(2)The 1968 Earthquake off the Coast of Tokachi (May 16, 1968, M 7.9)
The source region of this interplate earthquake covered a broad stretch in the open sea east of Aomori Prefecture. Strong ground motion was felt over a wide area, primarily in the northern Tohoku region and southern Hokkaido. In the Tohoku region, this earthquake registered a seismic intensity of 5 in JMA scale in Aomori City and Hachinohe (Hatinohe) City (Fig.4-14). This earthquake also generated a tsunami that was 6 m at its highest point. The tsunami arrived at low tide, however, so only slight damage resulted (Fig.4-15). The earthquake was responsible for a total of 52 fatalities, 47 of which were in Aomori Prefecture. The primary damage included landslides and the collapse of cliffs and houses. Damage was extensive in areas with soft ground, including hilly areas comprised of volcanic deposit, reclaimed land, and low marshy areas (Fig.4-16, Fig.4-17). Another factor thought to have contributed to the damage was the soft ground caused by more than 200 mm of rainfall in three days prior to the event in eastern Aomori Prefecture. The largest aftershock (M 7.5) occurred about 12 hours after the main shock on May 16 and the M 7.2 aftershock occurred on June 12. Seismic intensity 4 in JMA scale was observed over a wide area from the northern Tohoku region to southern Hokkaido. The largest aftershock, which occurred on May 16, registered a seismic intensity of 5 in JMA scale in Urakawa and Hiroo in southern Hokkaido. The frequency of aftershocks increased for a time on June 12, but gradually diminished (Fig.4-18).
The 1952 Earthquake off the Coast of Tokachi (M 8.2) occurred in the area adjoining the northeast side of the site of the 1968 Earthquake off the Coast of Tokachi.