Defining fault avoidance zones and associated geotechnical properties using MASW: a case study on the Springfield Fault, New Zealand

Abstract

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 ~5m 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.