Kaiser, A.E.; Hill, M.P.; Wotherspoon, L.; Bourguignon, S.; Bruce, Z.R.; Morgenstern, R.; Giallini, S. 2019 Updated 3D basin model and NZS 1170.5 subsoil class and site period maps for the Wellington CBD : Project 2017-GNS-03-NHRP. GNS Science consultancy report 2019/01. 48 p. + appendix. https://doi.org/10.21420/4khg-2d09
Abstract
Soil deposits in sedimentary basins are well-known for amplifying earthquake ground motion. Models of the 3D subsurface and maps of key geotechnical parameters are important to mitigate these effects and guide robust engineering design practices. Furthermore, they can help us better understand ground motion amplification and damage patterns observed in recent earthquakes in Wellington. Following the 2016 Mw 7.8 Kaikoura earthquake, we have updated the central Wellington 3D basin model and low amplitude natural site period and NZS 1170.5 site subsoil class maps for central Wellington. The new maps incorporate significant new geotechnical and geophysical data collected since the original work of Semmens et al. (2010), including 700+ new boreholes records collated from a range of sources and 400+ new geophysical measurements collected as part of this project. We have compiled an upgraded borehole database comprising 1,427 boreholes, 429 of which intersect greywacke basement. The database allows us to better define key geotechnical engineering units within our 3D model, in particular shallow loose sediments and areas of hydraulic and engineered fill in the uppermost 30 m. The surface geology map has also been updated based on boreholes, aerial photography and LiDAR digital terrain models. The result is a new (v2.0) 3D geological and velocity model for central Wellington. We have also compiled a new geophysical site period database from microtremor horizontal to-vertical spectral ratio (HVSR) surveys. The database includes an estimate of quality for each individual site period measurement. The geophysical information complements the borehole information by providing key constraints on site period and basin structure in deeper parts of the basin where boreholes to basement are sparse. Finally, we draw together the large quantity of complementary geological and geophysical information in an innovative scheme to provide updated maps of natural site period and site subsoil class. Our results indicate that site period is longer than previously thought in the upper Thorndon basin. This implies that the depth to greywacke basement is likely deeper than previously mapped, and site Class D is the most appropriate NZS1170.5 subsoil classification. Furthermore, the variability of site period estimates in the Thorndon area, indicates that complex amplification effects are present, likely associated with the Wellington Fault basin edge and lateral 3D subsurface variations. In waterfront areas, including CentrePort, site period is mapped in the 1–2 s period range corresponding to that of large amplifications observed during the Kaikoura earthquake. Further acquisition of deep boreholes intersecting basement in the Thorndon and CentrePort area and on the footwall adjacent to the Wellington fault, would provide valuable confirmation of the deeper basin structure in this area. The Te Aro basin to the south is now mapped with more steeply dipping basement on its western and eastern flanks. This is consistent with the new discovery of Aotea Fault offshore, which is inferred to extend beneath the eastern edge of the basin. Geological and geophysical estimates of site period in the Te Aro basin are generally in excellent agreement and allow well-constrained and smoothly varying site period contours to be mapped. Our study highlights that areas above the steeply-sided basin edges adjacent to deep soils may experience longer period site response than implied by considering only the 1D profile beneath the site. (auth)
