Rebecca McMahon

Rebecca is a geotechnical engineer for Beca, based in Nelson. She has  worked as a consultant since 2012 and completed a Masters of Engineering through University of Auckland between 2016-2018 while working part-time. 






​Nelson-Tasman Dynamic Site Characterisation.

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Research Title:
Nelson-Tasman Dynamic Site Characterisation.
​Project Summary:
Deep soil and gravels deposits are present in the Nelson-Tasman region which may lead to amplification of ground shaking and basin effects. However, prior to this research there was little information available on the dynamic properties of the deposits across the Nelson-Tasman region, and the potential site amplification effects. Existing geotechnical subsurface information from around the region was collated to characterise the geological and geomorphological features of the area. Active source and two-dimensional passive arrays were used to develop shear wave velocity profiles for selected sites across the region. Moutere Gravel, a prominent geological formation in Nelson-Tasman, comprises deep deposits of clay-bound gravel and was found to have a relatively high shear wave velocity, thought to be due to the over-consolidation of the unit. However, Moutere Gravel formation is not considered to be bedrock as it is an uncemented gravel even at depth. Port Hills Gravel is present below much of Nelson and also has high shear wave velocities. This is due to the gradual transition from clay-bound gravel to conglomerate, at which point this deposit can be classified as rock. The information collected in this study will inform site classification across the region and improved inputs into seismic design, improved physics-based ground motion simulation, and regional earthquake response planning in Nelson-Tasman ​