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Mortimer N.,Institute of Geological & Nuclear Sciences
Journal of Structural Geology | Year: 2014

Most of the South Island of New Zealand lies within an Eocene-Recent continental shear zone related to Pacific-Australia plate motion. Macroscopic finite strain in this shear zone has, in the past, been tracked through the deformation of the Dun Mountain Ophiolite Belt. This paper identifies additional sub-vertical basement strain markers including: Buller-Takaka Terrane boundary, Darran Suite and Jurassic volcanic belt within the Median Batholith, Taieri-Wakatipu-Goulter Synform axial trace, Esk Head Melange and bedding form surfaces within the Buller, Takaka and Torlesse terranes. An analysis of the oroclinal bend over the entire Zealandia continent shows that it is a composite feature involving pre- as well as post-Eocene bending of basement structures. Satisfactory paleogeographic reconstructions of Zealandia cannot be made without the use of substantial regional scale, non-rigid intracontinental deformation. © 2013 Elsevier Ltd. Source

Reyners M.,Institute of Geological & Nuclear Sciences
Earth and Planetary Science Letters | Year: 2013

Recent work involving relocation of New Zealand seismicity using a nationwide 3-D seismic velocity model has located the subducted western edge of the Hikurangi Plateau. Both the thickness (ca. 35km) and the area of the plateau subducted in the Cenozoic (ca. 287,000km2) are much larger than previously supposed. From ca. 45Ma, the westernmost tip of the plateau controlled the transition at the Pacific/Australia plate boundary from subduction to the north to Emerald Basin opening to the south. At ca. 23Ma, curvature of the subduction zone against the western flank of the buoyant plateau became extreme, and a Subduction-Transform Edge Propagator (STEP fault) developed along the western edge of the plateau. This STEP fault corresponds to the Alpine Fault, and the resulting Pacific slab edge is currently defined by intermediate-depth seismicity under the northernmost South Island. Alpine STEP fault propagation was terminated at ca. 15Ma, when the western edge of the plateau became parallel to the trench, and thus STEP fault formation was no longer favoured. Wholesale subduction of the plateau at the Hikurangi subduction zone began at ca. 10Ma. The development of a subduction décollement above the plateau mechanically favoured deformation within the overlying Australian plate continental crust. This led to inception of the Marlborough fault system at ca. 7Ma, and the North Island fault system at 1-2Ma. At ca. 7Ma, the western edge of the converging plateau again became more normal to the trench, and there is evidence supporting the development of a second STEP fault beneath the Taupo Volcanic Zone until ca. 3Ma. Both episodes of STEP fault development at the plateau edge led to rapid slab rollback, and correspond closely with episodes of backarc basin opening to the north in the wider Southwest Pacific. The Cenozoic tectonics of New Zealand and the Southwest Pacific has been strongly influenced not only by the resistance to subduction of the buoyant Hikurangi Plateau, but also by the shape of its western edge and changing angle of attack of this edge at the plate boundary. © 2012 Elsevier B.V. Source

Zhao J.X.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2010

Attenuation models derived from recorded ground motions are still important elements of probabilistic seismic hazard studies. Engineers use empirical attenuation models to derive the displacement demand for a site of interest from an earthquake at a given location. Many attenuation models have been published for different parts of the world and for different types of earthquakes. Most models have a simple function of constant or magnitude-dependent geometric spreading, and seldom consider well-known seismological effects such as Moho reflection for shallow crustal earthquakes, multiple travel paths and constructive interference for subduction earthquakes, and special characteristics of volcano zones. The reason for not accounting for such effects may be the desire for simplicity in the attenuation functional forms for engineering applications and a lack of records from which to reliably identify these effects quantitatively. In this article, a large set of strong-motion records obtained from dense recording networks in Japan is used to derive geometric attenuation functional form and a possible manner to model the effect of volcanic zones. A liberal approach is taken to introduce a relatively large number of parameters that can account for known seismological effects while retaining a fairly simple attenuation functional form, based on analyses of residuals from simple models similar to those published previously. Preliminary results are reported here, together with the proposed geometric attenuation function forms and plausible explanation of the physical process that leads to the proposed geometric attenuation functions. The proposed model shows a large increase in the maximum likelihood from the random effects methodology, the elimination of bias in the distribution of residuals with respect to source distance, and much improved fitting for well-recorded earthquakes. Source

Bradley B.A.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2010

Acceleration spectrum intensity (ASI), defined as the integral of the pseudospectral acceleration of a ground motion from 0.1 to 0.5 sec, was originally proposed as a ground-motion intensity measure (IM) relevant for the seismic response of concrete dams over two decades ago. ASI may be a desirable IM in emerging performance-based earthquake engineering frameworks because its consideration of a range of spectral periods makes it useful for concurrent prediction of acceleration and displacement demands in individual structures and also for regional loss estimation where short-period structures are typically prevalent. This article presents a theoretical basis for predicting ASI, based on prediction equations for spectral acceleration, both for individual sites and spatially distributed regions. ASI is found to have a better predictability than conventional ground-motion IMs such as elastic pseudospectral acceleration at a specific period. Furthermore, for site-specific applications conditional response spectra are derived, which can be considered as the correct target response spectra for ground-motion selection, and the features of these conditional spectra as a function of earthquake magnitude, source-to-site distance, and epsilon are examined. For spatially distributed applications, the intraevent correlation of ASI as a function of the separation distance of two sites is derived and compared to that of other common IMs. Source

Stewart M.K.,Institute of Geological & Nuclear Sciences
Journal of Hydrology | Year: 2012

Knowledge of the sources and flowpaths of water in the Christchurch groundwater system will be vital to future management of the system. To gain such knowledge, oxygen-18 ( 18O), tritium ( 3H), carbon-14 ( 14C), and chemical concentrations have been measured on deep and shallow groundwaters since 1970. 18O measurements show that seepage from the Waimakariri River is the dominant source of the groundwater. Early 3H measurements (in the 1970s) showed non-zero concentrations in the deep groundwaters, but these were discounted at the time as due to "at most a few percent of very young water" However, reinterpretation in light of the 14C ages in this work has revealed much younger ages for the deep waters than was previously believed, with average ages of 38years in 1971, 71years in 1976, 98years in 1985, greater than 120years in 1986, and greater than 150years in 1993-1994.Because the Waimakariri River was identified as the major source of the deep groundwater, the river's 14C concentration between 1986 and 2006 was modelled by combining the records of its two carbon sources (biogenic carbon and atmospheric CO 2). This allowed the initial 14C concentrations of the groundwaters to be unequivocally determined and their mean 14C ages estimated using the same flow model as was applied to the 3H measurements. The resulting mean 14C ages are in the range 5-1500years. The long sequence of measurements reveals that the mean ages of the deep Christchurch groundwater have changed markedly during the study. The pre-exploitation rate of turnover of water in the system is not known, but was probably quite slow. By the 1970s, ages in the deep system (Aquifers 4 and 5) had become relatively young right across Christchurch (with mean ages of 60-70. years) indicating mainly lateral inflow of young Waimakariri River water because of groundwater abstraction. Mean ages measured since have gradually increased showing increasing upflow of much older water from depth - this water has 10-15% rainfall recharge and is sourced from the inland plains region. There is now (in 2006) a steep gradient in age from west to east across Christchurch (from 300. years to 1400. years) showing that a large body of much older, deeper water is stored on the seaward side of the system where the deep aquifers are blind. This body will yield good quality water for many years, but eventually it is likely to be replaced or bypassed by younger (a few hundred years old), Waimakariri River-dominated but surface recharge-bearing, water from inland. © 2012 Elsevier B.V. Source

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