Wotherspoon LM, Kaiser AE, Manea EF, Stolte AC. 2022. National Seismic Hazard Model: Site Characterisation Database summary report. Lower Hutt (NZ): GNS Science. 54 p. (GNS Science report; 2022/28). doi:10.21420/363X-CK83.
Abstract
This report summarises the development of the 2022 Site Characterisation Database as part of the 2022 National Seismic Hazard Model (NSHM) update. The database provides a set of site characterisation parameters defined at past and present GeoNet seismic instrument locations. The datasets and methodology used to assign site parameter values are presented, including how these have evolved since the 2017 version of the database (Kaiser et al. 2017). The site database includes key site parameters required to run and test commonly used empirical ground-motion models. In addition, new parameters have been added in 2022, including the lognormal standard deviation of each key parameter to represent measurement uncertainty. Quality parameters were assigned to each key parameter set (similar to the 2017 database) to reflect the level to which the available data is able to constrain their values. Available data sources ranged from site-specific investigations and regional velocity models through to national maps and geologic knowledge. The number of sites increased from 497 to 870 between the 2017 and 2022 versions of the database, with the inclusion of short-period and broadband seismometer stations of the national network. A large number of these new sites are located on rock deposits (with a measured or assumed shear-wave velocity greater than 750 m/s and an unconfined compressive strength greater than 1 MPa). The quality of the data used to inform the database improved, particularly for the original 497 strong-motion sites, as a result of new regional studies and national horizontal-to-vertical spectral ratio analysis. The fundamental site period (T0) was the most well-constrained of all the site parameters, with almost half of the database classified based on the highest-quality measurements. 58% of the sites in the database were assigned a fundamental site period representative of soil response above rock. Although there was an improvement in the quality of the parameters representing the time-averaged shear-wave velocity in the uppermost 30 m of the profile (Vs30), very little site-specific data were available, with almost no information for rock sites. The majority of these classifications were based on national maps or geologic interpretation. Depth-based parameters (Z1.0 and Z2.5) had the lowest quality overall, with very few direct measurements available to constrain these values. Overall, this database has provided an increase in the parameters available to characterise potential site effects, improved the quality of the parameters with the addition of new datasets and provided a detailed summary of the methodology and references used as part of its development. (The authors)
