Measuring near-surface geophysical properties as an aid in estimating soft-soil response to earthquakes

The response of soils to earthquake shaking depends on a number of geological, geophysical and geotechnical characteristics, many of which can be directly or indirectly determined through the application of near-surface geophysical techniques. The last 30 years has seen the development and application of new methodologies to earthquake-prone areas in support of hazard zonation mapping and site-specific investigations. Broad-band amplification of ground motion as well as resonance amplification depends on soil thickness, internal stratigraphy of surficial materials, shear wave velocity structure, as well as seismic impedance contrasts and attenuation within overburden and across the overburden/bedrock boundary. Compressional and shear wave seismic reflection, refraction and borehole methods can be utilized to support geotechnical I-D (or greater) modelling of ground motion response. These methods, as well as ancillary electrical, electromagnetic and ground probing radar techniques, can also be applied to detect and map soil deformation effects resulting from paleo-earthquake activity. This paper reviews current applications of near-surface geophysical techniques to soft soil site response investigations with examples drawn from recent work in Canada. Three-dimensional shear wave velocity structure has been mapped throughout an area of thick Holocene and Pleistocene age sediments in the Fraser River delta near Vancouver, Canada, using P-wave reflection, shear wave refraction, downhole shear-wave velocity measurements, surface wave inversion methods and P to S conversion of deep seismic reflection data. The resulting maps reveal sub-surface structural anomalies unknown from previously drilling or from surface geological mapping. This geophysical compilation has been used to model variations in resonance periods; these in turn compare favourably with site periods measured from micro-tremor surveys. Investigations of paleo-earthquake sites in eastern Canada using near-surface geophysical techniques have shown the correlation of thick low shear wave velocity overburden with areas of surface disturbance of cohesive soils. Using geophysical parameters obtained from surveys, modelling of ground motion amplification of the thick soft-soil basin has confirmed areas of large amplification associated with geological evidence of faulted and folded stratigraphy and other liquefaction features within the unconsolidated sediments. In adjacent areas where paleo-earthquake shaking has resulted in widespread landsliding, geophysical surveys, including P-wave reflection, electrical, electromagnetic and ground-probing radar, have been used to determine geotechnical sensitivity of the failed cohesive sediments and to predict areas of future hazards.