This thesis presents new insights into Alpine Fault structures at the drill site of the Deep Fault Drilling Project (DFDP)-2B at Whataroa in New Zealand. Despite the challenging conditions for seismic imaging within a glacial valley filled with sediments and steeply dipping valley flanks, several structures related to the valley itself as well as the tectonic fault system are imaged.
The Alpine Fault at the West Coast in New Zealand is a major plate boundary forming a significant geohazard as large earthquakes (magnitude 7-8) occur regularly and the next earthquake is expected relatively soon. A major effort has been made to study the fault characteristics through scientific drilling in the Deep Fault Drilling Project (DFDP) Alpine Fault with the deepest DFDP-2B borehole located in the Whataroa Valley.
A great variety of seismic data are newly acquired. First, the WhataDUSIE (Whataroa Detailed University Seismic Imaging Experiment) data set is a ~5 km long 2D profile acquired in 2011 prior to the drilling. As the 2D profile could not fully explain the 3D structures in the Whataroa Valley, an extended surface and borehole data set was acquired in 2016 after the drilling. This data set consists of shorter 2D lines (< 3 km), a dense 3D-array, and vertical seismic profiling (VSP) using the DFDP-2B borehole including the fibre-optic cable.
3D seismic data proved to be essential to understand the complex 3D structures of the glacial valley and the major fault. First-arrival travel time tomography and prestack depth migration (PSDM) are applied to obtain a P-wave velocity model and seismic images of the subsurface (<5 km). In this complex setting, the Fresnel volume migration (a focusing PSDM method) proved to best obtain structural information about the subsurface.
Analysing the results of the seismic data processing, two major outcomes are achieved: improved knowledge about the glacial structures of the Whataroa Valley and structural images of the Alpine Fault zone.
The Whataroa Valley is an overdeepened glacial valley with details of the basement topography visible in the seismic images. A deep trough is identified south of the DFDP-2B borehole with horizontal layering of the sediments. Valley flanks are identified in both the seismic images and the P-wave velocity model, particularly the western valley flank. Thus, Quaternary and glacial processes can be analysed with the help of the newly derived seismic images.
The Alpine Fault is directly imaged with the seismic data, which is the first time in this region at shallow depths (<5 km). Several shorter fault segments between depths of 0.2 km and 2.2 km dipping 40-56° to the southeast are directly imaged. Further identified reflectors and faults are interpreted to represent Alpine Fault structures in the form of a damage zone and induced faults adding further complexity to the fault zone.
In conclusion, the 3D seismic results presented in this thesis provide new insights into the Whataroa subsurface. Hence, the new results form a good basis for a deeper understanding of the Alpine Fault structures and underlying processes which is important for potential future drilling but also for the estimation of the geohazard in the region.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:74299 |
| Date | 08 April 2021 |
| Creators | Lay, Vera |
| Contributors | Buske, Stefan, Stern, Tim, TU Bergakademie Freiberg |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1002/2016JB013534, 10.1029/2019JB018519 |
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