Earthquake of 1987, off Chiba, central Japan and possible triggering of eastern Tokyo earthquake of 1988

Abstract

A moderate earthquake ( M 6.7) occurred off eastern Chiba, central Japan, on December 17, 1987 as a consequence of the internal deformation of the Philippine Sea plate. This event helped in clarifying the geometry of the easternmost part of the Philippine Sea plate as well as the mode of earthquake occurrence in this region. Three months later, another moderate earthquake ( M 6.0) took plate beneath eastern Tokyo. This event was generated at the slab-slab collision zone at about 90 km depth, and is considered to be triggered by the former earthquake. A similar phenomenon was observed for the Izu ( M 6.7) of June 29, 1980 and the mid Chiba ( M 6.1) of September 25, 1980 earthquake pair, and the off Ibaraki ( M 7.0) of July 23, 1982 and the southern Ibaraki ( M 6.0) of February 27, 1983 earthquake pair. These are the only three earthquake pairs of M ≥ 6 which occurred in the Tokyo metropolitan area since 1974. Thus, over these 15 years, all M-6 interplate earthquakes in this specific region are preceded by nearby M-7 earthquakes. This suggests that M-7 shallow events enhance stress in the deeper plate collision zone beneath the Tokyo metropolitan area and trigger M-6 interplate events.