Southern Geophysical Ltd

Bromley, New Zealand

Southern Geophysical Ltd

Bromley, New Zealand
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Kamuhangire R.,Aurecon | Plunket T.,Aurecon | Ruegg C.,Southern Geophysical Ltd.
Proceedings of the 5th International Conference on Geotechnical and Geophysical Site Characterisation, ISC 2016 | Year: 2016

The major events of the Canterbury Earthquake Sequence (CES) caused significant land and building damage in Christchurch between September 2010 and December 2011. The extensive damage necessitated detailed ground investigations and liquefaction hazard assessments to inform repair and rebuild options. Relying solely on intrusive testing to provide sufficient information for ground characterisation, risk assessments and design is often costly, particularly for large sites, and can be limited by site access. Combining traditional intrusive methods with non-intrusive geophysical investigations has proven to be an economical and time-saving approach and can aid in delineating abrupt changes in ground stratigraphy. This paper presents a case study site where a combination of Multi-channel Analysis of Surface Waves (MASW) profiles and Cone Penetrometer Tests (CPTs) were used to outline the extents of an in-filled palaeochannel. The contrasting land and building damage at the site is described, together with the importance of a detailed desktop study. The MASW and CPT results are correlated with observations of damage, and the effectiveness of combining the two investigation techniques is highlighted. © 2016 Australian Geomechanics Society.

Duffy B.,University of Canterbury | Campbell J.,University of Canterbury | Finnemore M.,University of Canterbury | Finnemore M.,Southern Geophysical Ltd | Gomez C.,University of Canterbury
Engineering Geology | Year: 2014

The Springfield thrust fault at Dalethorpe, west Canterbury, New Zealand, provides a test case to explore the correlation between shear wave velocities at a range of scales, and direct field observations of distributed deformation and outcrop properties. The Springfield fault ruptures to the surface through hard Torlesse greywacke, overlain on a flight of late Quaternary glacio-fluvial terraces by ~. 5. m of gravel. Fault slip has displaced all but the lowest terraces, revealing the geometry and location of faulting. We used multi-channel analysis of surface waves and meter-scale cross-hole measurements to map shear wave velocities below the lowest, apparently undisplaced, terrace. We correlated these surveys with geotechnical parameters measured at outcrops and investigated relationships in the laboratory. Both field and laboratory results indicate that the shear wave velocity of Torlesse greywacke declines sharply with increasing fracture density. Field surveys further indicate that relatively unweathered, high velocity greywacke is being exhumed in a bivergent wedge between two opposite-facing thrusts. The fracturing and low shear wave velocities are focused in a wide, low-velocity damage zone that has developed in the hanging wall of the main thrust, and a smaller but similar feature in the hanging wall of the backthrust. This is consistent with the geomorphology of the site. Our correlations of geomorphic indicators of deformation with fault zone velocity structure provide a useful method with which to characterize the distribution of cumulative strain. This type of analysis has utility for land use planning on, or close to, active faults, especially where they are obscured by fluvial deposits. © 2014 Elsevier B.V.

Carpentier S.F.A.,ETH Zurich | Green A.G.,ETH Zurich | Langridge R.,Institute of Geological & Nuclear Sciences | Hurter F.,ETH Zurich | And 4 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2013

The transpressive Alpine Fault is the boundary between the Pacific and Australian plates across the South Island of New Zealand. Earthquakes on the Alpine Fault and related structures pose a serious risk to many urban centers, including the city of Christchurch. Although it is a major feature on satellite images, the Alpine Fault is a difficult target for surface studies along much of its length; it mostly traverses densely forested and mountainous terrain and where it occurs in the lowlands it is usually covered by recent sediments. To investigate the Alpine Fault at a rare accessible location (Inchbonnie), we have acquired high-resolution seismic reflection data along five 380-1200 m long lines. Images produced from these data reveal a glacially overdeepened valley containing a thick sequence of diverse glacigenic sediments that have been disrupted by three en echelon strands of the principal Alpine Fault and several secondary fault strands. Based on their seismic facies, the sedimentary sequence is interpreted to comprise basal lacustrine beds overlain successively by alluvial-colluvial deposits that possibly include the remnants of large landslides, deltaic-fan units, and braided river gravels. Whereas the principal Alpine Fault strands disrupt the entire post-glacial sedimentary section and likely offset basement at depths up to 400 m, most of the secondary faults either merge with the principal fault strands at shallow depths or are surficial features limited to the sedimentary section. Key Points Seismic reflection imaging reveals a system of broad en echelon faulting Five parallel seismic lines lead up to a pseudo 3-D model of the Alpine Fault A consistent interpretation is made of paleosedimentation in a glacial valley ©2012. American Geophysical Union. All Rights Reserved.

Carpentier S.F.A.,ETH Zurich | Green A.G.,ETH Zurich | Langridge R.,Institute of Geological & Nuclear Sciences | Boschetti S.,ETH Zurich | And 5 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

High-resolution GPS and ground-penetrating radar (GPR) data are used to detect and identify hidden faults along a stretch of the transpressional Alpine Fault (South Island, New Zealand) immediately north of its junction with the Hope Fault. At this location, the Alpine Fault emerges from the basement into a sequence of variably thick late Holocene gravel deposits. Geomorphology and trenching already mapped three principal fault strands and two distinct step over zones at the study site. Our GPR images reveal numerous additional secondary fault strands throughout the region, only some of which are obvious at the surface or in the trench walls. According to the GPR data, the main fault-generated disturbance zone has a width ranging from ∼40 to ∼200 m. The secondary fault strands outside of the step over zones likely represent the branches of positive flower structures, whereas the faulting pattern around the step over zones is best explained in terms of linked Riedel shears. Systematic northeastward increases in the width of the main fault-generated disturbance zone and corresponding increases in principal fault-scarp height are the likely consequences of older terraces in the northeast being disrupted and offset by more earthquakes than younger terraces in the southwest. The pattern of complex faulting in this region is distinct from the system of alternating strike-slip and reverse faults characteristic of the Alpine Fault to the south and from the rather simple sequence of faults mapped to the north. GPR surveying has added new information on the distribution and nature of faulting at our study site. Copyright © 2012 by the American Geophysical Union.

Carpentier S.F.A.,ETH Zurich | Green A.G.,ETH Zurich | Doetsch J.,ETH Zurich | Dorn C.,ETH Zurich | And 5 more authors.
Journal of Applied Geophysics | Year: 2012

Prior to the recent highly damaging M 7.1 earthquake near the city of Christchurch on the South Island of New Zealand, we recorded coincident high-resolution seismic and ground-penetrating radar (GPR) data across parts of the northwest Canterbury Plains. The seismic reflection images reveal a vast network of interconnected faults and folds below a seemingly undisturbed flat surface. To complement the seismic images, which only provide limited information on the very shallow subsurface (i.e., <. 20. m), we have now processed and analysed the GPR data. The migrated GPR images are dominated by complex reflection patterns characteristic of glaciofluvial sediments. Such sediments eroded from the Southern Alps are observed at the surface throughout our study site. Although it is difficult to distinguish between complexities associated with complicated sedimentation processes and disruptions and offsets of GPR reflections associated with recent movements on faults and folds, we identify a number of regions where the GPR data are consistent with tectonic deformation of Holocene sediments. Two of these regions straddle an interpolated connection between active faults mapped at the surface. In a third region, the development of river terraces imaged in the GPR data may have been affected by slip on newly discovered underlying faults. The most significant near-surface deformation, which is apparent on a coincident seismic reflection image, P-wave tomogram and GPR image, is observed on the flank of a major anticline that appears to have been thrust close to the surface along a reverse fault. Some of the faults and folds resolved in our seismic and GPR data may have been reactivated during the recent period of intense seismicity. © 2011 Elsevier B.V..

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