COLLISION OF FOREARCS AND OVERLAPPED DEEP SEISMIC ZONE IN THE TRANSITIONAL ZONE BETWEEN THE NORTHERN HONSHU AND KURILE ARCS

Abstract

Tectonic relation between the colliding forearc and descending lithosphere beneath the Hokkaido transition zone is presented. Structure and state of stress inferred beneath Hidaka mountains and off Urakawa regions show a possibility that the crust of the northern Honshu forearc is downthrusting beneath Hidaka mountains-the southwestern end of the Kurile forearc. Urakawa-oki earthquake of 1982 (JMA magnitude 7.1) which occurred in the forearc-junction region is concluded to be a result of this colliding movement. The collision is inferred to be mainly ascribed to southwestward movement of the Kurile forearc which may be originated from oblique subduction of the Pacific plate beneath the Kurile forearc. To understand the tectonic relation between colliding structure of Hidaka mountains and off Urakawa regions, and configuration of intermediate-depth seismic zone, more than 4, 200 hypocenters were redetermined. To look through the configuration of the deep seismic zone in detail, the hypocenters were projected on vertical planes and on different slant planes perpendicular to the dipping deep seismic zone. Hypocenter distribution in the intermediate-depth seismic zone shows that the seismic zones of the northern Honshu and the Kurile arcs are overlapping beneath the transitional zone of the two arcs. It is inferred that the overlap is ascribed to anomalous southwestward migration of the obliquely subducted Pacific plate beneath the Kurile forearc.
It is concluded that beneath the forearcs in the arc-junctions, crustal structure, geometry of the contorted plate and state of the stress may be significantly controlled by the direction of the oceanic plate motion.