Seismicity in the source areas of the 1896 and 1933 Sanriku earthquakes and implications for large near-trench earthquake faults

In the northern part of the Japan Trench, the 1933 Showa-Sanriku earthquake (M-w 8.4), an outer-trench, normal-faulting earthquake, occurred 37 yr after the 1896 Meiji-Sanriku tsunami earthquake (M-w 8.0), a shallow, near-trench, plate-interface rupture. Tsunamis generated by both earthquakes caused severe damage along the Sanriku coast. Precise locations of earth-quakes in the source areas of the 1896 and 1933 earthquakes have not previously been obtained because they occurred at considerable distances from the coast in deep water beyond the maximum operational depth of conventional ocean bottom seismographs (OBSs). In 2015, we incorporated OBSs designed for operation in deep water (ultradeep OBSs) in an OBS array during two months of seismic observations in the source areas of the 1896 and 1933 Sanriku earthquakes to investigate the relationship of seismicity there to outer-rise normal-faulting earthquakes and near-trench tsunami earthquakes. Our analysis showed that seismicity during our observation period occurred along three roughly linear trench-parallel trends in the outer trench region. Seismic activity along these trends likely corresponds to aftershocks of the 1933 Showa-Sanriku earthquake and the M-w 7.4 normal-faulting earthquake that occurred 40 min after the 2011 Tohoku-Oki earthquake. Furthermore, changes of the clarity of reflections from the oceanic Moho on seismic reflection profiles and low-velocity anomalies within the oceanic mantle were observed near the linear trends of the seismicity. The focal mechanisms we determined indicate that an extensional stress regime extends to about 40 km depth, below which the stress regime is compressional. These observations suggest that rupture during the 1933 Showa-Sanriku earthquake did not extend to the base of the oceanic lithosphere and that compound rupture of multiple or segmented faults is a more plausible explanation for that earthquake. The source area of the 1896 Meiji-Sanriku tsunami earthquake is characterized by an aseismic region landward of the trench axis. Spatial heterogeneity of seismicity and crustal structure might indicate the near-trench faults that could lead to future hazardous events such as the 1896 and 1933 Sanriku earthquakes, and should be taken into account in assessment of tsunami hazards related to large near-trench earthquakes.