资源环境科技发展态势分析平台

A subset of megathrust earthquakes produce anomalously large tsunamis for their magnitude. All of these recorded 'tsunami earthquakes' in the past 50 years had extensional aftershocks in the upper plate. These include the two largest and most destructive earthquakes of that period, the 2004 Sumatra-Andaman and the 2011 Tohoku events. Evidence from the region of Tohoku indicates that normal fault slip in the upper plate during the earthquake may have contributed to the tsunami size. Here we present a numerical model that shows how a reduction of the dip of a subducting slab, on a timescale of millions of years, can result in an extensional fault failure above a megathrust earthquake on timescales of seconds to months. Slab dip reduction bends the upper plate so that the shallow part fails in extension when a megathrust rupture relieves compressional stress. This results in a distribution of extensional aftershocks comparable to that seen above the Tohoku megathrust. Volcanic arc migration and uplift data for Tohoku and several other tsunami earthquakes is consistent with slab dip reduction. The collection of more such data might identify other areas of tsunami hazard related to slab dip reduction.

Tsunami generation by megathrust earthquakes is enhanced by extensional faulting in the upper plate when the subducting slab shallows, according to numerical modelling and observations from the Sumatra-Andaman and Tohoku earthquake-tsunami events.