Geodetic strain determinations from the Okarito and Godley‐Tekapo regions, central South Island, New Zealand

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Abstract
This paper presents geodetic strain rate determinations from the Okarito area of South Westland aid the Godley Valley region in western Canterbury. In the Okarito area, the triangulation network extends from c. 10 km west of the Alpine Fault to 1 km east of the fault. To examine the distribution of strain about the fault, 1 broke the data into two subnetworks. One of the networks includes all stations located west of the Alpine Fault, and the other includes two stations located west of the fault and two stations located near the fault on the east. Geodetic strain rates for the two networks are similar. In the western network, the maximum (engineering) shear strain rate is 0.85 ± 0.15 μrad/yr and the azimuth of principal axis of relative contraction is 119° ± 4, whereas in the e.istern network the maximum (engineering) shear strain rate is 0.62 ± 0.11 μrad/yr and the azimuth of the principal axis of relative contraction is 115° ± 6. Errors are to the 1 SE level of confidence. Results of the strain determinations indicate that high shear strain rates are not restricted to networks that cross the surface trace of the Alpine Fault. In the Godley Valley, the maximum (engineering) shear strain rate is 0.28 ± 0.14 urad/yr and the azimuth of the principal axis of relative contraction is 116° ± 14, similar to other measurements from the eastern side of the Alps. The strain rate from the northern half of the area is much higher, however. A subnetwork extending from the northern end of the Godley Valley (within a few kilometres of the Main Divide) to the confluence with the Macauley River gives 0 62 ± 0.19 for the maximum (engineering) shear strain rate and 92° ± 9 for the azimuth of the principal axis of relative contraction. The Godley earthquake of 1984, with a moment magnitude of 6.1, occurred within a few kilometres of the subnetwork and may possibly have affected the results. The azimuth of the principal axis of relative contraction is within ±2 SE of 110° for the whole central Southern Alps. This direction is consistent with transpressional movement on the Alpine Fault and the NUVEL‐1 relative plate motion vectors for the Pacific‐Australian plate pair.