千葉県東方沖地震の概要  [in Japanese] Characteristic Feature of the 1987 East Off Chiba Prefecture Earthquake  [in Japanese]

The East Off Chiba Prefecture Earth quake (M6.7) on December 17, 1987 was located at 35°22'N, 140゜30'E, about 10 km off the east coast of Chiba Prefecture with focal depth of 58 km. The intensity of ground motions V on JMA scale was recorded at Chiba, Chosi and Katsuura, Chiba Prefecture, Central Japan. The earthquake, indentified as an interplate earthquake of the Philippine Sea plate, was accompanied by the after shock activity which propagated at from the depth of 55 km near the lower surface of the plate to the depth of 25 km near the upper surface of the plate. The right-lateral strike slip fault of the earthquake cut across the northeastern edge of the Philippine Sea plate in the NNW-SSE direction. The Eastern Part of Tokyo Earthquake on March 18, 1988 (M6.5) occurred on the plate boundary between the Philippine Sea plate and the Pacific plate. The coincidental occurrence of the two major earthquakes in the southern part of the Kanto district was interpreted as caused by a mechanical coupling between the two plates beneath the district. The 1987 East Off Chiba Prefecture Earth quake was preceded by the long continued earthquakes warms off the coast of Kujyukuri, Chiba Prefecture since 1971 as a possible precursory activity accosiated with a stress concentration at the fault area of the earthquake.