A 5000 yr record of coastal uplift and subsidence reveals multiple source faults for past earthquakes on the central Hikurangi margin, New Zealand

Abstract

Prehistoric records of subduction earthquakes are often distinguished by evidence of synchronous widespread coastal deformation, the extent of which negates the plausibility of alternative source faults. At the Hikurangi subduction margin in New Zealand, untangling the record of subduction interface ruptures is complicated. Large earthquake age uncertainties inhibit unique solutions of along-strike correlations, and complex patterns of coastal deformation caused by upper-plate faulting prevent reliable indication of source faults. In this work, we improved paleoearthquake reconstructions on the central Hikurangi margin with a new, well-constrained 5000 yr earthquake record from Pakuratahi Valley near Napier, North Island, New Zealand. Evidence of laterally extensive paleoenvironmental changes is consistent with coseismic subsidence and coseismic uplift in large earthquakes. Radiocarbon dates on fragile terrestrial macrofossils and tephra isochrons were used to construct robust age models that yielded earthquake ages of 4839−4601 calibrated (cal.) yr B.P., 3630−3564 cal. yr B.P., 2687−2439 cal. yr B.P., and 1228−823 cal. yr B.P. Integration of these ages with refined earthquake chronology from nearby Ahuriri Lagoon indicated that the next large earthquake impacting the Napier area is more likely to cause coastal subsidence than uplift. Drawing on correlations with cotemporal evidence elsewhere on the central margin, we infer that the overall patterns of coseismic deformation could be generated by either rupture of the subduction interface or upper-plate faults, or both. This inability to separate source faults for past earthquakes limits the efficiency of forecasting future earthquakes. Similar problems of intertwined paleoearthquake signatures likely apply to other plate boundaries, where we recommend cautious interpretation of coastal deformation to accurately address the hazard from both types of source faults.