Size and timing of large prehistoric earthquakes on the Wairau Fault, South Island

Abstract

The Wairau Fault is the northwesternmost of the active Marlborough Faults, a system of right-lateral strike-slip faults in the northern South Island of New Zealand that serve to transfer slip from the right-lateral Alpine Fault in the South Island to the Hikurangi subduction zone in the North Island. Although the slip rates for the Marlborough faults have been reasonably well-constrained, the paleoseismic history has only recently been addressed. Recent work on the Awatere, Clarence, and Kekerengu Faults has begun to answer these questions, but, until now, no direct investigations of the earthquake event history of the Wairau Fault have been undertaken.

In 1999, the Earthquake Commission of New Zealand funded the Institute of Geological and Nuclear Sciences to undertake an investigation of the rupture history of the Wairau Fault. The aim of the project was to constrain the timing of recent (Holocene, <10,000 years) fault ruptures. To obtain these data, the Institute proposed to excavate three trenches across the fault, with the hope of mapping the fault and stratigraphy in the trenches, determining the number of fault ruptures that had occurred, and, by collecting carbonaceous material from the faulted and unfaulted sediments, obtain radiocarbon dates that could constrain the absolute ages of past rupture events. These results would then be used to infer the recurrence interval of faulting and the time elapsed since the last fault rupture.

This report presents the results of the first paleoseismic study of the Wairau Fault and puts some preliminary constraints on the timing of Holocene earthquakes on this northernmost active fault of the Marlborough Fault System.