UPPER PLATE FAULTING ABOVE A SHALLOW SUBDUCTION MEGATHRUST EARTHQUAKE: MECHANICS AND IMPLICATIONS OF THE EXTREME SURFACE FAULTING DURING THE 2016 KAIKOURA, NEW ZEALAND, EARTHQUAKE

In most subduction zones, the upper plate above the zone of plate interface coupling and shallow rupture is offshore and thus underwater, obscuring the deformation. Although it is commonly assumed that that this region experiences broad elastic rebound similar to upper plate deformation farther onshore, recent observations from the 2016 Kaikoura, New Zealand, earthquake at the southern end of the Hikurangi subduction zone indicate a more complex deformational behavior. The Kaikoura earthquake was notable for producing widespread co-seismic surface ruptures, including some with extremely large fault offsets of up to 12 m. We have found that the extent of extreme surface ruptures is limited to the region immediately above the deeper subduction megathrust rupture zone. We explain this correlation with deformational models showing how upper-plate co-seismic extension (elastic rebound) unclamps these faults, and megathrust rupture decouples the base of these faults where they truncate against the plate interface. The combination of these processes leads to near-complete strain release in the upper plate above the rupture zone during a shallow megathrust earthquake. These conditions capable of producing significant surface rupture during megathrust earthquakes likely occur elsewhere along shallow, coupled sections of subduction zones, but in many cases these regions lie offshore and have been difficult to observe directly. We find geologic and seismic evidence that this upper-plate faulting process has occurred further north along the Hikurangi (New Zealand) subduction zone during the 1855 Wairarapa earthquake; as part of the 1960 Valdivia, Chile, event (the largest instrumentally recorded earthquake); and potentially during past earthquakes in the Cascadia subduction zone, as revealed by plate-motion-parallel displacement rates on an active upper-plate fault evidenced by offset marine terraces. Our results indicate that paleo-seismic interpretations of upper-plate faulting in subduction zones should include considerations of the linkage between periods of extreme surface faulting and megathrust events.