Integrated onshore–offshore paleoseismic records show multiple slip styles on the plate interface, central Hikurangi subduction margin, Aotearoa New Zealand

Abstract

Recent subduction earthquakes have shown that weakly-coupled sections of the plate interface fault may be susceptible to a range of different interface slip styles, meaning that seismic hazard is difficult to constrain. On the central Hikurangi subduction margin in Aotearoa New Zealand, plate motion over the last 20 years is thought to have been released aseismically via creep and slow slip events, but geologic evidence indicates seismic slip during large earthquakes may have occurred in the past. Due to patchy preservation and large age uncertainties, correlations of coastal coseismic deformation are unable to distinguish between similarly-timed earthquakes on upper plate faults and widespread synchronous rupture on the plate interface. To develop a more spatially and temporally continuous archive of paleoearthquakes, we present a 10,000-year record of submarine turbidites. Using high-resolution radiocarbon dating and age-depth modelling to generate precise core chronologies, we correlate unique episodes of synchronous turbidite triggering within four discrete submarine distributary systems that collectively span ca. 175 km of the central margin. Agreement with the timing of coseismic deformation onshore indicates that turbidites were likely triggered by ground shaking, and that both proxies can be combined to better examine individual earthquake ruptures. For the last 5000-year period, our onshore-offshore correlations show that paleoearthquakes vary in size, location and the combination of source faults involved. Our findings support that seismic slip on the plate interface occurred during some events which may also be part of larger multi-section subduction earthquakes. Based on this, we infer conditional stability on the weakly-coupled central Hikurangi margin that allows the plate interface to host both interseismic slow slip and seismic slip in large earthquakes. Combining the merits of onshore-offshore proxies to investigate variability in paleoearthquake patterns at other subduction margins should be prioritised to help reveal the full scope of interface slip behaviour and associated hazard.