(4)The 1994 Earthquake far off the Coast of Sanriku (December 28, 1994, M 7.5)


The source region of this interplate earthquake extended over a wide area from east to west, from the open sea off the area near the border of Aomori and Iwate Prefectures to a location close to shore near the Japan Trench. Strong ground motion was felt, primarily in the area near Hachinohe, Aomori Prefecture, where seismic intensity 6 in JMA scale was observed (Fig.4-24). The tsunami observed by the tide station reached a maximum height of 55 cm at Miyako in Iwate Prefecture. This is small for a destructive earthquake occurring in the open sea off Sanriku, and it did not result in tsunami damage. The ground motion however, did cause damage, including collapsed houses in Hachinohe. There were a total of 3 fatalities (Fig.4-25, Fig.4-26).

The largest aftershock (M 7.1) occurred on January 7, 10 days after the main shock near the coast in northern Iwate Prefecture. Seismic intensity 5 in JMA scale was observed in Hachinohe, and the aftershock caused numerous injuries and collapsed houses. There was considerable aftershock activity, with the largest aftershock occurring on January 7. The frequency of aftershocks gradually decreased thereafter (Fig.4-27, Fig.4-28).

Observations of the crustal movement using the Global Positioning System (GPS) detected movement of several centimeters to the east in all areas from Hokkaido to Tohoku accompanying the occurrence of the Earthquake far off the Coast of Sanriku.

The source region of this earthquake partially overlapped that of the 1968 Earthquake off the Coast of Tokachi, east off Aomori prefecture (M 7.9). This underscores the importance of studying the recurrence time of interplate earthquakes.