The Rupture Mode of the Shallow Large-Slip Surge of the Tohoku-Oki Earthquake

The remarkable feature of the M-w 9.1 Tohoku-Oki earthquake of 2011 was a late, shallow, surge that propagated from about 12 km depth all the way to the trench. This surge had very large slip and was depleted in high-frequency radiation. It was followed by normal faulting in the outer wedge; this requires that the postsurge basal shear stress be close to zero. Explanations for the surge propagating through the velocity-strengthening region by a thermal weakening mechanism fail to produce the near-total stress drop required by the postseismic extension. The surge propagated in a region of material contrast across the plate interface. The combination of a strong nucleation pulse from down dip, velocity strengthening, and the bimaterial effect satisfies the conditions for wrinkle pulse propagation. A wrinkle pulse rupture mode can produce an instability by overcoming velocity strengthening with the normal stress reduction produced by the bimaterial effect. It also leads to total stress drop, satisfying the extension constraint and explaining the prodigious slip amplitude of the surge. Because the surfaces are detached in wrinkle pulse mode, asperity contact is reduced, which explains the dearth of high-frequency radiation during the surge. This behavior appears to be a common feature of many of the greatest subduction earthquakes.