(5)The Rikuu Earthquake (August 31, 1896, M 7.2)


This earthquake occurred close to the eastern margin of the Yokote Basin fault zone extending in a north-south direction near the border between Akita and Iwate Prefectures. Strong ground motion occurred, primarily in these two prefectures. The extent of the damage and other factors indicate motion of seismic intensity 5-6 in JMA scale near the focal region, and up to seismic intensity 7 in JMA scale in part (Fig.4-29). This earthquake claimed the lives of 209 people, primarily in Akita Prefecture (Fig.4-30, Fig.4-31).

Surface slipping occurred with this earthquake in the northern part of the eastern margin of the Yokote Basin fault zone in Akita Prefecture (the Obonai, Shiraiwa, Ota, and Senya faults) and part of the western margin of the Shizukuishi Basin - the eastern margin of the Mahiru mountains(Shizukuishi-bonchi-seien Mahiru-sanchi-toen) fault zone in Iwate Prefecture. For example, the eastern side of the Senya fault, which is part of the eastern margin of the Yokote Basin fault zone, uplifted 3.5 m higher than the western side of the fault at its highest point. The slip that occurred at the western margin of the Shizukuishi Basin - the eastern margin of the Mahiru mountains fault zone is known as the Kawafune (Kawahune) fault. The western side of this fault uplifted 2 m higher than the eastern side of the fault at its highest point.

Surveys on active faults make it clear that the Rikuu Earthquake occurred on a reverse fault. It is thought that a large earthquake occurred on the Senya fault about 3,500 years before the Rikuu Earthquake and that a 40 km segment along the northern section of the eastern margin of the Yokote Basin fault zone was active during this earthquake. There was no activity in the southern section.

Strong foreshock activity was observed beginning on August 23, eight days before the main shock. The maximum foreshock was an earthquake of M 6.4, which caused damage in some areas. The conditions of the aftershocks are shown in (Fig.4-32).