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.

特大推力地震的一部分会产生异常大的海啸。在过去的50年中，所有这些记录在案的“海啸地震”都在上板块产生了余震。其中包括那个时期的两次最大，破坏性最大的地震，即2004年的苏门答腊-安达曼地震和2011年的东北地震。来自东北地区的证据表明，地震期间上板块的正常断层滑动可能是海啸发生的原因。在这里，我们提供了一个数值模型，该模型显示了数百万年的时间尺度上俯冲平板倾角的减小如何在数秒至数月的时间尺度上导致超大推力地震以上的伸展断层破坏。平板倾角减小使上板弯曲，从而当大推力破裂减轻了压应力时，浅部无法延伸。这导致了伸展余震的分布与东北巨推力以上的分布相当。东北和其他几次海啸地震的火山弧迁移和隆升数据与平板倾角的减少相一致。收集更多此类数据可能会确定与减少板倾角有关的海啸危害的其他领域。