[SCG61-06] Seismicity in the trench-outer rise region along the Japan Trench based on repeated OBS observations: Implications for large outer-rise earthquakes
The earthquakes observed by the repeated OBS observations were characterized by the shallow normal-faulting earthquakes. Most of the earthquakes occurred at depths shallower than 20 km, which corresponds to the oceanic crust and uppermost part of the oceanic mantle, although some events with normal-faulting focal mechanisms were observed at depths of 40 to 50 km below the sea surface. These observations suggest that the extensional stress regime extends down to a depth of 40 km within the Pacific plate. The shallow earthquakes within the oceanic crust and the uppermost part of the oceanic mantle generally showed trench-parallel linear trends along the horst-and-graben structures even though the horst-and-graben structures can be divided into small segments, both parallel and oblique to the trench. Therefore, large normal-faulting earthquakes might have complicated rupture process along the segmented normal-faults. Compared to the earthquakes at depths shallower than 20 km, earthquakes at depths of about 40 km occurred in the limited area. The seismicity at deeper depths generally coincide with areas of low Vp, which may suggest the localized hydration along the normal-faults extending into the oceanic mantle. Spatial heterogeneity of the seismic velocity structure might be an indicator of potential faults of large outer-rise normal-faulting earthquakes.