The 1968 may 23 inangahua, New Zealand, earthquake: An integrated geological, geodetic, and seismological source model

Abstract

The 1968 Inangahua, New Zealand, earthquake occurred in the West Coast Basin and Range Province, northwest of the main plate boundary zone in northern South Island. At MS 7.4, it is not the largest known earthquake in the province, but it has been the subject of thorough seismological, geological, and geodetic documentation. Re interpretation of past observations and more recent data, in the light of new structural and tectonic theories, has produced a new source model for the earthquake. The data suggest that at least 4 m of reverse slip occurred on a fault plane dipping 45° to the northwest beneath the northern part of the Grey‐Inangahua Depression, an area previously inferred to be on the footwall of major reverse faults bounding the ranges on either side of the depression. The seismogenic fault may have propagated north and south across older geological structures in recent times. Faulting within basement is occurring on pre‐existing faults and is accommodating some of the compressional component of oblique relative motion across the plate boundary in northern South Island. Discontinuous coseismic fault ruptures are mainly interpreted as secondary features formed in response to widespread shortening within the sedimentary cover (flexural slip folding) imparted by the deeper primary faulting. Ongoing uplift across a late Quaternary fault trace at Manuka Flat possibly represents postseismic slip over the upper part or northern end of the 1968 rupture plane. Although the Inangahua earthquake source mechanism is consistent with the regional late Quaternary tectonic pattern, the regional rate of seismicity is high (perhaps representing clustering) in comparison with average fault slip rates and recurrence intervals in the province. © The Royal Society of New Zealand 1994.