Blick, G.H.; Read, S.A.L.; Hall, P.T. 1985 Ostler Fault Zone progress report on surveying results 1966 - 1985 levelling and tilt levelling. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EDS 103 90 p.
Abstract: Several surveying patterns have been installed since 1963 to monitor for possible ground deformation across and within the Ostler Fault Zone. A triangulation network, two long levelling routes, six tilt levelling patterns, and two short levelling routes have been surveyed on a regular basis. This report presents and analyses the results obtained from the levelling routes and tilt levelling networks. All work described in this report (installation, survey and analysis) has been carried out by New Zealand Geological Survey (NZGS) and Ministry of Works and Development (MWD) staff. The levelling profiles over the period 1966-1984, and the tilt levelling pattern outer radials over the period 1977-1985, record vertical movements that suggest uplift and backtilting (or warping) is occurring about the western, upthrown, sides of the Ruataniwha and Y Fault Traces of the Ostler Fault Zone. The uplift is a maximum adjacent to the fault traces at a rate of 0.4-1.5 mm/year, and decreases westwards to near zero 400 to 800 m from the traces. The vertical changes detected by the levelling are similar in style to the deformation preserved geologically from past movements in the Ostler Fault Zone but differ in the amount of displacement across the fault zone. One of the tilt levelling pattern inner triangles detected tilts of up to 25 microradians in the six months prior to the magnitude 4.6 Pukaki earthquake of 17 December 1978. Recommendations are made for the continued monitoring for vertical movements in the fault zone. Analysis of the triangulation and trilateration data will be the subject of a separate report. (auths)
