Aftershock distribution of the 1994 Sanriku-oki earthquake (M-w 7.7) revealed by ocean bottom seismographic observation

The 1994 Sanriku-oki earthquake (M-w 7.7) ruptured the entire seismogenic z one of the Pacific plate subduction beneath northeastern Japan are. We depl oyed 18 ocean bottom seismographs and recorded its aftershocks, which span the whole seismogenic zone, with unprecedented accuracy. We calculate hypoc enters and mechanisms to improve our understanding of the plate subduction geometry and plate coupling. The rupture of the mainshock initiated at the updip end of the seismogenic zone defined as the trenchward limit of the af tershock and background seismicity. The rupture continued to slip along a s hallow dipping ( < 10 degrees) interface for similar to 60 km, and then sli pped across an asperity where the dip angle starts to increase at similar t o 143 degrees E. Here aftershocks are few, suggesting complete release of s train. The rupture continued another 100 km or so to the downdip end beneat h the coastline, where the plate dip angle reaches 30 degrees to smoothly c onnect to the dip angle of the Wadati-Benioff zone. Coseismic slips are lar ger westward of 143 degrees E, where the mantle wedge comes into contact wi th the subducting plate. The stress redistribution due to the mainshock see ms to have caused aftershocks with normal faultings to occur offset above a nd below the plate boundary.