"Rapid" geophysical characterisation of New Zealand sedimentary basins using the horizontal-to-vertical spectral ratio method

Abstract

Earthquakes in New Zealand and internationally have demonstrated the influence of sedimentary basins on site amplification. However, apart from a handful of detailed site characterisation studies, few basins in New Zealand have been characterised to a level that basin effects can start to be quantified. This report presents a ‘rapid’ geophysical characterisation approach that can allow for faster dissemination of basin characteristics that can feed into research and practice while more refined models are developed. This method is based on representative deep shear wave velocity profiles from surface wave testing and estimates of fundamental site period across the basin based on horizontal-to-vertical spectral ratio testing. Using the relationship between the shear wave velocity profile and site period, basin depths can be estimated at each site period testing location, informing the development of basin models.

As part of this project, new regional geophysical site investigation studies were undertaken in Waikato, Hauraki Plains and Hawke’s Bay. Deep shear wave velocity profiles and site period estimates across these regions have improved understanding of the properties of regional soil deposits and the regional variability in dynamic site characteristics. In Waikato fundamental site periods in excess of 5 seconds were recorded, while in the Hauraki Plains, site periods were greater than 6 seconds in some areas, suggesting deep basins in both regions. Fundamental site period estimates in the Napier area were less than 2 seconds in most locations, reducing closer to the surrounding hills and Bluff Hill. South of Napier and across much of the remainder of the Heretaunga Plains, some of the H/V peaks were likely representative of a shallower impedance contrast, and not the overall soil profile to bedrock. Variation in depth to bedrock in this area aligned well with the surrounding topography.

New deep shear wave velocity profiles from south of Napier were used in combination with the site period estimates in the Napier area to estimate the basin depth using the ‘rapid’ method. This was able to provide insight into the structure and shear wave velocity of the deposits in the basin under Napier. Site period estimates from Waikato and the Hauraki Plains were used in combination with new deep shear wave velocity profiles from Waikato to estimate the basin depth in these regions. These were compared against models constrained using regional topographic data. There were clear differences between the two model types in some areas, with the ‘rapid’ method likely providing a better representation of the basin structure, based on comparisons with other investigation data, and provide additional insight into the basin structures. The ‘rapid’ method was applied to existing site period estimates and deep shear wave velocity profiles from Canterbury, where a high resolution basin model already exists that has been constrained by a number of datasets. The depth estimates from both models were in good agreement, demonstrating the usefulness of the method in providing initial information on basin structure that can inform regional site classification for seismic design, regional near surface shear wave velocity models, and the development of velocity models for physics-based ground motion simulation.