Unbending and horizontal fracture of the subducting Pacific plate, as evidenced by the 1993 Kushiro-oki and the 1981 and 1987 intermediate-depth earthquakes in Hokkaido

Abstract

A large number of aftershocks of the 1993 Kushiro-oki earthquake ( M JMA = 7.8), 1987 Hidaka earthquake ( M JMA = 7.0), and 1981 Western Hidaka earthquake ( M JMA = 7.1) have been observed by the seismic network of Hokkaido University. These three earthquakes are the largest events that have occurred in the depth range from 100 km to 200 km under the Pacific side of Hokkaido for 109 years. They migrated from west to east in the last 12 years. Their focal mechanism solutions represent typical down-dip extension in the lower zone of the double seismic zone in the descending Pacific plate. We investigated aftershock distributions of the three intermediate-depth earthquakes to study physical properties of the descending slab. Aftershocks of the Kushiro-oki earthquake spread horizontally over an area of 50 km × 40 km. A vertical cross-section of this aftershock distribution shows that it extends from the lower seismic zone almost to the upper seismic zone. It shows a slightly concave shape of about 35 km in length. Aftershocks of the Hidaka and Western Hidaka earthquakes also extend horizontally over areas of 10 km × 10 km and 8 km × 12 km, respectively. These main shocks occurred in the lower seismic zone. The horizontal aftershock distributions of all three large earthquakes indicate that the horizontal nodal planes of the focal mechanisms, rather than the vertical nodal planes, are the fault planes. Such a characteristic suggests that the descending lithosphere under Hokkaido has been unbent macroscopically over the last 12 years. Unbending and the long horizontal faults of the intermediate-depth earthquakes may be caused by mantle flow in the asthenosphere beneath the Pacific plate. We suggest that the descending slab of about 100 km thickness constitutes two layers: a seismic, brittle upper layer and an aseismic, ductile lower layer. Small ruptures may tend to occur easily along both boundary surfaces of the upper layer under the action of forces, especially unbending, and form the double seismic zone.