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Paleoecological insights into subduction zone earthquake occurrence, eastern North Island, New Zealand

Ursula Cochran^,1, Kelvin Berryman¹, Judith Zachariasen², Dallas Mildenhall³, Bruce Hayward⁴, Kate Southall⁴, Chris Hollis⁵, Peter Barker⁶, Laura Wallace⁷, Brent Alloway⁷ and Kate Wilson⁸

¹ GNS Science, P.O. Box 30-368, Lower Hutt 6315, New Zealand
² URS Corporation, Suite 800, 1333 Broadway, Oakland, California 94612, USA
³ GNS Science, P.O. Box 30-368, Lower Hutt 6315, New Zealand
⁴ Geomarine Research, 49 Swainston Road, St. Johns, Auckland 1072, New Zealand
⁵ GNS Science, P.O. Box 30-368, Lower Hutt 6315, New Zealand
⁶ Barker Consulting, P.O. Box 27-106, Wellington 6001, New Zealand
⁷ GNS Science, P.O. Box 30-368, Lower Hutt 6315, New Zealand
⁸ School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6015, New Zealand

Paleoecological investigations of three Holocene marginal-marine sedimentary sequences provide information on vertical tectonic deformation in a transect across the forearc basin adjacent to the Hikurangi subduction zone, New Zealand. The elevation of maximum postglacial sea level indicators at Te Paeroa Lagoon and Opoho is between 4 and 6 m below present mean sea level, indicating net subsidence since 7200 yr B.P. Opoutama is closer to the Hikurangi Trench and appears to lie near the edge of the zone of subsidence, as evidence for vertical movement there is equivocal. Some of the subsidence at Te Paeroa Lagoon and Opoho is likely to be a result of compaction. However, a component of subsidence probably happened coseismically in two events at ca. 7100 and 5550 yr B.P. Event signatures consist of tsunami deposits overlain by chaotically mixed, reworked sediment that appears to have filled rapidly created accommodation space at marine inlet sites 10 km apart. Large offshore earthquakes are suggested by the coincidence of tsunami inundation with sudden subsidence. Forward elastic-dislocation models indicate that the observed subsidence could be achieved in M_w 7.9 earthquakes on either the subduction interface or the Lachlan Fault, which would involve synchronous uplift of Mahia Peninsula. Combined rupture of the interface and the Lachlan Fault, either simultaneously in a M_w 8.1 earthquake, or consecutively, could explain larger amounts (>1.5 m) of coastal subsidence.

Key Words: Holocene • Wairoa • Mahia Peninsula • Hawke Bay • Lachlan Fault • accommodation space • earthquakes • microfossils • sea level • subduction zone • subsidence • tsunami

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