Abstract
This paper presents the development of representative shear wave velocity profiles for the prominent geologic formations in the Nelson-Tasman region of New Zealand. Shear wave velocity (VS) profiles to depths of up to 100 m were developed at over 50 sites using a combination of active source and passive source surface wave testing. Using this data and regional geologic information, VS-depth functions were developed for six of the prominent geologic formations. Comparison with existing VS-depth functions from New Zealand and international studies highlighted the significantly higher shear wave velocities of the deposits in this region. VS exceeded 750 m/s for the Moutere Gravels and Port Hills Gravels at relatively shallow depths, representative of rock deposits. However, while the Port Hills Gravels transition to a conglomerate rock below depths of 30 m or less, the Moutere Gravel formation is an uncemented clay-bound gravel. The young gravel and sand deposits have VS higher than those from other regions. As the region is thought to have undergone cycles of geologic uplift, the resulting over-consolidation of these deposits could explain the high VS. Horizontal-to-vertical spectral ratio testing was not able to characterise the fundamental site period across the region, likely due to the weak impedance contrast that would exist at the gravel-rock interface at depth. These outcomes highlight the importance of regional geotechnical and geophysical characterisation to constrain the salient features that control potential seismic site amplification and site classification.
Highlights
In New Zealand, recent earthquakes have demonstrated the influence of near surface soil profile characteristics and sedimentary basin structures on site amplification, with these often referred to as site effects
This paper presents the development of representative VS profiles for the major geologic formations in the NelsonTasman region
horizontal-to-vertical spectral ratio (HVSR) testing was performed at 52 sites to explore trends in the fundamental period of vibration of soil above bedrock across the Nelson and Tasman region
Summary
In New Zealand, recent earthquakes have demonstrated the influence of near surface soil profile characteristics and sedimentary basin structures on site amplification, with these often referred to as site effects These site effects are important on the local site scale for the design and assessment of structures; and on the regional scale to assess impacts on distributed infrastructure and understand the effects of 3D basin geometry on ground motions. The 2013 Cook Strait EQs [7] and 2016 Kaikōura earthquakes further demonstrated the influence of local site conditions and sedimentary basins on ground motions in Wellington [8] Key to understanding these effects is characterisation of the shear wave velocity (VS) of different geologic deposits and their stratigraphic characteristics, both locally and across wider basin structures
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