Tectonics of the 1931 hawke's bay earthquake

Abstract

The Ms7.8 Hawke's Bay earthquake of 1931 February 3 (New Zealand local time) was felt throughout most of New Zealand andcaused extensive damage at Napier, Hastings, and throughout Hawke's Bay. Surface deformation accompanying the earthquake resulted in a >90 km long, 17 km wide asymmetric dome trending northeast and extending from southwest of Hastings to northeast of the Mohaka River mouth. Maximum uplift of 2.7 m occurred near the mouth of the Aropaoanui River close to the location of major aftershock activity, while maximum subsidence of 1 m was recorded at Hastings, tothe southeast of the steeper, southeast-facing side of the dome. Observed surface faulting of about 15 km in length was confined to the southwestern end of the dome where near-surface rocks had sufficient strength and suitably oriented pre-existing fractures to accommodate slip.Elsewhere, the elastic nature of the rocksresulted in surface folding above aburied causative fault. Fault scarps with up to 4.6m vertical separation and 1.8 m dextral strike slip were not well preserved in 1984; only about 3 km of fault trace could be confidently recognised. Present geological techniques for the recognition of past earthquakes would fail to determine the extent of surface deformation and the magnitude of the1931earthquake.Fault modelling from elevation changes and retriangulation data suggest that the 1931 earthquake was generated byslip at a reverse-dextral fault that dips steeply to the northwest beneath Hawke's Bay. Interpretation of seismic reflection data from Hawke's Bay and beneathHawke Bay confirm the location of a major fault along theline of zero uplift in 1931. The 1931 earthquake deformation pattern is typical of imbricate faults within the coastal part of the accretionary wedge ofthe Australian plate. Thesefaults propagate upward from the subduction zone of the Pacific plate and generate most of the large earthquakes in this part of New Zealand. © 1990 Taylor & Francis Group, LLC.