Abstract

Subduction megathrust ruptures that breach the trench, such as in the 2011 M = 9 Tohoku-oki earthquake, can be very tsunamigenic. However, whether buried ruptures are intrinsically less tsunamigenic has not been fully investigated. Here, we conduct this investigation by studying the mechanics of seafloor deformation and the resultant tsunami runup. With a trench-breaching rupture, deformation is dominated by the rigid-body translation of the frontal upper plate, and seafloor uplift is enhanced by the horizontal motion of the sloping seafloor. With a buried rupture, the rigid-body motion is reduced, but the shortening of the upper plate due to seaward slip termination causes elastic thickening to enhance tsunamigenic seafloor uplift. By combining a finite-element deformation model and a shallow-water equation tsunami model, we systematically test various subduction zone geometrical and slip parameters to study the trade-off between rigid-body translation and elastic thickening in causing seafloor uplift and tsunami runup. To isolate the effect of slip depth, we compare scenarios with the same peak slip and rupture width. We find that very shallow ruptures, including those breaching the trench, are generally less tsunamigenic than deeper ruptures. Given peak slip, tsunamigenic potential is maximized if the rupture is not too shallow or too deep but is buried to moderate depths. Our model tests using variable hypothetical rupture depths suggest that, with a slip magnitude as large as in the 2011 Tohoku-oki earthquake, tsunami runups as high as the observed would occur even if the rupture were fully buried.