Large-scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge

Slow slip events have become recognized in the last decade as an important mode of fault slip, and are most widely observed at subduction zones. Many episodes of tectonic tremor (related to slow slip) have been triggered by distant earthquakes due to dynamic-stress changes from passing seismic waves. However, there are few clear examples of large, geodetically detected slow slip events triggered by distant earthquakes. Here we use analyses of seismic and geodetic data to show that the magnitude 7.8 Kaikoura earthquake in New Zealand in 2016 triggered a large slow slip event between 250 and 600 km away. The slow slip was shallow, at less than 15 km deep, and spanned more than 15,000 km(2) of the central and northern Hikurangi subduction margin. The slow slip initiated immediately after the earthquake, lasted one to two weeks and was accompanied by a swarm of seismicity. We show that changes in dynamic stress in the slow slip source area ranged from 100 to 600 kPa-approximately 1,000 times greater than the static-stress changes of 0.2 to 0.7 kPa. We therefore propose that the slow slip event was triggered by dynamic-stress changes caused by passing seismic waves. Furthermore, the dynamic-stress changes were greatest on the shallow subduction interface, at less than 10 km depth, in a region overlain by a sedimentary wedge that acts as a waveguide, trapping seismic energy and probably promoting triggering of slip. This suggests that shallow slow slip events are more easily triggered by dynamic-stress changes compared with deep events.