Vs30 Assessments: Understanding the Underground

Abstract

Understanding the vulnerability of a construction site to seismic shaking is fundamental to robust building design in New Zealand. The recent release of the updated National Seismic Hazard Model reflects this, with an integral parameter in assessing a site’s seismic hazard being the velocity of shear seismic waves within the uppermost subsurface at the site (Vs30), which can impact shaking. With uncertainty currently remaining regarding how the NSHM will be incorporated into updated building codes, we present methods and examples of how reliable shear wave velocity assessments can be determined across a range of different sites. Examples are given of how a balance of non-invasive geophysical techniques such as multi-channel analysis of surface waves (MASW) with invasive geotechnical investigations such as seismic CPTs can be used to give a comprehensive assessment of Vs30 across sites with varying characteristics and challenges. The incorporation of non-invasive seismic techniques in site investigations allows for large areas to be assessed in a time-efficient manner, as well as enabling investigations to be undertaken in sections of difficult terrain and where invasive techniques are not economically viable. Meanwhile, targeted invasive investigations act as vital tie points and ground truthing for the seismic profiles measured. With an optimal balance of the complimentary techniques, a robust pseudo-3D map of Vs30 across any site can be developed in a time and cost-efficient manner, and with minimised environmental impact. Such understanding of a site’s vulnerability to shaking allows for buildings and infrastructure to be appropriately designed to minimise seismic risk.