GENERATING PRECISELY-DATED EARTHQUAKE RECORDS ON THE CENTRAL HIKURANGI SUBDUCTION MARGIN, NEW ZEALAND

On the Hikurangi subduction margin, New Zealand, well-dated earthquake records are required to establish whether the weakly coupled central margin ruptures in large earthquakes independently from the locked southern margin. The existing record of subduction earthquakes on the central margin record from Ahuriri Lagoon reports 8 subduction earthquakes in the last 7000 years however, it is obtained from a large estuarine system with oceanographic and topographic complexities resulting in spatially heterogeneous preservation. Evidence for each earthquake at Ahuriri only occurs in one or two cores and is dated with just a few radiocarbon ages, and all events from across the system are combined into a single age model to produce earthquake ages. A preliminary study from a site called Pakuratahi Valley, just 10 km north, yields asynchronous earthquake ages which is problematic when attributing a subduction interface source as both sites should record the same earthquakes.

To distinguish whether the disparity in earthquake ages between Ahuriri and Pakuratahi is due to imprecise dating or if, in fact, the sites record different earthquakes, we develop a new earthquake chronology from the Pakuratahi Valley. The reduced paleoenvironmental complexity of this site has produced laterally continuous stratigraphy, displaying compelling evidence of both coseismic subsidence and uplift characterised by peat-silt and silt-peat contacts. Our approach to ensure robust calculation of earthquake ages includes i) scrutinous selection of material for radiocarbon dating targeting short-lived, fragile organics, ii) consideration of stratigraphic context of targets and airfall versus reworked tephra horizons, and iii) sequential dating through peat to reduce uncertainties. The precisely dated Pakuratahi earthquakes will then be compared with the records from Ahuriri and offshore turbidite paleoseismology to better understand subduction earthquake behaviour of the central and southern Hikurangi subduction zone.