FIELD INVESTIGATION OF HOLOCENE EARTHQUAKE UPLIFTED PALEO-SHORELINES AT WAIMARAMA AND ARAMOANA, SOUTHERN HAWKES BAY, NORTH ISLAND, NEW ZEALAND

The Hikurangi subduction margin along the east coast of New Zealand’s North Island accommodates oblique convergence of the Pacific Plate westward beneath the Australian plate at 45 mm/yr. Along the central segment of the margin, evidence of active forearc uplift is preserved along the southern Hawke’s Bay coastline within a series of uplifted Holocene marine terraces, extending from Cape Kidnappers, south to Blackhead.

Field studies of uplifted terraces at Waimarama and Aramoana were conducted in 2018 and 2019 under the Research Experience for Undergraduates (REU) Program of the NSF SHIRE Project (Seismogenesis at Hikurangi Integrated Research Experiment). Topographic profiles were surveyed across terraces using a laser rangefinder and handheld GPS unit to identify the number, extent, and elevation of terrace surfaces at each location. LiDAR data from Land Information New Zealand (LINZ) was processed in ArcGIS to generate terrace maps, and aerial drone imagery was collected in the field to aid in terrace interpretations. Two uplifted marine terraces (I, II) are identified along the Waimarama coastline, as well as at least one fluvial terrace along Puhokio Stream. At Aramoana, three marine terraces (I, II, III) were interpreted from field survey data.

To constrain the ages of prehistoric coseismic uplift events, marine shells were collected for carbon-14 dating of bedrock terrace platforms and overlying sediments. Preliminary calibrated AMS ages from Waimarama range from 5180-8330 ybp for Terrace I, and 1360-3430 ybp for Terrace II, consistent with earlier studies (Hull, 1987; Miyauchi et al., 1989; Ota et al., 1990). Additional pending ages for both Waimarama and Aramoana will allow for further correlation of terraces and earthquake event ages.

Tectonic uplift along the central Hikurangi margin is the net result of a complex interaction between megathrust slip at depth and upper-plate faulting. Uplifted Holocene paleo-strandlines are interpreted as preserving single earthquake uplift events. Results of this study 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). Both of these event types pose a significant seismic and tsunami hazard for New Zealand residents.