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Assessing Seismic Hazard on Offshore Fault Sources Using New Coastal Record Techniques: Example From the Central Hikurangi Subduction Zone, New Zealand

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems II

Type: Oral

Date: 4/21/2021

Presentation Time: 02:30 PM Pacific

Description: 

A large proportion of the world’s population live on coasts, and in tectonic settings these communities are increasingly vulnerable to seismic and tsunami hazards. Characterising earthquake fault sources in coastal settings can be challenging, particularly if faults are wholly submarine and so cannot be studied by conventional paleoseismic techniques. Nearshore faults are particularly challenging to characterise because of the difficulties in obtaining geophysical data in shallow water and yet are likely to present the highest hazard. Conventional techniques studying coastal paleoearthquake records such as coseismic marine terraces are increasingly being complemented by new techniques such as lidar, UAV and high-resolution dating using Bayesian statistical modelling. We present a paleoseismic study of marine terraces using these techniques at Aramoana, southern Hawke’s Bay region, New Zealand, in the central Hikurangi Subduction Margin. This area overlies a partially locked part of the subduction interface and has a system of poorly-constrained upper plate thrust faults mapped ~6 km offshore. Three Holocene marine terraces are well expressed in lidar and UAV data and a trench confirms each is underlain by a subhorizontal shore platform separated by large (1-4.5 m) steps, a morphology interpreted to have formed by shore platform erosion interrupted by three earthquakes. OxCal modelling of 38 radiocarbon ages from the beach deposits in the trench constrain three discrete clusters of shell ages in the last 7000 years, with uncertainties of 300-400 years. Some of these correlate with paleoearthquake ages from other marine terrace and subsiding lagoon sites up to 75 km north of Aramoana, but not with earthquake ages from the northern Hikurangi Subduction Margin, suggesting the earthquake sources are not great (M8+) subduction earthquakes. Results from simple elastic dislocation models suggest that the earthquake sources are unlikely to be individual nearshore faults but could be multi-fault ruptures on the upper plate faults or localised subduction earthquakes.

Presenting Author: Nicola Litchfield

Student Presenter: No

Authors