2022 – Impact of new industry guidelines and new seismic hazard knowledge on seismic hazard assessments for dams in Australia and New Zealand

Paul Somerville, Jeff Bayless, Andreas Skarlatoudis, Scott Condon AECOM, Los Angeles, California

This paper describes rapid improvements in the modelling of seismic hazards in Australia and New Zealand and in the appropriate representation of probabilistic hazard results in seismic response analyses following new industry guidelines. We have reviewed site-specific seismic hazard calculations for dam sites in all of the states and territories of Australia and in both the North and South Islands of New Zealand, using updated regulations and seismic source models, and compared them with results using earlier regulations and models at the same site. Changes in the Australian National Committee on Large Dams guidelines (ANCOLD, 2019) including the use of MASW seismic velocity profiling of dam foundations, and the use of time histories spectrally matched to Conditional Mean Spectra in place of probabilistic Uniform Hazard Spectra, have improved the reliability of seismic hazard analyses. The 2014 NGA West 2 ground motion models indicate that the ANCOLD (2019) requirement to provide ground motions for bedrock conditions, for which nonlinear soil response is negligible, can be met at sites with VS30 as low as 600 m/s. Changes in earthquake source models in the Geoscience Australia 2018 National Seismic Hazard Assessment have in almost all cases reduced seismic hazard levels below previously estimated levels at the same dam sites in Australia. The peak acceleration of the Safety Evaluation Earthquake (SEE) at many dam sites in Australia now falls below the notional threshold of 0.1 – 0.15g for the triggering of liquefaction. The strong ground motions recorded from the 21 September 2021 Mw 5.9 Woods Point earthquake provided an unprecedented opportunity to compare recorded ground motions with ground motion models being used in Australia, and are in fairly good agreement with those models. A database of strong ground motion recordings in Australia is being used to select and rank ground motion models, and to validate ground motion simulations for the development of improved models. In New Zealand, impending changes in the GNS Science New Zealand seismic hazard model may also result in significant changes in seismic hazard levels. The first New Zealand Community Fault Model (CFM) was released this year, and a revised New Zealand National Seismic Hazard Model, based on the CFM and replacing the current model developed in 2010, is due to be released later this year. New Zealand has an extensive program of validation of ground motion simulation methods against ground motions recorded from a wide range of earthquake magnitudes for both shallow crustal and subduction earthquakes.