Seismotectonic source characterisation in low seismicity regions of New Zealand: Otago case study
Authors: Mark Stirling, Jonathan Griffin, Ella van den Berg
Paper number: 18/SP767
Abstract
The Otago region has seen very few earthquakes over the last two centuries, and this is due to the region being well to the east of the main boundary of the Australian and Pacific tectonic plates. A formidable task is therefore to decide where major earthquakes will occur in the future. The experience of Christchurch showed that a major earthquake can occur out of the blue, and have devastating consequences. With this concern in mind our work over the period 2018-2020 has focused on using geological data and methods to find out how often large earthquakes have occurred over geologic time on two major Otago faults. These are the Hyde and Northwest (NW) Cardrona faults, which would cause damaging shaking in Dunedin, Wanaka and Queenstown if they produced major earthquakes. We excavated two trenches across the Hyde Fault and one across the NW Cardrona Fault, and worked out when major earthquakes occurred on them. We found that two major earthquakes occurred on the Hyde Fault about 23 and 10 kyrs ago. The study is the first of its kind to be carried out on the Hyde Fault. Our study of the NW Cardrona study showed that three major earthquakes occurred on the fault, about 38, 28, and 3-14 kyrs ago. The NW Cardrona trench was a re-excavation of a trench originally excavated for hydro-electric dam site development in the 1980s, when sediment age dating techniques were largely unavailable. The results of these studies will be used in the 2022 update of the New Zealand national seismic hazard model.
Technical Abstract
This report is a precis of work carried out over the period 2018-2020 on characterising active fault sources for seismic hazard assessment in the Otago region. Specifically, the earthquake recurrence behaviour of the Hyde and Northwest (NW) Cardrona faults are characterised from paleoseismic investigations and preparation of trench logs. The Hyde Fault study reveals that two ground rupturing earthquakes occurred on the fault at 22.8+1.8 and 10.2+1.3 kyrs, respectively, which loosely constrains a recurrence interval of c. 9.5-16 kyrs. The study represents the first paleoseismic investigations ever carried out on the Hyde Fault. The NW Cardrona study reveals three events to have ruptured the fault, at: 37.8 ± 4 -39.3 ± 4.4; 27.2 ± 2.8 - 28.2 ± 2.6; and 3.3 ± 0.6–14.3 ± 1.4 kyrs. These data constrain a recurrence interval of 11-13 kyrs. The NW Cardrona trench is a re-excavation of a trench originally excavated for hydro-electric power development in the 1980s, when dating techniques were largely unavailable. The results of these studies will be used to update earthquake recurrence estimates for the 2022 update of the national seismic hazard model.
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