According to the observation of JMA, the earthquake swarm was occurred off the Urup Is. in December 1991, and the length of swarm was 150km along the Kurile trench. A tsunami associated with the largest earthquake (M=6.8) was observed with the maximum double-amplitude 10-20cm and wave-period about 15min at the Hokkaido and Sanriku coasts. The maximum amplitudes were 43cm at Shikotan Is. and 30cm at Ogasawara-Chichijima Is. The diagram of the attenuation of wave-height with distance showed that the tsunami magnitude was m=1 on the Imamura-Iida scale. This value is two-grade (5 times for wave-height) high comparing with the normal tsunami of the same earthquake magnitude, suggesting that the tsunami was generated by the low-frequency earthquake. The estimated tsunami source is about 100km long. The present swarm area is situated on the source area of the abnormal tsunamis (m=1.5-2) accompanied by aftershocks of the giant Kurile earthquakes in 1918 and 1963.
On the basis of the grade of magnitude for the tsunamis generated from Hokkaido to Urup Is. during the last 100 years, 1893-1992, the radiated energy, E_t, at each region divided by 300km segment is investigated using the empirical formula, log E_t=0.7m+19.1ergs (HATORI, 1979). As the result of calculations, the cumulative energy radiated at each region is the order of 10²¹ergs. The amount of energy in the Urup region being 3.7×10²¹ergs is largest. This value is a little larger than that of the energy radiated from the Sanriku region. Although the 1993 Kushiro-Oki earthquake (M=7.8) of the intra-plate type was occurred, a gap of tsunami source exists still between Kushiro and Nemuro, East Hokkaido.