The contribution of geophysical techniques to site characterisation and liquefaction risk assessment: Case study of Nafplion City, Greece

Abstract

Abstract We present an example of how geophysical methodologies can considerably contribute to seismic and liquefaction risk assessment in an area where urban development is planned. The inspection for possible hidden faults by geophysical methods is particularly critical, since such a possibility could practically hinder the town planning in this area. However, even if no primary threats exist within the area, the response of the foundation soil to various scenarios of historical earthquakes, which have affected the place in the past, must be examined. The geophysical methodologies could also assist this analysis, contributing with the calculation of the amplification of the seismic motion from the bedrock up to the surface. The investigation area of Nafplion, Greece, was suspected to have high liquefaction potential since the foundation soil consists of loose sandy silt with a very shallow aquifer. The implementation of gravity and seismic methods considerably aided the investigation for possible seismic faults. Special emphasis was given to seismic depth migration and particularly to the construction of valid velocity models, in order to precisely calculate the dip characteristics of the structures. Shallow seismic techniques were also applied to provide the near-surface velocity structure, which is a prerequisite for assessing the liquefaction risk. In particular, our case study provides an example of how seismic methods (seismic reflection, seismic refraction, seismic modelling, MASW, multichannel analysis of microtremors and crosshole investigations) when combined with geotechnical borehole testing, enhance the reliability of their output and allow the coverage of wide areas in a cost-effective way in comparison to standard borehole tests. Data and information provided by the application of the geophysical methods were subsequently incorporated in the liquefaction risk assessment at several selected sites within the study area. Factors of safety against liquefaction and liquefaction potential values were computed for three scenario earthquakes that were selected on the basis of the known seismic impact of past earthquakes in the town of Nafplion. We found that liquefaction probability can reach values as high as 80% at some sites depending on the selected earthquake scenario. The formations most prone to liquefaction are detected at depths between 5 and 10 m. This information can be helpful for making risk-based design decision in this region.