Transition from Subduction to Strike-slip: Insights from a well-expressed example in central New Zealand

Central New Zealand (southern North Island/northern South Island) occupies a complex transition from subduction to strike-slip at the southern termination of the Hikurangi subduction zone. This transition was also the site of the 2016 M7.8 Kaikoura earthquake, which is the most complex earthquake ever recorded, involving rupture of over a dozen faults. I will discuss how this transition from subduction to strike-slip occurs, through a joint interpretation of geologic and geodetic data. Approximately 2 cm/yr of convergence is accommodated at the Hikurangi Trough offshore the southern North Island, which decreases rapidly southward offshore the northern South Island (to a few mm/yr), where the majority of the relative plate motion is transferred onto strike-slip faults within the upper plate. This transfer of slip is facilitated via easterly-trending strike-slip faults (such as the Boo Boo fault in Cook Strait). Although the shallow megathrust offshore the northeastern South Island accommodates low rates of convergence, kinematic models based on geodetic data indicate that a large component of relative plate motion must be accommodated on the subduction interface at depth, down-dip of the major upper plate faults. This, along with geodetic evidence for interseismic coupling on the subduction interface beneath the northeastern South Island has implications for the potential southward extent of subduction interface earthquake rupture in central New Zealand. InSAR and GNSS evidence for large amounts of afterslip on the subduction interface following the Kaikoura earthquake also indicates the subduction zone continues as an important boundary well into the northern South Island.