A Geodetic Study of the Otago Fault System of the South Island of New Zealand

Abstract

The Otago Fault System in the southern South Island of New Zealand is an area dominated by actively growing asymmetric anticlines above buried reverse faults. Geological studies indicate that the average total shortening rate across the system is 2–3 mm/year. This region was the subject of several early geodetic studies based on terrestrial techniques, which reported high (0.5 ppm/year) strain rates that are difficult to reconcile with geological estimates of fault slip rates.

We have analysed approximately 5 years of recent satellite-based geodetic data from approximately 20 stations distributed evenly over a broad corridor extending across the full extent of the Otago Fault System. We divided the data into three sub-networks to study the variation of geodetic strain over the region. The resulting strain rate tensors show a transition, with the northern subnetwork adjacent to the Alpine Fault showing predominantly a shear strain with a small transpressional component and a rate of 30 ppb/year while in the southernmost subnetwork, the strain rate tensor is consistent with uniaxial contraction at rates of 20ppb/year. Tectonically, in the northern part of our study area, the geodetic strain data are broadly consistent with elastic strain accumulation on the Alpine Fault while in the south, the strain rate tensors are consistent with the Otago Fault System. The principal axis of contraction for the southern subnetwork is about 15 ppb/year, which is consistent with geological estimates of rates of total shortening across the Otago Fault System, but is oblique to the northeast trending ranges.