Existence of interplane earthquakes and neutral stress boundary between the upper and lower planes of the double seismic zone beneath Tohoku and Hokkaido, northeastern Japan

Using data from the recently constructed nationwide dense seismic network, we determined the hypocenters and focal mechanisms of many intermediate-depth intraslab earthquakes within the Pacific slab beneath northeastern Japan. The results show that in addition to the upper and lower planes of the double seismic zone, a considerable number of intraslab earthquakes also occur between the two planes. This interplane earthquake activity is not homogeneously distributed in space, being high beneath eastern Hokkaido and the fore-arc regions of southeast and central Tohoku. The focal mechanisms of the interplane earthquakes tend to be of the down-dip compressional type (DC) in Tohoku and the Hokkaido corner, but down-dip tensional type (DE) in eastern Hokkaido. Upper plane earthquakes are characterized by DC-type stress while lower plane earthquakes are DE-type in both Tohoku and eastern Hokkaido. The existence of interplane earthquakes enables estimation of the position of the neutral plane between the upper-plane DC stress and the lower-plane DE stress. We did so by applying stress tensor inversions to focal mechanism data obtained in the present study and Japan Meteorological Agency (JMA) data. The results show that the neutral plane is located about 22 km beneath the upper plate interface beneath Tohoku, but only about 10 km beneath the upper surface beneath eastern Hokkaido. This difference in the location of the neutral plane may be due to the difference in buoyancy force exerted by the less dense metastable olivine wedge that is a result of the oblique plate subduction beneath Hokkaido. Comparison of large intraslab earthquakes beneath the two regions shows that their aftershock areas are limited by the neutral plane, suggesting that large earthquake ruptures are confined to either the DC or DE stress field, and do not go beyond the neutral plane. (C) 2010 Elsevier B.V. All rights reserved.