Ylona van Dinther, Lukas Preiswerk, Taras Gerya

The 1960 M9.5 Valdivia and 1964 M9.2 Alaska earthquakes caused a decimeters-high secondary zone of uplift a few hundred kilometers landward of the trench. We analyze GPS data from the 2010 M8.8 Maule and 2011 M9.0 Tohoku-Oki earthquakes to confirm the existence of a secondary zone of uplift due to great earthquakes at the megathrust interface. This uplift varies in magnitude and location, but consistently occurs at a few hundred kilometers landward from the trench and is likely predominantly coseismic in nature. This secondary zone of uplift is systematically predicted by our 2D continuum visco-elasto-plastic seismo-thermo-mechanical (STM) numerical simulations, which physically-consistently model the dynamics at both geodynamic and seismic cycle timescales. Through testing hypotheses in both simple and realistic setups, we propose that a superposition of two physical mechanisms could be responsible for this phenomenon. First, a wavelength is introduced through elastic buckling of a visco-elastically layered fore-arc that is horizontally compressed in the interseismic period. The consequent secondary zone of subsidence is elastically rebound during the earthquake into a secondary zone of relative uplift. Second, absolute and broader uplift is ensured through a mass conservation-driven return flow following accelerated slab penetration due to a megathrust earthquake. The dip and width of the seismogenic zone and resulting (deep) coseismic slip seem to have the largest affect on location and amplitude of the secondary zone of uplift. These results imply that stick-slip modulates subduction and corner flow rates and that visco-elastic layering is important for inversion of interseismic and coseismic fault displacements.