LIDAR MAPPING OF EARTHQUAKE UPLIFTED PALEO-SHORELINES: GLENDHU ROCKS TO FLAT POINT, SOUTHERN WAIRARAPA COAST, 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. At the southern end of the margin, pronounced forearc uplift occurs along the Wairarapa coast onshore of the subducting Hikurangi plateau. In this area, flights of uplifted Holocene marine terraces and beach ridges occur along a narrow coastal lowland, preserving a record of prehistoric coseismic uplift events.

In January 2017, we participated in the Research Experience for Undergraduates (REU) Program of the NSF SHIRE Project (Subduction at Hikurangi Integrated Research Experiment). We conducted reconnaissance fieldwork at multiple coastal sites to identify future targets for detailed study. Here, we report results of Airborne LiDAR mapping and correlation of uplifted paleo-shorelines. We use LiDAR data sourced from Land Information New Zealand (LINZ), and processed in ArcGIS, to generate shoreline maps. Real Time Kinematic (RTK) GPS surveying was conducted at one site (Glenburn Station) for calibration of LiDAR-derived shoreline elevations. Prior field mapping and radiocarbon dating (Berryman et al., 2011) are used to guide our LiDAR mapping efforts.

From Glendhu Rocks, northeast to Flat Point, up to four uplifted terraces and beach ridges occur along the coast. At some sites, the terrace risers converge indicating the loss of a younger riser to erosion. Mapping and lateral correlation is made difficult by the presence of abundant landslide debris and alluvial fan deposits. Near Flat Point, dune deposits obscure the expression of terraces and ridges.

Tectonic uplift along the southern 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. Ongoing mapping, surveying, and age dating 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.