抄録
Based on tide-gauge records, the Hyuganada tsunami of Aug. 7, 1984 is investigated. Adding the present data, the pattern of tsunami activity off the east coast of Kyushu since 1899 is discussed on a space-time diagram. The magnitude (Imamura-Iida scale) of the 1984 tsunami is estimated to be m=-1. This grade is small for an earthquake having magnitude of M=7.1. The source area of tsunami estimated by means of an inverse refraction diagram agrees with the aftershock area, and the source length is 25km in the N-S direction. The initial motion of the tsunami was in an upward direction at Hyuga-Shirahama, but at Tosa-Shimizu and Muroto, the initial disturbance began with a down motion. It suggests the west side of the sea-bottom uplifted and the east side subsided.
According to the geographic distribution of the source area of the Hyuganada tsunamis generated during the last 86 years (1899-1984), the tsunami sources are parallel to the bathymetric line and their region is divided by three groups: A) is located near the Kyushu coast, B) about 50km east from the Kyushu coast and C) off the Shikoku coast. A tsunami of each group was generated on extension of the major axis of the former source. Tsunami magnitudes (Imamura-Iida scale, m=0-1.5) of the A and C groups are larger than that of the B group. From the pattern of generating cycle, the northern part of the A group is considered a region of relatively high tsunami risk as a type of the 1941 earthquake.
