Every major infrastructure project needs to consider potential site-specific hazards: e.g. landslides for roads; the environmental impact of power generation facilities; wind loading for buildings. Earthquakes are a hazard for these and many other assets including dams, nuclear power stations, resource pipelines and critical data centres.
Where can I get an Earthquake Hazard Assessment for a Dam Safety Review?
The Seismology Research Centre (SRC) routinely produces earthquake hazard assessments for Australian dams. In 2019 ANCOLD published Guidelines for Design of Dams and Appurtenant Structures for Earthquake to provide data and tools to aid in the safety review of existing, new, or upgraded dams. Seismic hazard is a complex science involving many sources of uncertainty. The SRC earthquake hazard team employs a range of different approaches and tools to address and quantify uncertainty inherent in seismic hazard work.
Our preferred approach primarily involves applying the Probabilistic Seismic Hazard Assessment (PSHA) methodology, but elements of Deterministic Hazard Assessment (DSHA) may also be considered depending on the consequence category of the dam and the proximity of any fault sources designated as active. Where a static analysis of the dam is sufficient the outcomes of our standard assessment are aimed to fulfil all necessary inputs to the engineering analysis. Standard outcomes are primarily peak ground acceleration (PGA) recurrence estimates and Uniform Hazard Response Spectra (UHRS) values for a range of Annual Exceedance Probability (AEP) values. Additional analyses such as topographic/slope effects, multiple damping ratios and complex sensitivity analysis may be required for particular sites.
For dynamic analysis of a structure, suites of time history records may be generated from strong motion recordings of real earthquakes that are scaled to a hazard intensity measure corresponding to an Operating Basis Earthquake (OBE) or Safety Evaluation Earthquake (SEE). The target spectra may be a UHRS or a suite of conditional mean spectra (CMS) depending on a range of factors.
In addition to carrying out site-specific seismic hazard assessments (SHA) for dams, tailings dams and tailings storage facilities, the SRC has also carried out similar assessments for pumped hydro schemes, bridges, offshore and onshore oil and gas platforms and related processing plants, ash dams, sensitive cultural and historic sites, highways, pipelines, railways, hospitals, and tall or otherwise significant buildings. For any critical infrastructure that needs to be engineered to withstand strong ground motion, the SRC can advise what level of ground shaking may be expected.
The cost to undertake an earthquake hazard assessment can vary wildly depending on the scope of works, from tens of thousands of dollars for a basic assessment, to hundreds of thousands of dollars for multi-site projects or multi-hazard assessments.
How does a site-specific seismic hazard assessment differ from a National Seismic Hazard Assessment?
Site-specific seismic hazard assessments (SSHA) primarily differ from regional or National Seismic Hazard Assessments (NSHA) in terms of the scale and level of detail involved. NSHA necessarily involve a more broad brush approach to the derivation of the PSHA inputs such as earthquake source model magnitude recurrence quantification (activity rates and b-values), local site conditions etc. A site-specific assessment will often involve production of a project-area earthquake catalogue, derivation of a small scale earthquake recurrence model for the site in question, and include investigations into the neotectonics and geology of the site. This considers local site conditions including soil or rock type and topography factors that may modify seismic wave amplitudes.
Although NSHA and SSHA both follow broadly similar methodologies, it is perfectly valid for site-specific estimates to vary from regional-scale hazard results, but the differences should be explainable in terms of the selection and justification of the inputs used. We regard an NSHA as a benchmark establishing both best practice methodologies and as a gauge as to how reasonable the site-specific hazard estimates may be.
What’s involved in a site-specific seismic hazard assessment?
A number of authors have outlined the essential processes involved in the production of a PSHA for critical structures, including U.S.NRG (2018), ANCOLD (2019), Baker et al. (2021) and Bommer (2022) amongst others. Ultimately what is included in any seismic hazard assessment needs to be appropriate for the structure it relates to and the outcomes required for proper analysis of that structure. Even in our current risk-averse society, expending the same amount of effort on producing an SHA for a structure with a limited operational lifespan and a small at-risk population as one would on investigation for a nuclear facility or waste repository, is not an efficient allocation of resources. For many structures, the use of local building codes or national/regional level seismic hazard assessments may be an acceptable approach, so long as it is understood that at certain locations the hazard assessments may be either underestimated, or overly conservative, depending on the input assumptions.
Site-specific seismic hazard assessments should be a “tailored fit” approach to the task rather than a “one size fits all” approach and will necessarily include different aspects of seismic hazard assessment such as probabilistic fault displacement hazard assessment, topographic amplification or site-specific effects as well as different outputs on a case to case basis.
Is a Probabilistic Fault Displacement Hazard Assessment different to a PSHA?
The SRC undertook our first ever Probabilistic Seismic Hazard Assessment in 1976 for a liquid natural gas storage tank located near Dandenong and that study, as with all PSHA studies, primarily investigated the primary hazard associated with earthquakes, i.e. strong ground motion. In addition to strong ground motion, there are secondary hazards arising from earthquakes including surface rupture, tsunami, liquefaction, landslides, seiches, flooding, fires and more.
A Probabilistic Fault Displacement Hazard Assessment (PFDHA) uses a similar methodology to a PSHA to quantify the hazard arising from surface displacement of a fault. This is often of particular importance for linear structures like pipelines, railways and highways. Both approaches use earthquake recurrence or rupture models: the PSHA then uses ground motion models to quantify the amount of ground shaking at various locations; while PFDHA use ground rupture models to quantify the probability and extent of primary and dispersed rupture at various locations.
In 2017 the SRC carried out our first PFDHA that considered primary fault rupture only for a tailings pipeline in PNG. We now include both primary and dispersed rupture hazard in these studies as a matter of course. If you have questions about seismic hazard assessments, contact our team.
