Abstract
Offshore from the densely populated city of Dunedin lies the obscure Green Island Fault. Prior research into the offshore reverse fault has categorised it as an independent structure that is probably active. However, most of these studies have inaccurately disclosed the fault’s characteristics and potential seismicity. In this thesis, I use a combination of seismic analysis techniques to re-evaluate the true seismic potential of the Green Island Fault by accurately constraining its extent, dip, displacement, and past activity. This investigation aims to re- construct a better understanding of the Green Island Fault characteristics and use this knowledge to determine its maximum moment magnitude (Mw).
To successfully answer the aim of this thesis, seismic datasets gathered from a multichannel seismic survey 19PL064 using the R.V. Polaris II, were processed and interpreted alongside roughly 1200 km2 area of existing single channel and multi-channel data along the inner Otago shelf. From this survey, 28 seismic lines were processed through the GLOBE Claritas software package, followed by an in-depth interpretation of fault horizons and associated anomalies through 2D and 3D analysis within the IHS Kingdom software.
Through the successful 2D and 3D interpretation of high-resolution seismic datasets, the characteristics of the Green Island Fault were constrained. Within the data, the fault’s visible offset extended 8.23 km, with an inferred extent of 16 km based on deformation evident near White Island. However, identified between the Akatore and Green Island faults are offset horizons and a possible flower structure of positive or hybrid structure type, supporting the idea of a genetic linkage between the two seismic structures.
Due to the successful placement of seismic lines during the survey, true dip of the fault was determined near its centre. By tying shore parallel and perpendicular lines, a dip of 31° was calculated. Using case study evidence, this value was found to fall within the dip ranges expected for the Akatore Fault.
The active state of the Green Island Fault has been called into question in past studies due to its lack of seafloor surfaces features. However, it is found that surfaces features associated with the fault have been truncated over time due to changing erosional and depositional environments between glacial and inter-glacial periods. Based the combined effects of stirring from the Southland Current and tides, and storm induced currents along the inner shelf, constant erosional and depositional cycles have caused Holocene sediments as young as 6.5 ka to bury the truncated fault. This, along with the presence of a small fault scarp near the fault’s southern tail, suggest the fault is active.
Displaced lithologies of the Green Island Fault were constrained through the support of case study evidence. The fault’s inshore flank contained a shallow dipping beds sequence that is interpreted to consist of the Otago Schist, Abbotsford Formation, Green Island Loose Sand, Burnside Mudstone, with possible evidence of the younger Caversham Sandstone close to the fault. Along its offshore flank, a visible anticline was interpreted to consist of mainly Abbotsford Formation with evidence Otago Schist below. Using trajectory estimates of interpreted Abbotsford Formation horizon offsets either side of the fault, a maximum displacement of 120 m was calculated for the southern 40% of the fault.
Based on the Characteristics described above, a maximum moment magnitude could be calculated for the Green Island Fault. Based on its independent extent (16 km), a maximum Mw range of 6.62 to 6.97 was calculated. However, the lower value calculated may not be able to generate enough energy to form the surface rupture present at the fault’s southern tail. Because of this, a maximum Mw range was calculated for the extent of the Akatore and Green Island Faults combined equating 7.07 to 7.42. Based on the energy required to produce a surface rupture and the similarities in fault characteristics, this thesis suggests that that Akatore and Green Island faults are not independent bodies but possibly genetically linked via a positive or hybrid flower structure, and that the Green Island Fault’s surface rupture may be the product of a seismic event generated by their combined extent.