Darby, D.J. 1987 Geodetic deformation analysis regionally for northern Hawke's Bay, and locally for the Mohaka River area. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EDS 105 82 p.
Abstract: An exhaustive search of land surveying archives for the northern Hawke's Bay area has revealed original records of horizontal observations which were not previously known to exist, and others for which only adjusted data on plans had previously been recovered. These new data are valuable supplements to the data used by others for deformation studies. The motivation for the search was in part the study of an area relevant to the assessment of seismotectonic hazard for power development of the Mohaka River. This is not necessarily an area for which the best records may be found, and because of losses in the fire following the 1931 Hawke's Bay earthquake, may possibly be the area in New Zealand where the records are the most chaotic. Because of this major earthquake and the 1932 Wairoa event, older surveys of relatively poor precision are of less use for the determination of spatially and temporarily uniform deformation rates than in other parts of the country. It is nevertheless important to have access to all possible data for the study of these events and the region in which they occurred. Reliable data from a total of 23 surveys between 1876 and 1983 have been recovered. The best method of estimating surface strain components depends upon the nature of available data. The two most appropriate methods, simultaneous adjustment and derivation from estimated displacements, are used here. These methods are fully exploited in an effort to extract reliable deformation estimates from the original observations which were not made for this purpose. Though the two methods sometimes yield different statistical confidence intervals for estimated parameters, some conclusions appear highly significant independently of their method of derivation. These include deformation associated with the two major earthquakes, and these results are quite compatible with those already published. For the 1931 Hawke's Bay event a tensor shear strain of 12+/-4 ppm relatively contractional at 120+/-70 is estimated regionally. For the 1932 Wairoa event a shear strain of 24+/-4 ppm relatively contractional at 78+/-50 is estimated for the epicentral area. For the periods before and after these events results differ from those previously published. there is some evidence that before the 1930's the regional strain regime was relatively contractional north-south. Since then, the shear strain in the area studied appears to be relatively contractional at 139+/-60, at a rate of 0.10+/-0.03 ppm/yr which is of the order expected in this tectonic environment. Other results are less well determined and relate mainly to deformation immediately before and immediately after the major earthquakes. The study of dislocation models for the major earthquakes is being undertaken and will be reported separately. It may lead indirectly to a better description of the strain regime before those events (auth)
