FIELD INVESTIGATION OF HOLOCENE EARTHQUAKE UPLIFTED PALEO-SHORELINES AT CLIFTON AND OCEAN BEACH, SOUTHERN HAWKES BAY, NORTH ISLAND, NEW ZEALAND

The Hikurangi margin is located along the east coast of New Zealand’s North Island where the Pacific plate subducts obliquely beneath the Australian plate at 45 mm/yr. Resultant coastal tectonic uplift occurs from ductile and brittle deformation of the overriding Australian plate as compressional stress is accommodated by folding and seismic slip along the deep megathrust and shallow upper-plate faults. Uplifted Holocene marine terraces are interpreted as preserving single earthquake uplift events, thus much of the North Island’s east coast preserves a geologic record of Hikurangi margin paleo-seismicity.

Tectonic activity along this plate boundary poses a major seismic and tsunami hazard for North Island residents, but it’s destructive potential is largely unknown because of New Zealand’s short historical earthquake record. The NSF SHIRE project (Seismogenesis at Hikurangi Integrated Research Experiment) is a collaborative research effort to study subduction zone dynamics and to expand the North Island’s paleo-seismic record. The 2018 and 2019 SHIRE Project REU program led CPP’s geomorphology research team to Clifton and Ocean Beach in the Cape Kidnappers area of southern Hawke’s Bay to conduct a terrestrial investigation of uplifted Holocene marine terraces.

This study reveals the presence of at least one earthquake uplifted Holocene marine terrace at Clifton and Ocean Beach. LIDAR data from Land Information New Zealand (LINZ) was processed in ArcGIS to map the terrace and shore deposits at these sites. Field topographic surveying and aerial drone imagery confirm the location, extent, and elevation of uplifted paleo-shorelines. Mollusk shells were retrieved from terrace deposits for radiocarbon dating. Preliminary calibrated AMS ages from southern Ocean Beach range from 2340-3160 ybp, consistent with prior studies of nearby sites at Cape Kidnappers and Waimarama (Hull, 1987; Miyauchi et al., 1989). Additional pending ages for Clifton and northern Ocean Beach will allow for further correlation of terraces and earthquake event ages. Ongoing terrace mapping, surveying, and age dating for additional sites along the Hikurangi margin may help differentiate between very large margin-wide megathrust earthquakes (M8.0-9.0+) and smaller more localized upper-plate thrust events (M7.0-8.0).