Investigating subduction earthquake geology along the southern Hikurangi margin using palaeoenvironmental histories of intertidal inlets

Three marine inlets across the southern Hikurangi margin, New Zealand, are investigated for evidence of palaeocoseismic subsidence, a signal associated with great subduction earthquakes. Microfossil analyses and radiocarbon-dated shells from core samples show that none of the sites have subsided since 2000 cal yr BP. Pauatahanui Inlet has remained at the present elevation since 7000 cal yr BP, while Big Lagoon and south-eastern Lake Wairarapa both subsided c. 4 m between 6000 and 2000 cal yr BP. At Big Lagoon most or all of the subsidence was tectonic. At south-eastern Lake Wairarapa, however, sediment compaction caused some or all of the subsidence. Neither of the subsided sites contain sedimentary or palaeoenvironmental transitions suggestive of sudden, coseismic relative sea-level rise. One possible palaeotsunami deposit was found at Big Lagoon, coincident with a colluvial deposit, and two liquefaction layers were identified in south-eastern Lake Wairarapa.