Near-surface expression of active deformation on the shallow Otago continental shelf

Author:

Bruce, Callum David

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Abstract:

The position of the offshore extent of the active Akatore Fault, a NE- SW trending reverse fault that ,runs along the coast SW of Dunedin is poorly constrained. The Akatore Fault is associated with several other offshore coast-parallel faults based on previously obtained shallow controlled-source seismic data. Accurate identification and positioning of these possibly active fault structures is crucial for producing accurate models of earthquake associated hazards in the nearby Dunedin urban area. Several earthquakes, including those of 1974 and 1989, are attributed to movement on these faults. Single-channel electro-acoustic 'Boomer' and CHIRP seismic reflection data were collected along with CAATI side scan sonar (C3D) along 89 lines. 484 km of 'Boomer' seismic data were collected over ~550 km2 ; only 293 km of Chirp seismic data and side scan sonar were collected due to weather conditions. The majority of lines were collected along NW -SE azimuths, running from just outside the surf zone (<10 m water depth) to a maximum of 28 km offshore (~75 m water depth). Survey lines were spaced approximately 250 m apart near shore and up to 5 km apart for offshore lines. One 'Boomer; line ties the 1978 Takapu-1A well. Most lines image distinct east-dipping reflections that correlate to the nearby CretaceousTertiary sequence. 'Boomer' subsurface penetration is generally only limited by the presence of multiple reflections. Primary reflections were recorded from sub-surface depths of up to 90 m. In several areas, internally reflective basement schist was interpreted. Several significant structures were imaged within the survey area: (1) the Akatore Fault, (2) the Green Island Fault, and (3) the Takapu Anticline. The Akatore Fault was imaged very near shore in the southern portion of the survey, and a minimum displacement of 55 m was calculated. Offset on the Green Island Fault, a large high-angle reverse fault was relatively well constrained to ~200 m (east side up). The data suggest that the Green Island Fault may be a step-over from the Akatore Fault. High-resolution imaging of the Takapu Anticline suggests that the Takapu Fault does not reach within ~80 m of the seafloor.

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ix, 74, 3 leaves : ill. (some col.), maps ; 30 cm

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OU geology Identifier:

2010Bruce

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Location (WKT, WGS84):

POLYGON ((170.546021910724846 -45.909302666524503,170.267271458731557 -46.152546149895599,170.110861646954135 -46.090114280022846,170.386507440564372 -45.906239878493096,170.546021910724846 -45.909302666524503))

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http://download.otagogeology.org.nz/temp/Abstracts/2010Bruce.pdf

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Citation

Bruce, Callum David, “Near-surface expression of active deformation on the shallow Otago continental shelf,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/517.

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