Focal Mechanisms of Intermediate-depth Earthquakes Beneath Southeastern Hokkaido, Japan Implications of the Double Seismic Zone

In this study, the focal mechanism solutions and spatial distributions of 100 intermediatedepth earthquakes are examined and discussed. These earthquakes occurred from 1985 to 1993 mostly in the upper and lower planes of the double seismic zone beneath the southeastern part of Hokkaido. The focalmechanism solutions along with the spatial distribution of their epicenters as a function of depth and the minimum- and maximum-compressive stress directions confirm the following essential features; A doubleplaned deep seismic zone between the depths from 60 to 120 km has been clearly revealed in this region and the number of earthquakes located in the upper plane are less than those for the lower one. Low-angle thrust fault mechanism is dominant to a depth of about 55–60 km in the zone. At depths deeper than about 60 km focal mechanism solutions are mostly characterized by down-dip extension type or down-dip compression type in both lower and upper planes of the zone. Most of the earthquakes deeper than 60 km have dip-slip components and some of the solutions have significant strike-slip components, as well. Although the downdip extensional-type events are dominant, seven events that are located on the lower plane are constrained as down-dip compression mechanism. The T axes of earthquake focal mechanisms on the lower plane show extension at about N40 E and N35 W, and the dip-angle between 25 and 70 , almost coincide with the dip of the subducting Pacific Plate in this region. We also investigate the statistical features of the stress directions of the earthquakes occurring both on the lower and upper plane after converting all the events onto the plane having the same strike and dip angle as the subducting lithosphere in the region. Seventy-two percent of the minimum compressive stress directions are oriented mainly in the NS or NNE- SSW directions (within 45 of the dip direction) at the plate surface of the lower plane and are almost parallel to the dip of the local slab in this region. The number of earthquakes located in the upper plane is less than that for the lower one. Those which occurred on the upper plane show various types of mechanism solutions. Weak systematic orientation is found in the direction distribution of stress in the upper plane.