Geological evidence for past large earthquakes and tsunamis along the Hikurangi subduction margin, New Zealand

Abstract The Hikurangi subduction margin, New Zealand, has not produced large subductionearthquakes within the short written historic period (~180 years) and the potential of the plate interface to host large (M > 7) to great (M > 8) earthquakes and tsunamis is poorly constrained. The geological recordof past subductionearthquakes offers a method for assessing the location, frequency and approximate magnitude of subductionearthquakes to underpin seismic and tsunami hazard assessments. We review evidence of Holocene coseismic coastal deformation and tsunamis at 22 locations along the margin. A consistent approach to radiocarbon age modelling is used and earthquake and tsunami evidence is ranked using a systematic assessment of the quality of age control and the certainty that the event in question is an earthquake. To identify possible subductionearthquakes, we use temporal correlation of earthquakes, combined with the typeof earthquakeevidence, likely primary fault source and the earthquake certainty ranking. We identify 10 past possible subductionearthquakes over the past 7000 years along the Hikurangi margin. The last subductionearthquake occurred at 520–470 years BP in the southern Hikurangi marginand the strongest evidence for a full marginrupture is at 870–815 years BP. There are no apparent persistent rupture patches, suggesting segmentation of the marginis not strong. In the southern margin, the type of geological deformation preserved generally matches that expected due to rupture of the interseismically locked portion of the subductioninterface but the southern termination of past subductionruptures remains unresolved. The pattern of geological deformation on the central marginsuggests that the region of the interface that currently hosts slow slip events also undergoes rupture in large earthquakes, demonstrating different modes of slip behaviour occur on the central Hikurangi margin. Evidence for subductionearthquakes on the northern marginhas not been identified because deformation signals from upper plate faults dominate the geological record. Large uncertainties remain in regard to evidence of past subductionearthquakes on the Hikurangi margin, with the greatest challenges presented by temporal correlation of earthquake evidence when working within the uncertainties of radiocarbon ages, and the presence of upper plate faults capable of producing deformation and tsunamis similar to that expected for subductionearthquakes. However, areas of priority research such as improving the paleotsunami record and integration of submarine turbiditerecords should produce significant advances in the future.