Stephenson, W.R.; Barker, P.R.; Bruce, Z.; Beetham, R.D. 2011 Immediate report on the use of microtremors (SPAC measurements) for assessing liquefaction potential in the Christchurch area. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/25 28 p.
Abstract: As a result of the Mw 7.1 September 4th 2010 Darfield earthquake, there was extensive liquefaction, lateral spreading and related ground, building and infrastructure damage in the greater Christchurch area. GNS Science has assessed the liquefaction potential of selected sites using the non-invasive SPatial AutoCorrelation of microtremors (SPAC) method. Seventeen sites were visited and measurements made and analysed. The sites were chosen so that a range of ground conditions could be investigated. Sites where GeoNet strong-motion recorders are located were included. This report briefly describes the SPAC method, outlines the initial analysis of the data and concludes that the technique is a practical and useful non-invasive tool for assessing liquefaction potential. Results can reliably allow determination of the shear wave velocity and thickness of an uppermost layer and less reliably the shear wave velocity and thickness of a second layer. Additional analyses of collected data can allow the determination of site period using the Horizontal to Vertical Spectral Ratio (HVSR – Nakamura) technique. The SPAC data show that where the shear-wave velocity is less than 200m/sec in uniform, loose sands and silts, the potential for liquefaction is high. SPAC sites with a shear wave velocity of less than 200 m/s liquefied in either or both of the Darfield or Christchurch earthquakes. Other geotechnical investigation techniques, such as Cone Penetration Tests (CPT) and MASW, have been briefly looked at to confirm the interpretation of our results. (auth)