Aftershock distribution of the 1994 Sanriku‐oki earthquake (<i>M_w</i> 7.7) revealed by ocean bottom seismographic observation

Abstract

The 1994 Sanriku‐oki earthquake ( M w 7.7) ruptured the entire seismogenic zone of the Pacific plate subduction beneath northeastern Japan arc. We deployed 18 ocean bottom seismographs and recorded its aftershocks, which span the whole seismogenic zone, with unprecedented accuracy. We calculate hypocenters 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 aftershock and background seismicity. The rupture continued to slip along a shallow dipping (&lt;10°) interface for ∼60 km, and then slipped across an asperity where the dip angle starts to increase at ∼143°E. Here aftershocks are few, suggesting complete release of strain. The rupture continued another 100 km or so to the downdip end beneath the coastline, where the plate dip angle reaches 30° to smoothly connect to the dip angle of the Wadati‐Benioff zone. Coseismic slips are larger westward of 143°E, where the mantle wedge comes into contact with the subducting plate. The stress redistribution due to the mainshock seems to have caused aftershocks with normal faultings to occur offset above and below the plate boundary.