A Vs30 Map for New Zealand based on Geology, Topographic Slope and Direct Measurements
Motivation Vs30 (the time-averaged shear wave velocity in the uppermost 30 metres of a given location) is an important engineering parameter used in research and practice. Vs30 is not straightforward or inexpensive to measure directly. Mapping Vs30 as a continuous quantity falls within the realm of geostatistical problems – where a limited set of observations in limited locations are used to attempt to understand the quantity elsewhere. These observation points are often geographically and geologically clustered, viz.: more measurements are collected in the built environment (in or near cities and towns), and cities & towns are often located in places with specific geologic characteristics (such as near alluvial deposits and water sources, for their ease of construction and transportation, among other considerations). This study represents a second proxy Vs30 mapping (Figure 1) for New Zealand after Perrin et al. (2015). Similar work has been carried out for other regions, e.g. North America. Most approaches to Vs30 mapping share some common characteristics: using a limited set of observed Vs30 values for a region, and finding correlations with readily available proxy data (geology & topography) to constrain estimated values of Vs30. This work aims to add to the knowledge of Vs30 mapping for New Zealand including, e.g., interpolation to resolve local discrepancies with available data (Figure 2) and developing a continuous estimate of model uncertainty. The resulting Vs30 and uncertainty maps will be valuable as a first-order resource for classifying uncharacterised sites, and as inputs to ground motion simulation research.
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