Evaluating the Tsunamigenic Potential of Buried Versus Trench‐Breaching Megathrust Slip. Issue 8 (26th August 2022) - Carvajal, Matías
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. Plain Language Summary: During subduction earthquakes, sudden seafloor deformation due to megathrust slip raises seawater to generate tsunamis. If the slip breaches the trench, seawater is raised by the seaward motion of the sloping seafloor. If the near‐trench part of fault refuses to slip, in a scenario called buried rupture, the seafloor will bulge to raise seawater. Here we combine computer models of seafloor deformation and tsunami propagation to study how the two types of rupture control tsunami runup. We find that, given slip size and location, their tsunamigenic potential is similar owing to a trade‐off mechanism. With buried rupture, tsunami generation is weakened by lessened motion of the sloping seafloor but strengthened by increased seafloor bulging. The effect is the opposite for a trench‐breaching rupture. We also find that if a rupture is buried to an optimal moderate depth, the bulging effect and tsunami size are maximized. These findings clarify that the key reason for a large tsunami is how large, not just how shallow, the megathrust slip is. This knowledge helps to understand why some recent megathrust earthquakes, including the 2011 M = 9 Tohoku‐oki earthquake, were so tsunamigenic. For tsunami hazard assessment, the tsunamigenic potential of buried ruptures should not be underrated. Key Points: Tsunamigenic seafloor uplift due to megathrust slip reflects a trade‐off between rigid‐body translation and elastic thickening Moderately buried slip is usually more tsunamigenic than trench‐breaching slip of the same size, because of enhanced elastic thickening What determines tsunami size, such as in the 2011 Tohoku‐oki tsunami, is mainly how large, not just how shallow, the offshore slip is