Abstract
This thesis documents a seismological study of the Pisa Fault, a 47 km long reverse fault that runs along the western edge of the upper Clutha Valley and bounds the Pisa Range and the Cromwell-Tarras Basin (Beanland and Berryman, 1989). Previous studies have used the relative offsets of glacial outwash surfaces to infer aperiodic activity on the Pisa Fault, however, this hypothesis relied on poorly constrained ages for these surfaces.
The central objective of this project is to use a combination of high-resolution digital elevation models, field mapping, and cosmogenic (CRN) and optically stimulated luminescence (OSL) dating of the faulted fluvial terraces (T1 through to T5) to determine a fault slip rate and earthquake recurrence interval for the Pisa Fault. The processed dating samples provided estimates of the age of the terrace surfaces, where T1 was found to have an age of 20.8 ± 3.1, T2 was found to have an age of 45.6 ± 6.4, T3 was found to have an age of 62.0 ± 8.8, and T4 was found to have an age of 125.3 ± 37.1. T5 was unable to have the dating results completed before submission of this study. These age constraints of displaced surfaces allowed the slip rate (0.20-0.34 mm/yr) to be quantified, as well as the elapsed time since last earthquake on the fault from comparison of the ages of displaced vs unfaulted surfaces. The estimated earthquake magnitude for the Pisa Fault is estimated to be between Mw 7.2-Mw 7.5, with a recurrence interval of 5100-17000 years. Earthquake magnitude and recurrence interval estimates are paramount for understanding the hazards that may be present, and they provide information for assessing the risk associated with the infrastructure and population that are in proximity to the fault. In the case of the Pisa Fault, the nearby towns of Cromwell and Wanaka would be threatened by Pisa Fault earthquakes. Due to the lack of knowledge surrounding this fault, population and infrastructure were allowed to develop in the region without understanding the hazard presented by the fault. The results of this project will be invaluable for providing earthquake hazard information for the fault, which will have many applications, e.g. allowing a management plan to be improved for large companies that require nearby systems to function and allow the definition of steps to be taken to minimize the impact of a future large magnitude earthquake.