Faulting caused by earthquakes beneath the outer slope of the Japan trench.

Abstract

We determined fault plane solutions and focal depths for seven events which occurred in the Japan trench outer slope, using comparison between synthetic and observed long- and short-period teleseismic seismograms. The five normal fault solutions obtained in this study have nearly vertical nodal planes parallel to the trench and their depth range, 2-16 km below the seafloor, is in accord with bending of the oceanic lithosphere prior to subduction. The occurrence of both types of normal faulting, downthrow of both the continental and oceanic sides, is likely to be associated with the formation of horst-graben structures in the outer trench slope near the trench axis. One normal fault solution (31 km below the seafloor) that is significantly deeper than the other normal fault solutions has a nearly vertical nodal plane striking parallel to the ENE magnetic anomaly lineations in the region. This event is also characterized by its lower stress drop or slower rupture than the other events. This event would represent a reactivation of the initial structural fabric inherited from the accretion at the mid-oceanic ridge and indicates that the bending stress is small at this depth. One reverse fault solution, 41 km below the seafloor, is in accord with the compressional bending stress in the deeper portion of the lithosphere. These events indicate that a neutral surface of bending stress is around 30 km depth in this region at least for the past few decades, consistent with the elastic thickness of lithosphere in adjacent regions. This implies that the lithosphere is not entirely in deviatoric tension in this region, suggesting that the 1933 Sanriku earthquake may be a large bending event.