An Investigation of the Tsunami of Sept. 6, 1971 Generated off Southwestern Sakhalin

Abstract

Based on tide gauge records at Hokkaido and Sakhalin, a tsunami accompanying the southwestern Sakhalin earthquake on Sept. 6, 1971 (M=6.9, JAM) is investigated. The southern end of the tsunami source is located at about 15km north of Moneron I. (Kaiba I.). The initial motion of the tsunami waves was in an upward direction at most stations, suggesting the uplift of the sea bottom in a southern part of the tsunami source. The source dimension of tsunami is inferred to be 70km long in the NE-SW direction and the area is 2.2×10³km². The tsunami energy of approximately 3×10¹⁸ ergs is calculated from a first wave (crest and trough) observed at Wakkanai. This value corresponds to the lower limit of Imamura-Iida's tsunami magnitude m=0.5. The tsunami magnitude, in turn, is estimated by the author's method, which makes use of the wave attenuation with distance. The average vertical displacement of 20cm might have occurred in the source to account for the estimated tsunami energy. Generally speaking, the present tsunami is of the standard magnitude in the statistical relation to earthquake magnitude. It is noticed that the tsunami fronts arriving the Shakotan peninsula have propagated in deep waters and the wave amplitudes are relatively high due to the refractive effect.