The Victorian earthquake has highlighted the potential to underestimate risks in Australia due to its long seismic cycles and national data limitations, catastrophe modelling firm RMS says.

The Geoscience Australia (GA) National Seismic Hazard Assessment 2018 (NSHA2018) features an earthquake catalogue with a relatively short history compared to regions such as Japan or Europe, RMS says.

“One major difference between our model and the GA model is that we include slowly moving active faults,” RMS Senior Director, Model Development Marleen Nyst says in a post on the firm’s website.

“RMS considers the geological records of thousands to tens of thousands of years of fault activity in the region, capturing potential active faults that have not had earthquake activity in the period covered by the GA earthquake catalogue.”

Dr Nyst says the firm’s clients have sought to understand differences between the RMS approach and outcomes from Geoscience Australia’s 2018 assessment, including for NSW.

“The NSHA2018 is a landmark model, and its findings are incorporated within the RMS model.” she says.

“But it pays to understand the way the NSHA2018 was built, the event catalogue used, and how the lack of significant historical events can tend to underestimate rates in high-exposure areas such as Sydney.”

RMS says in Australia tectonic change is slow compared to an active region such as California and the full seismic cycle could extend tens or hundreds of thousands of years.

The 5.9 magnitude earthquake that shook Melbourne in September was the largest onshore earthquake in Victoria in recorded history.

“It reminds us that if RMS only used the earthquake catalogue to model seismic hazard in Australia, the frequency of these larger events would probably not be very accurate and likely be too low,” Dr Nyst says.

The Geoscience Australia 2018 major update included scientific advances in magnitude measurement, leading to an overall reduced seismic hazard estimate that brought the country into line with other stable tectonic regions.

But GA notes significant events are still a possibility and building codes in some similar tectonic environments overseas take a more conservative approach to reduce risks in the case of rare events.

Information from the GA hazard assessment is used by private and public organisations, including the Australian Building Codes Board, and feeds into earthquake design codes put out by Standards Australia.