Possibility of a Large Earthquake near Odawara, Central Japan, Preceding the Tokai Earthquake

Abstract

Near Odawara on the northwestern coast of Sagami Bay in central Japan, along the northernmost convergent boundary of the Philippine Sea plate, destructive seismic faultings occurred five times during the last 400 years, in 1633, 1703, 1782, 1853, and 1923, with a recurrence interval of about 70 years. The 1703 and the 1853 events were followed by the 1707 Hoei and the 1854 Ansei-Tokai earthquakes, two antecedents of the Tokai earthquake expected along the Suruga trough, respectively. The 1923 event, the largest, has proved a left-lateral thrust faulting of 1.8 × 1027 dyne-cm seismic moment along a plane westerly dipping beneath Odawara from a steep submarine scarp on the western edge of the Sagami trough (“West-Sagai-Bay fault”) based on the coseismic crustal deformation. The 1633, 1703, 1782, and 1853 Odawara earthquakes are interpreted as partial or entire ruptures of this fault based on the macroseismic and tsunami data. The WestSagami-Bay fault is considered a nascent convergent boundary between the Philippine Sea and the continental plates based on its repeated ruptures, the interseismic subsidence of its hanging-wall side before 1923, the uplifted marine terraces at Matsushima Island on its hanging-wall side, and the slip vector of its foot-wall side during the 1923 event. Probably it has been developed in consequence of the collision of the Izu Peninsula with Honshu Island. The current convergence between the Philippine Sea plate and Honshu Island in this region is accommodated by a dual boundary along the Suruga trough and the West-Sagami-Bay fault with the “Izu Transform Belt” between them. The space-time regularities of the rupture of the West-Sagami-Bay fault and between the ruptures of this fault and along the Suruga trough can be basically understood by the mechanics of this dual convergence system, and the Odawara earthquake is considered a triggering factor of the Tokai earthquake. The rupture scale of the West-Sagami-Bay fault, which appears complementary to that along the Suruga trough, varies with the time considerably, probably because the rate of stress accumulation over the entire fault fluctuates due to its juvenility among the complex subduction-collision system. But the seismic slip of the nucleation area, presumably just beneath Odawara, seems to recur periodically. Judging from the temporal regularities and considerable amount of strain accumulation on the northwestern coast of Sagami Bay estimated from geodetic survey data, a large earthquake may occur near Odawara during the 1990’s or 2000’s and the Tokai earthquake in Suruga Bay may follow it in a few years.