SAHKE - Seismic Array Hikurangi Experiment



    GNS Science and Victoria University of Wellington in conjunction with the University of Tokyo (Earthquake Research Institute) and the University of California, is installing a temporary dense array of seismometers across the Wellington – Wairarapa region of the lower North Island. This project consists of two main parts. The first part, SAHKE I, consists of a series of seismometer arrays in the region between November 2009 and April 2010. These seismometers recorded local, regional, and teleseismic earthquakes in addition of offshore seismic airgun blasts. This allowed scientists to image the deeper structures associated with the subduction zone in this region of New Zealand. The second part of this experiment, SAHKE II, will take place between May and June 2011, when a dense transect line of seismometers will record signals from local seismic blasts. This data will allow scientist to produce a high-resolution image of the shallow structures, such as faults, regions of stress build-up, and regions of weakness, within the region. The combination of these two datasets will provide details of the substructure of this region, the structural and mechanical stresses associated with subduction zone in this region, and regions of high risk for frequent earthquake hazards. This information will then be integrated with geological data to resolve issues of the continental structure, evolution and dynamics of the lower North Island and New Zealand as a whole.


    Tectonic Setting of the southern North Island


    The Wellington region lies at the southern end of the Tonga-Kermadec-Hikurangi subduction zone. It forms part of the westward subduction of thick and bathymetrically elevated oceanic Hikurangi Plateau (Pacific Plate) beneath the Australian Plate that initiated 20 – 25 Ma ago (Ballance, 1976). In the southernmost North Island, the contemporary oblique plate convergence of ~42 mm/yr can be broken down into ~30 mm/yr of margin-orthogonal motion and ~28 mm/yr of margin-parallel motion (Beavan et al., 2002). The margin-orthogonal component is accommodated by thrust faulting and related folding in the onshore and offshore parts of the Hikurangi Margin’s upper plate. In the southern part of the North Island, this takes place by slip on NE-striking reverse faults and folds that occur in an offshore accretionary wedge, and by slip on the subduction megathrust beneath these faults, which is strongly “coupled” (Barnes and Mercier de Lepinay, 1997; Nicol et al., 2002; Nicol et al., 2007). The margin-parallel component of plate motion is accommodated by (1) dextral-slip on the NNE-striking faults of the North Island Dextral fault belt (NIDFB), including the Wellington and Wairarapa faults (e.g. Beanland, 1995; Van Dissen and Berryman, 1996; Mouslopoulou et al., 2007), (2) clockwise vertical-axis rotation of eastern parts of the North Island (e.g. Wallace et al., 2004; Nicol et al., 2007; Rowan and Roberts, 2008), (3) strike-slip on active ENE-striking structures in Cook Strait (such as the Boo Boo Fault), and by (4) oblique-slip on other NE-striking offshore faults, including the subduction megathrust which has accommodated a large proportion (>80%) of Neogene and Quaternary crustal plate boundary convergence (Nicol and Beavan, 2003; Nicol et al., 2007). Seismicity data suggest that faults of the overriding plate, including the Wairarapa fault, intersect the subduction megathrust at depths of 20-30 km beneath the southernmost part of the North Island (Reyners, 1998; Beavan and Darby, 2005).
    The wedge of accreted sediment in the southern part of the Hikurangi Margin is imaged using seismic reflection data, as a thick (> 3 km) but narrow (<80 km wide) prism that encroaches onshore but widens to the north (Davey et al., 1986; Lewis and Pettinga, 1993; Henrys et al., 2006; Barnes et al., 2010). The accretionary wedge comprises a backstop of Mesozoic greywacke rocks and pre-existing passive margin sediments, that range in age from mid Cretaceous to Palaeogene and have been thrust faulted and back tilted since 25 Ma (Lewis and Pettinga, 1993). These rocks, deposited on the active margin of Gondwanaland, are now exposed in the Wellington region (Begg and Johnston, 2000) and along the axial ranges of the North Island. Intervening slope basins on the margin comprise Miocene to Recent trench-fill turbidite sediments overlying the interplate décollement. Correlation of the seismic stratigraphy with published stratigraphy of the Hikurangi Plateau sedimentary sequence (Davy et al., 2008) constrains the ages and stratigraphy of the Mesozoic and Cenozoic sequence subducting beneath the décollement (Barnes et al., 2010). This sequence is inferred to include Cretaceous volcaniclastics, pelagic and clastic sedimentary rocks, and late Cretaceous–early Oligocene (70–32 Ma) nanofossil chalks with alternating clays.
    In summary, substantial progress has been made over the last decade in our understanding of a diverse range of tectonics and subduction interface processes at the Hikurangi margin (see Wallace et al., 2009). Marine geophysical studies reveal major along-strike changes in the style of tectonism, including along-strike changes from a strongly accretionary environment (southern Hikurangi margin) to subduction erosion (northern Hikurangi margin) (Davey et al., 1986; Lewis and Pettinga, 1993; Collot et al., 1996; Barnes and Mercier de Lepinay, 1997; Lewis et al., 1998; Collot et al., 2001; Barker et al., 2009; Barnes et al., 2010). The change from accretion to erosion is also accompanied by a northward decrease in thickness of sediment on the subducting Pacific Plate (Lewis et al., 1998) and a three-fold northward increase in the rate of convergence at the trench (Wallace et al., 2004). Recent geodetic studies reveal that there are major along-strike variations in the depth to the down-dip limit of the likely seismogenic zone (Wallace et al., 2004; Wallace et al., 2009). The availability of both contemporary and long-term slip measurements and a paleoseismic record for the southern Hikurangi Margin provides a superb opportunity to address questions for the along-strike heterogeneity of kinematic behaviour at subduction zones worldwide. The SAHKE project aims to understand the frictional properties and geometry of the subduction interface beneath the southern segment of the Hikurangi Margin and results will be compared to similar studies completed along other segment of the margin (e.g. Henrys et al., 2003; Sutherland et al., 2009) and to studies of other subduction zones - and ultimately, therefore, make predictions of earthquake behaviour along the margin.
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