THE SPATIAL DISTRIBUTION OF EARTHQUAKE HYPOCENTERS AND THE THREE-DIMENSIONAL VELOCITY STRUCTURE IN THE KANTO-TOKAI DISTRICT, JAPAN

Abstract

We examined the spatial distribution of earthquake hypocenters and the three-dimensional velocity structure of P-wave for the Kanto-Tokai District, central Japan, in order to investigate their relation. About 5, 000 earthquakes which occurred during the period from January 1980 to December 1981 were relocated. The double-planed structure of the intermediate-depth seismic zone is observed clearly beneath the Kanto District. The seismic zone underthrusting from Suruga trough toward the west and from Sagami trough toward the east were clearly traced throughout the troughs. Their dip angles are found to be steeper at the northern end compared with the southern part. These inclined seismic zones, however, could not be traced through the inland region to the north of the Izu Peninsula.
The three-dimensional P-wave velocity structure was investigated by applying an inversion method for local earthquake data. The velocity struc-ture between 0 and 145 km in depth was modeled by six layers. Each layer was divided into 10 (latitude)×6 (longitude) blocks, the size of which is 28×26 km2. A high velocity zone appears in and around the Izu Peninsula (northernmost tip of the Philippine Sea plate) in the first layer (0-20 km) and to the east of the Izu Peninsula in the second layer (20-45 km). Low velocity zones appear beneath Tokyo Bay and in the western region in the first layer and to the northwest of the Izu Peninsula in the second layer. Coincidence of the high velocity zone with the inclined seismic zone is found under Sagami trough but such a correlation is not found under Suruga trough owing presumably to poor resolution.