Improved age control for a fourteenth century earthquake and tsunami from Okupe Lagoon, Kapiti Island

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Cochran, U.A.; Litchfield, N.J.; Clark, K.J.; Ries, W.F.; Villamor, P.; Howarth, J.D.; Watson, C.; Strong, D.T. 2015 Improved age control for a fourteenth century earthquake and tsunami from Okupe Lagoon, Kapiti Island. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/28 37 p. + 4 appendices

 

Abstract: Analysis of six new cores from Okupe Lagoon on Kapiti Island replicates previous evidence for an earthquake and tsunami signature and vastly improves the age control for the event which is now known to have occurred at 620-564 cal. yr BP (calibrated years before present) or 1330-1386 AD. The signature consists of a concentration of brackish-marine diatoms within a unit of pebbles, wood, and plant fragments, and a change in depositional environment to a less saline but deeper water lagoon interpreted as marine inundation, catchment disturbance, and subsidence respectively. We analyse twelve radiocarbon ages from two cores to obtain the most likely age for this event. There are no known paleoearthquakes of this age in the wider Wellington or Marlborough regions and the event timing does not coincide with evidence for rupture of the Hikurangi subduction interface at 520-470 cal. yr BP from Big Lagoon in Marlborough. A likely source of the earthquake at 620-564 cal. yr BP is surface rupture of the Rangatira Fault. With a digital elevation model from new lidar data, we document onshore traces of the active Rangatira Fault that has previously only been known from offshore studies. Given there is a scarp height of 1-2 m across recent (< 5000 year old) geomorphic features, we infer surface rupture of the fault within that time period. Several features of Okupe Lagoon’s stratigraphy are more simply explained by rupture of the Rangatira Fault at 620-564 cal. yr BP than by rupture of a distal fault: a thousand year old marker horizon is 0.4 metres lower on the eastern (down-thrown) side of the fault than it is on the west, the 620-564 cal. yr BP earthquake signature only occurs on the eastern (down-thrown) side of the fault, the earthquake resulted in deepening of the eastern arm of the lagoon. We hypothesize that an earthquake on the Rangatira Fault at 620-564 cal. yr BP caused these features when the eastern side of the fault dropped with respect to the west (sense of movement indicated by scarp geometry) and a tsunami was triggered in the lagoon because of offset of the lagoon floor resulting in a strong flow of water from west to east that scoured and deposited the wood, pebbles and diatoms that make up the sedimentary part of the earthquake signature. Although this scenario fits the available data well, we did not know about the fault scarp when we designed our field sampling, so we do not have an adequate distribution of data across the fault to absolutely confirm this scenario in this study. Geological evidence for rupture of the southern Hikurangi subduction interface at 520-470 cal. yr BP at Big Lagoon is not replicated in this study as the earthquake signature we targeted for further investigation from Okupe Lagoon is older and most likely from a different source. However, a weaker signal of catchment disturbance observed in the new cores has an age of 540-494 cal. yr BP which overlaps with the age of evidence at Big Lagoon and may be the result of strong shaking from a subduction earthquake but, without evidence of vertical deformation, does not provide independent evidence of one. Recent related research from Ahuriri Lagoon in Napier indicates that the two most recent earthquakes involving subsidence at that site overlap in time with the earthquakes at Big Lagoon. Therefore, rupture of the southern and central parts of the Hikurangi subuduction interface together in great earthquakes cannot be ruled out – improved age control at both sites and, ideally, further evidence at intervening sites, is required to address this possibility. (auth)