Mechanical strain gauge measurements at Te Marau

D J Darby,N D Perrin

doi:10.5459/bnzsee.19.2.104-110

Abstract

In 1981 an array of five steel rods linking four monument was installed across the trace of the Wellington Fault, at the site of the Te Marau water storage project. This array was instrumented with electronic displacement transducers and data loggers to serve as a horizontal strainmeter. Corresponding measurements have been made from time to time using a Whittemore mechanical strain gauge. Though these measurements are consistent with a model of right lateral shear deformation at an average rate of (5 ± 2) x 10-6 /yr (engineering units) since 1982, some or all of this estimate may be due to flexure of the rods; it may therefore be considered as an upper limit to deformation at the site. Verification by precise surveying methods may be possible if there is a sufficiently broad zone of deformation.

Highlights

The Wellington fault has been described by Lensen [1958]; its strike near Te Marua is about 060° and, citing Berryman, Brown and Wood [1983] give an average right lateral offset rate of 7.4 mm/yr over the last 140,000 years
The strainmeter at the site of the Te Marua water storage project was installed in 1981 to measure deformation about the fault, and is described by Brown and Wood [1983]. It was assembled in a 10 m deep trench, backfilled, and consists of an approximately horizontal array of five stainless steel rods; two of these cross the trace of the Wellington fault diagonally, two are parallel to it, and the fifth is nearly perpendicular to it (Figure 1 )
A shear model which ignores rod flexure indicates the mechanical strain gauge measurements are consistent with right lateral shear deformation on the Wellington fault at an average rate of (5 ± 2) x 1 0 ~ 6 / y r since 1 9 8 2

Summary

Introduction

The Wellington fault has been described by Lensen [1958]; its strike near Te Marua is about 060° and, citing Berryman (in p r e p . ) , Brown and Wood [1983] give an average right lateral offset rate of 7.4 mm/yr over the last 140,000 years. G is the gauge measurement for rod r in r s survey s; g° is the ideal initial measurement for r rod r, in the absence of random or modelling errors, and must be estimated; d is the apparent linear dilatation s compared to the first survey for each subsequentsurveys (s > 1 ) ; these are to be estimated and are discussed in detail below; y (i = 1,2) a r e the two {tensor) strain 1 rate components, assumed constant in time, which describe uniform shear strain; they are to be estimated; t , is the time of survey s after the S 1 first (s > 1 ) ; L r is the length of rod r; 6r is the azimuth of rod r; e is the random error for rod r in survey s; errors are assumed to have a normal distribution with mean zero, and a standard deviation which will be estimated from the residuals.

Results

Conclusion