We have studied earthquakes along the northern Honshu arc, Japan, which were previously thought to have occurred within the downgoing slab, landward of the trench axis and seaward of the double seismic zone. We used P-wave synthetic seismograms to determine precise source depths and mechanisms. We examined six earthquakes in the zone from 50km to 150km landward of the trench axis whose published focal mechanisms showed normal faulting or T axes subparallel to the dip of the slab. These events were thought to have occurred within the slab due to their focal mechanisms. When we computed body-wave synthetics, however, five of these six events were shown to be of thrust type and located at the plate interface. Our symthetics confirm that the sixth event (March 16, 1969, m_b=5.5, M_s=5.5), located about 100km landward of the trench axis, definitely occurred within the slab 15km below the plate interface. We obtain a focal mechanism with T axis dipping at an angle of 60°. The location of this event is about 50km seaward from the edge of the double seismic zone. The bending-unbending theory of an elastic-plastic plate cannot explain the stress axis of this event; however, the gravitational pull or sagging of the downgoing slab is consistent with this stress orientation.