1996 年 48 巻 4 号 p. 451-461
A tsunami was generated by the 1994 Hokkaido Toho-Oki earthquake and was observed at tidal stations on the Pacific coast of the Japanese Islands. It was observed also at several tidal stations on Hokkaido coast of the Okhotsk Sea, where the arrival of the initial wave was recorded earlier than the expected time: for example, the initial motion of the tsunami was recorded at Utoro 17 minutes after the mainshock, while that the theoretically expected tsunami arrival time from the source in the south sea region of Shikotan Island is 80 minutes after it. We tried to estimate the location of the source of the early coming waves by drawing the inverse refraction diagram from each station. We suggest two probable locations of the source; one is the area 80 kilometers north of Abashiri, and the other is the area 40 kilometers north of the tip of Shiretoko Peninsula. The seismic activity at both of those areas is poor. So, we cannot judge that the early coming waves were generated by some another events. Both of those areas are situated in the continental slope regions and with steep slopes. We suppose that a land slide at either of those areas caused the early coming waves.