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measured by geological studies over many seismic cycles (e.g., 10 kyr to millions of years). To address this question we compare geological rates of extension, shortening, fault slip and vertical axis rotations, derived from cross sections, seismicreflection lines, displaced landforms and paleomagnetic data and measured over the last 5 Ma, with results from modelling of GPS velocities from the Hikurangi Subduction margin in the North Island. GPS velocities in the North Island are found to be influenced by two fundamental processes (1) elastic strain due to interseismic coupling on faults in the region, and (2) long-term tectonic block rotations (Wallace et al., 2004). It is the latter component of the GPS velocity field that can be directly compared to geological estimates of block rotation and fault slip rates. Regional analysis of the Hikurangi margin suggests that within the uncertainties of the data the rates of deformation for both data sets are, in most cases, approximately equal and independent of the sample period. Geological and geodetic datasets both indicate rapid clockwise rotation (2-4 °/Ma) of the eastern North Island. Extension and contractional strains result from the rotation. Margin-parallel velocity gradients arising from the rotation of the eastern North Island account for up to 70% of the margin-parallel component of Pacific/Australia relative plate motion. Stable deformation rates over time periods ranging from 5-10 years to millions of years are consistent with the constant rates of plate motion observed over similar temporal scales and demonstrate the utility of GPS velocity fields for constraining longer-term tectonic deformation (i.e. > 1 earthquake cycle duration). The apparent absence of faults at the ground surface accommodating aseismic creep and of large magnitude earthquakes over the last 10-15 years, suggests that in the upper crust contemporary geodetic strains are predominantly elastic. These contemporary strains are spatially distributed across much of the North Island, however, the compatibility of block-rotation rates and fault-slip rates from geological studies and those from modelling of GPS data indicates that elastic strains will, in the future, mainly be converted to narrow zones of permanent deformation on, or close to, the largest faults (i.e. those faults large enough to be resolved in the regional geology data). Elastic strain release is most likely to take place during large magnitude earthquakes. Wallace et al. 2004. JGR 109, B12406, doi:10.1029/2004JB003241. ORAL FROM KAIKOURA TO KAITORETE: NON- INVASIVE GEOPHYSICAL MAPPING OF MAORI BURIAL SITES David C. Nobes 1 , Leah Bateman, Caroline Butland, Mark Flintoft, Francie Gaiger, Joanna Lea, Scott Wilkinson & Harry M. Jol 2 1 Department of Geological Sciences, University of Canterbury, P.B. 4800, Christchurch 2 Department of Geography & Anthropology, University of Wisconsin-Eau Claire, USA (david.nobes*canterbury.ac.nz) Sacred (tapu) sites need to be treated with care and sensitivity. Non-invasive, non-destructive geophysical surveying is an appropriate, optimal way to map the nature and extent of such sites, particularly burial sites – urupa. Since 1997, geophysical methods have been successfully used on urupa (Nobes, 1999). The response varies from site to site, depending primarily on the nature of the soil and the underlying bedrock. The results from four sites representing three physical settings are presented here: 1. The Oaro urupa is on top of a hill overlooking the South Pacific, and was the first urupa to be surveyed using geophysical methods (Nobes, 1999). The site’s clay soils overlie limestone bedrock. Magnetic, electromagnetic (EM) and ground penetrating radar (GPR) surveys were completed. The responses are clear and 2. unequivocal, once the influences due to fences and the clay are removed. The Koukourarata (Port Levy) and Wairewa (Little River) sites are in loess overlying the volcanic bedrock of Banks Peninsula. The EM responses are apparent but are not as obvious as for the magnetic surveys. The GPR surveys have clear anomalous responses that in most cases can be interpreted as graves (Bateman, 2003). 3. The Mangamaunu urupa is situated in coastal beach sands, next to State Highway 1, just north of Kaikoura and near the hamlet of Hapuku. Anomalous geophysical responses are all present but less obvious than for the Oaro, Koukourarata and Wairewa sites. Nonetheless, we can identify areas where the variable geophysical response is indicative of disturbance that is likely due to graves. Bateman, L., 2003. Applications of near-surface geophysics in the search for graves in Maori urupa. B.Sc. (Honours) project in Engineering Geology, Department of Geological Sciences, University of Canterbury, 69 pp. Nobes, David C., 1999. Geophysical surveys of burial sites: a case study of the Oaro urupa, Geophysics, 64(2): 357-367. POSTER 50 th Kaikoura05 -60- Kaikoura 2005
3D IMAGING OF THE FORE- AND BACKTHRUSTS OF THE GLEN LYON SEGMENT OF THE OSTLER FAULT David C. Nobes 1 , Douglas W. Burbank 2 , Colin Amos 2 & Kenneth Davis 2 1 Department of Geological Sciences, University of Canterbury, P.B. 4800, Christchurch 2 Department of Geological Sciences, University of California-Santa Barbara, USA (david.nobes*canterbury.ac.nz) A set of closely spaced ground penetrating radar (GPR) and GPS survey lines were acquired across the fore- and backthrusts of the Glen Lyon segment of the Ostler Fault. The project is part of a larger ongoing study of thrust fault processes. We collected 21 survey lines, 50-m long and 1 m apart, and approximately centred on the fore- and backthrust scarps. In addition, common midpoint (CMP) velocity surveys were completed at each site so that the profiles could be migrated, and the travel time converted to depth. The velocity structure at the two sites was similar, yet subtly different, and consistent with mostly unsaturated sandy and silty gravels. At the forethrust site, the velocity was 110 m/µs (± 10 – 15 m/µs) down to about 4 m depth, rising to 120 ± 10 m/µs before returning to 110 ± 10 m/µs from 5 to 8 m depth. In contrast, the backthrust velocities below the scarp were 110 ± 15 m/µs at shallow depths (< 2m), and 120 – 130 ± 15 m/µs at depths down to 6 m. Above the backthrust scarp, the velocities were more consistent with dry sands and gravels, of the order of 150 m/µs, but were not well constrained. Deeper velocities were not resolved at either site. The 3D GPR profiles reveal a backthrust with a relatively simple geometry, and dipping 65° ± 8° to the east. For the forethrust, on the other hand, there are from 2 to 4 individual fault displacements visible in the 21 GPR lines, with dips of the order of 56° ± 9° to the west-northwest. These results are consistent with measurements on outcrop (Davis et al., 2005) and GPR profiles from the Benmore segment of the Ostler Fault (Wallace et al., in submission), which yielded dips of 50° and 51°, respectively, ± 9°. The most recent main fault displacement almost reaches the surface near the top of the current forethrust scarp, with a thin wedge of sediment in front of the fault. The other fault displacements, each with its own sediment wedge, are located at successively greater depths and in front (to the east) of the current scarp. The forethrust position appears to be migrating westward toward the hinterland with each new rupture sequence. Davis, Kenneth, Burbank, Douglas W., Fisher, Donald, Wallace, Shamus & Nobes, David, 2005. Thrust fault growth and segment linkage in the active Ostler fault zone, New Zealand. Journal of Structural Geology, 27: 1528-1546. Wallace, Shamus C., Nobes, David C., Davis, Kenneth J., Burbank, Douglas W. & White, Antony. Threedimensional imaging of the Benmore segment of the Ostler Fault, South Island, New Zealand. Geophysical Journal International: in submission. ORAL ACTIVE FAULTING AND PALEOSEISMICITY OF THE OFFSHORE KAPITI-MANAWATU FAULT SYSTEM, SOUTHERN NORTH ISLAND, NEW ZEALAND S.D. Nodder 1 ,G.Lamarche 1 &J.-N.Proust 2 1 National Institute of Water and Atmospheric Research, Private Bag 14-901, Wellington. 2 Géosciences Rennes, UMR CNRS, Univ. de Rennes 1, 35042 Rennes cedex, France. (s.nodder*niwa.co.nz) Seismic hazards associated with active faulting in coastal regions may be under-estimated if recent offshore deformation is not accounted for in hazard analyses. At subduction margins, deformation occurs predominantly in the high-strain forearc basin, but can also extend considerable distance away into the back-arc environment. An example is described here in the offshore, low contractional strain Kapiti-Manawatu Fault System (KFMS), which is located along the eastern margin of the proto back-arc Wanganui Basin, 200 km behind the Hikurangi margin subduction front. High-resolution seismic reflection data show the presence of numerous sea-floor fault scarps and near-surface deformation of late Quaternary and possibly Holocene reflectors on high-angle (>60°), reactivated reverse and normal faults within the KMFS. Continuous sea-floor scarps, which are inferred to correspond to earthquake rupture segments, range from
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