source locations of two tsunamis. In case of the 1854 (on Dec. 24) tsunami, seismic and tsunami intensities along the Nankaido district are larger than those of the 1946 tsunami but the distribution patterns are similar to the 1946 tsunami and the source locations of two tsunamis are approximately estimated in the same region. The source areas of these tsunamis extend 200-250km along the Nankai trough. On the other hand, the source dimension of the 1707 tsunami seems to be of the order of 500km. The distribution pattern of inundation heights for the 1707 tsunami seems to be similar to the sum of two tsunamis of 1944 and 1946.