Abstract
What is the current condition of geologic hazards off the coast of Dunedin, New Zealand? Both the Akatore and Green Island Faults are thought to be active in this location. With earthquake events around Christchurch, New Zealand in 2010 and 2011, and around Kaikoura in 2016, what is the likelihood of earthquakes on the active faults around Dunedin, New Zealand? What is the extent of seismic investigations that have taken place in this region? What processing has been required for these seismic data?
To answer these questions, an area of roughly 1,000 km2 off the coast of Otago warrants further inquiry. While numerous offshore studies have been conducted along the coast from Tokomairiro Mouth to St Kilda, Dunedin, these studies have been analyzed individually and have remained relatively isolated from each other. Data from previous and current studies included in this thesis are single-channel seismic, multi-channel seismic, sediment, sidescan sonar, and multi-beam echo sounder datasets. These data will then be interrogated alongside each other.
Multi-channel seismic and multi-beam echo sounder datasets were collected specifically for this study using the R.V. Polaris II. Other datasets were collected by the University of Otago, Land Information New Zealand (LINZ), National Institute of Water and Atmospheric Research (NIWA), GeoNet, and the Otago Regional Council (ORC). The data were processed using Caris and GLOBE Claritas software packages before viewing them using ArcGIS and IHS Kingdom software.
By reviewing the processing of the multi-channel seismic data, improvements and suggestions are made for future data collection. These improvements and suggestions are geared toward streamlining the processing method and ensuring all possible signal is being recorded to improve the resolution of the marine geology. This improved resolution of the marine geology was combined with the other datasets to study the interactions between the bathymetry, geology, and sediments. The resulting acoustic images and maps provide answers to the questions concerning marine geologic hazards around Dunedin, New Zealand.
The Akatore Fault continues at least 6 km offshore from Tokomairiro Mouth. Evidence for movement and the amount of throw in this area are not possible to determine as the seismic data are obscured by the presence of the Otago Schist. Folding is observed numerous lithologic units north of the fault and is believed to be connected to the Akatore Fault. This fold continues on its southwest to northeast orientation before changing direction and heading further offshore.
As the fold continues, it transitions back into a fault, which is a step-over fault from the Akatore Fault. This second fault, named the Green Island Fault, has a minimum length of 9 km. Evidence of movement on the Green Island Fault within the last 6.5 ky is visible in both the seismic and sidescan sonar data. The total amount of throw is not known as the first multiple obscures the single-channel seismic data. Multi-channel seismic data require better positioning before it is possible to obtain increased depth penetration and improve the quality of the seismic data.
The quality of the seismic data would also be aided by improved resolution. Analysis of the frequencies observed during each survey suggests that a sampling rate of 0.15 ms is required to record all possible signal. Data collected using this sampling rate would be able to record frequencies up to 3,333 Hz. Initial results have shown the frequency component of the water column and marine geology to be between 100 Hz and 3,175 Hz.