(3)The 1993 Earthquake off the Southwest Coast of Hokkaido (July 12, 1993, M 7.8)
This earthquake occurred in the eastern margin of the Sea of Japan, and its source region ran north to south along an isobath of 3,000 m at the boundary between the Sea of Japan basin and Okushiri ridge at Okushiri Island (Okusiri To) west of the central section of Oshima Peninsula. The seismic intensity from this earthquake was 5 in JMA scale at Suttsu, Esashi, Otaru, and Fukaura in Aomori Prefecture (Fig.3-13). The focal region was near Okushiri Island and the western shore of the Oshima Peninsula, in which a tsunami arrived 4 or 5 minutes after the earthquake, and many people lost their lives. Field investigations showed that the maximum tsunami height was about 30 m high at Okushiri Island, and reached 7 m to 8 m at the western shore of Oshima Peninsula (Fig.3-14). In particular, the town area of the Aonae of Okushiri Island was devastated by the tsunami and the fires that broke out after the earthquake (Fig.3-15). Additional damage was caused by liquefaction and cracks in the soft-ground area on Oshima Peninsula. In total, 230 people were killed or missing, including one fatality in Aomori Prefecture, and 323 people were injured. Okushiri Island subsided from several centimeters to more than one meter during the Earthquake off the Southwest Coast of Hokkaido (Fig.3-16). However, topographical studies show this island has undergone upheaval in the past. If earthquakes caused this past upheaval, the clarification of any link between this earthquake and the subsidence is a task for future study.
The frequency of aftershocks gradually decreased (Fig.3-17). The largest aftershock (M 6.5) occurred on August 8 slightly east of the source region of the main shock (Fig.3-18).