TECTONICS OF THE 1931 HAWKES BAY EARTHQUAKE

The M(s) 7.8 Hawke's Bay earthquake of 1931 February 3 (New Zealand local time) was felt throughout most of New Zealand and caused 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, to the 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 accomodate slip. Elsewhere, the elastic nature of the rocks resulted in surface folding above a buried causative fault. Fault scarps with up to 4.6 m 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 the 1931 earthquake. Fault modelling from elevation changes and retriangulation data suggest that the 1931 earthquake was generated by slip 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 beneath Hawke Bay confirm the location of a major fault along the line of zero uplift in 1931. The 1931 earthquake deformation pattern is typical of imbricate faults within the coastal part of the accretionary wedge of the Australian plate. These faults propagate upward from the subduction zone of the Pacific plate and generate most of the large earthquakes in this part of New Zealand.