Abstract Title: Marine Terrace Evidence for Two Paleo-Earthquake Uplift Events Along the Southern Hawkes Bay Coastline, North Island, New Zealand

Abstract Submitted to: TECTONOPHYSICS

Abstract Text:

The Hikurangi subduction margin along the east coast of New Zealand’s North Island accommodates oblique convergence of the Pacic 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 set of uplifted Holocene marine terraces that wrap around the Cape Kidnappers headland, from Clifton in the north to Waimarama in the south.

Field studies of uplifted terraces at Clifton, Ocean Beach, and Waimarama 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 proles were
surveyed across terraces using a laser rangender and handheld GPS unit to identify the number, extent, and elevation of terrace surfaces at each location. Airborne LiDAR data from Land Information New Zealand (LINZ) was processed in ArcGIS to generate terrace maps, and aerial drone imagery was collected in the eld to aid in terrace interpretations. Two uplifted marine terraces (T1, T2) are identied along the Waimarama coastline, while only the lower of these two terraces (T1) is recognized at Ocean Beach and Clifton.

To constrain the ages of prehistoric coseismic uplift events, marine shells were collected for 14C dating of bedrock terrace platforms and overlying sediments. Preliminary calibrated AMS ages from Waimarama range from 5180-6570 ybp for T2, and 1860-3430 ybp for T1, consistent with earlier studies (Hull, 1987; Miyauchi et al., 1989; Clark et al., 2019). Ages for T1 at Ocean Beach and Clifton range from 1710- 3160 ybp. Earthquake events likely occurred near the younger end of these age ranges (E1≤1700 ybp; E2≤5200 ybp).

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 signicant seismic and tsunami hazard for New Zealand residents.