Outer trench-slope faulting and the 2011 M_w 9.0 off the Pacific coast of Tohoku Earthquake

The 11 March 2011 off the Pacific coast of Tohoku Earthquake (M_w 9.0) produced megathrust displacements of at least 40 m. The resulting tsunami devastated the Honshu coast southwest of regions struck by earthquakegenerated tsunami in 1611, 1896 and 1933. The 1896 Meiji-Sanriku earthquake was also an underthrusting earthquake, but the 1933 Sanriku-oki earthquake was a trench-slope normal faulting event; both generated inundation heights of 10 to 25 m along the coast of Iwate prefecture. Possible occurrence of a great outer trenchslope earthquake seaward of the 2011 Tohoku Earthquake along a southwestward extension of the 1933 fault zone is a concern. The second largest 2011 aftershock, an outer rise M_w 7.7 normal faulting earthquake occurred near the southern end of the 1933 rupture. Additional aftershock activity has been distributed along a trend below the trench and diffusely spread in the outer rise, seaward of the megathrust region where the largest slip occurred. Coulomb stress perturbations of at least 5-10 bars are calculated for outer rise normal fault geometries for mainshock slip models. Whether a future great trench slope event will occur is uncertain, but the potential tsunamigenic hazard can be gauged by the huge inundations accompanying the 1933 rupture.