Untangling Upper-Plate Fault and Subduction Earthquakes Using Holocene Marine Terraces, Cape Kidnappers Coast, Hawke’s Bay, New Zealand

Untangling sources of coseismic coastal uplift (or subsidence) is an ongoing challenge in subduction zones with abundant upper-plate faults. Attributing the cause of uplift to a particular fault is crucial to understanding subduction zone kinematics, seismic and tsunami hazard, and future sea level rise. In this study, we revisit Holocene marine terraces (MTs) along the Cape Kidnappers coast, New Zealand, above a portion of the Hikurangi subduction interface that is currently weakly coupled and outboard of coastal lagoons that contain records of coseismic subsidence attributed to subduction earthquakes. Untangling the sources of coastal uplift is crucial to understanding the size and likelihood of seismogenic slip on the currently weakly coupled portion of the central Hikurangi subduction margin. MTs are remapped and surveyed, 65 new radiocarbon ages are added to the previous 20, and ages are compared with surrounding sites. We confirm the presence of one main MT between Clifton and north Waimārama and three in south Waimārama. Coseismic uplift is dated at 6251–5911 (EQ1), 4787–4396 (EQ2) and 2231–1825 (EQ3a), and/or 2006–1665 (EQ3b) cal. B.P. The new ages do not support the widely cited landward younging of ages on the Kidnappers terrace. A lack of temporal correlation with subsiding sites in inner Hawke Bay but correlations with MT (uplift) sites to the northeast suggests the earthquake sources are upper-plate faults at or north of Waimārama. Estimated magnitudes are Mw∼6.8 (EQ1), Mw∼7.7 (EQ2) and Mw∼7.8 (EQ3 simultaneous rupture), or a series from Mw∼6.8 to 7.3 (EQ3 cascading rupture). The two interevent times (1124–3305 yr) suggest relatively long recurrence intervals for earthquakes on the near-shore Waimārama and Kidnappers faults and that multifault earthquakes are relatively rare.