EVIDENCE FOR PALEO-SUBDUCTION EARTHQUAKES AND TSUNAMI AT A MARGIN WITH PERVASIVE ACTIVE UPPER PLATE FAULTING: SOUTHERN HIKURANGI MARGIN, NEW ZEALAND

At the southern Hikurangi margin, New Zealand, we document evidence of coseismic subsidence and paleotsunami at 879-798 years BP, and a younger coseismic subsidence event at 519-463 years BP. Both events are attributed to rupture of the plate interface. The southern Hikurangi margin has had no significant (>M 6.5) subduction earthquakes in historical times, and this is the first evidence of prehistorical subduction earthquakes on this segment of the margin. The forearc structure of the southern Hikurangi margin is dominated by multiple active strike-slip faults that are capable of producing coastal deformation and tsunami similar to that expected in the geologic record for a subduction earthquake. Distinguishing the effects of upper plate faulting from plate interface earthquakes is a significant challenge at this margin.

Our study site is Big Lagoon, in the northeastern South Island where back-barrier salt marshes preserve a record of ~1000 years of marginal marine sedimentation. Foraminifera and diatoms are used to understand the paleoenvironments and 28 radiocarbon dates provide age control. Two prominent paleosols are present at 1 m and 0.7 m depth. The presence of tsunami sand overlying the lower paleosol, the sharp upper contacts of the paleosols and their association with a sudden change in paleo-elevation is evidence that each paleosol is associated with coseismic tectonic subsidence. We use correlation with local and regional upper plate paleoearthquake chronologies, elastic dislocation and tsunami modelling to determine the most likely seismic sources for each subsidence event.

The older event at 879-798 years BP correlates with the penultimate rupture of the Wairarapa Fault. Dislocation modelling suggests the Wairarapa Fault, an upper plate fault listric to the plate interface, would not produce tectonic subsidence at Big Lagoon, but rupture of the deep portion of the plate interface alone, or with the Wairarapa Fault, could produce the observed 0.5 m of subsidence recorded at Big Lagoon. The tsunami associated with this event was ≥3.3 m in height at the shoreline of Big Lagoon and inundated at least 360 m inland. The 519-463 year BP event does not correlate with any regional upper plate fault ruptures and is most likely a subduction earthquake that caused ~0.3 m of subsidence at Big Lagoon.