GA's updated procedure for calculating local magnitudes is now in production [SEC=OFFICIAL]
Trevor Allen
Hi all,
This is just a general note to advise end users of GA’s earthquake catalogue that our updated procedure for calculating local magnitudes (MLa) is now in production as of 1 July 2026 at 0:00:00 UTC.
Why are we making this change?
It has been known for some time that some earthquake magnitude calculations – especially at lower magnitudes – can be overwhelmed by large-amplitude, low-frequency background noise that is not part of the earthquake signal. Sometimes this noise is erroneously selected by the SeisComP algorithm that calculates the earthquake magnitude and leads to an overestimation of magnitudes for smaller (MLa ~< 3.0) events. This issue will likely have affected earthquake magnitudes calculated since the transition to the SeisComP software in mid-2018 (approx.)
What has changed?
GA has now initiated a magnitude-dependent pre-filter to calculate MLa. Specific details are provided on the “Event Details” tab by clicking on the information icon, 🛈
What does this change mean?
These changes will mean that we publish fewer earthquakes that meet GA’s reporting threshold (MLa 2.0, or significantly felt smaller events). GA’s 2023 National Seismic Hazard Assessment recognised this issue and independently corrected for these errors. However, the NSHA23 catalogue is limited to the end of 2022 and subsequent catalogue magnitudes have not been updated. Our next plan is to revise the magnitudes using the new procedure and publish them at earthquakes@GA for earthquakes occurring in the SeisComp era. This is a priority activity and will be undertaken as resourcing allows.
For more detail on the technical aspects of these changes, please see this GA Record: Recommended Procedures for Local Magnitude Estimation for Australian Earthquakes
Please let me know if you have any questions regarding these changes.
Cheers,
Trev
Trevor Allen
| Seismologist
Natural Hazards & Impacts | Community
Safety Branch
Place & Communities Division
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