Different stress directions in the aftershock focal mechanisms of the Kushiro-Oki earthquake of Jan. 15, 1993, SE Hokkaido, Japan, and horizontal rupture in the double seismic zone

Abstract

On 15 January, 1993, a large intermediate-depth earthquake with a magnitude of M=7.8, shook the eastern part of the island of Hokkaido, Japan, close to the northeastern Japan trench and to the Kuril trench; this earthquake is now referred to as the Kushiro-Oki earthquake. The main shock of the Kushiro-Oki earthquake showed a down-dip extension mechanism having horizontal and vertical nodal planes. The hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting Pacific plate. After the mainshock, more than 300 aftershocks were recorded by three seismograph networks. From these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks. The aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate; it did not, however, reach the upper plane. We estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone. Thirty-eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock, but 25 aftershock mechanisms had almost opposite directions to those of the mainshock. The hypocenters of these aftershocks were spatially mixed. This suggests that the stress state is strongly inhomogeneous in the aftershock region, and that horizontal movement occurs by the down-dip extensional stress which is stored in the region between two planes of the deep seismic zone. The remanent down-dip extensional stress was released as the consistent group of aftershocks. High aftershock activity may be attributed to the inhomogeneity of the material in the region. The previous four intermediate-depth earthquakes beneath Hokkaido Island as well as aftershock distributions of two of them have been examined for comparison with the Kushiro-Oki earthquake. The mainshocks of the four large intermediate-depth earthquakes are also located on the lower plane with a similar mechanism to that of the Kushiro-Oki earthquake. The aftershock regions of two of these earthquakes are also distributed horizontally. They indicate dominantly horizontal movement, with down-dip extension mechanisms at an intermediate depth in the subducting Pacific plate beneath Hokkaido Island.