(5)The 1982 Earthquake off the Coast of Urakawa (March 21, 1982, M 7.1)
This is one of the earthquakes that occurred in a rather deep location from the southern Hidaka Mountains to the area off the coast of Urakawa. The focal region was at a depth of roughly 40 km about 20 km southwest of Urakawa. The seismic intensity 6 in JMA scale was observed at Urakawa (Fig.3-20), and caused extensive damage to the coastal area near the Hidaka branch administrative office, near the focal region. Shaking corresponding to a seismic intensity 5 in JMA scale was felt locally in the Sapporo City and the Ishikari branch administrative office, which are about 100 km away from the focal region, and some damage occurred there. A total of 167 people were injured in the earthquake (Fig.3-21 and Fig.3-22). The area from the southern Hidaka Mountains to the area off the coast of Urakawa is the site of much seismic activity, but this earthquake was the strongest ever recorded in this area to date and also created the first tsunami of any earthquake in this area. The tsunami arrived at Urakawa about 4 minutes after the earthquake. Its height was observed at 135 cm at the tide station, but caused no damage. The frequency of aftershocks gradually decreased, but their frequency was greater than usual (Fig.3-23). The largest aftershock (M 5.8) occurred about 8 hours after the main shock.