HR: 14:10h
AN: NS23A-03    [Abstracts]
TI: High Resolution 3D Seismic Reflection Imaging Across the Northern Alpine Fault, New Zealand
AU: * Kaiser, A
EM: kaiseran@aug.ig.erdw.ethz.ch
AF: Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, 8093, Switzerland
AU: Campbell, F
AF: Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, 8093, Switzerland
AU: Stratford, W
AF: Department of Geography and Geology, University of Copenhagen Nørregade 10, Copenhagen, DK-1017, Denmark
AU: Horstmeyer, H
AF: Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, 8093, Switzerland
AU: Langridge, R
AF: GNS Science, PO Box 30-368, Lower Hutt, 5040, New Zealand
AU: Finnemore, M
AF: Department of Geological Sciences, University of Canterbury, PO Box 4800, Christchurch, 8140, New Zealand
AU: Ernst, J
AF: Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, 8093, Switzerland
AU: Nobes, D
AF: Department of Geological Sciences, University of Canterbury, PO Box 4800, Christchurch, 8140, New Zealand
AU: Green, A
AF: Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, 8093, Switzerland
AB: The Alpine Fault is a major transform structure that delineates the boundary between the Australian and Pacific plates through the South Island of New Zealand. Geological evidence suggests that ~470 km of dextral strike-slip movement has occurred along its length. Although it has not been affected by major ruptures during the 150 years of recorded history, large earthquakes (magnitude >7.5) have the potential to cause significant damage in inhabited regions. At our study site, the northern section of the fault juxtaposes Haast schist against west-coast sedimentary basement rocks. Recent lateral and smaller dip-slip components of movement are indicated by offset Quaternary river terraces at the site, and ground-penetrating radar data image a steeply dipping fault zone in the shallow subsurface (<15 m depth). The dip and fault structure at greater depths is uncertain. We have conducted a high-resolution 3D seismic reflection survey to image fault zone structure beyond the sedimentary cover into basement rock to ~200 m depth. An area of roughly 184 x 500 m was surveyed using a pseudo-3D shooting configuration that yielded ~50-fold data at a 4 x 2 m spacing. In addition, we recorded a 370 m-long ultra-high resolution seismic reflection line that provided ~60-fold data at 0.25 m intervals. Application of relatively standard seismic reflection processing techniques has yielded high-quality stacked sections that reveal sedimentary layering in shallow river gravels and a strong reflection from the sediment-basement contact. The dipping basement reflection is offset by ~40 m across the main trace of the Alpine fault. We speculate that basement faulting has offset an erosional surface that formed during the most recent major period of glaciation. Such an interpretation results in a Quaternary dip-slip rate in accord with other estimates along the fault in this region. A second basement discontinuity suggests the existence of a subsidiary fault strand approximately 300 m from the main fault.. We intend to employ more sophisticated processing techniques in an attempt to image more steeply dipping structures within the fault zone.
DE: 8010 Fractures and faults
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting