Abstract

Seismic reflection imaging shows a marked shallow kink at ∼12 km depth in the Pacific plate beneath the central North Island, New Zealand, that coincides with (1) a decrease in the amplitude of the plate boundary reflection, (2) the locus of prominent landward-dipping splay thrust faults in the overlying plate, and (3) the onset of seismogenesis on the subduction interface and within the subducted plate. We propose that the sharp change in the dip of the plate interface is indicative of the downdip transition from stable to unstable slip regimes. Earthquake focal mechanisms suggest the kinking is accomplished through simple shear on reactivated normal faults in the crust of the subducted plate, akin to the down-stepping motion of an escalator. The geological record of uplift in the overlying plate indicates the escalator has been operating for the last 7 m.y.

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