We are often asked why our earthquake locations and magnitudes are different to those published by Geoscience Australia (GA), and who is right. Well, it depends.
To determine an earthquake’s latitude, longitude and depth, you need to use at least three seismographs. The closer they are to an earthquake, the more accurate your solution will be. In some parts of Australia, the SRC has a higher density of seismographs than GA (mainly around Sydney, Brisbane, and across Victoria and Tasmania) but there are vast areas of Australia that are only covered by GA’s network.
The SRC publishes earthquake locations that have been located by a seismologist, but only after emergency notifications to our stakeholders have been completed. In the interests of rapid public information dissemination, GA will often publish an automatic earthquake location that has been signed off by a seismologist, which will be refined at a later time, meaning the location may change slightly.
So, GA’s locations will be more accurate in some parts of Australia, and SRC locations may be better for smaller earthquakes within our network at different times.
What about magnitudes? In the past GA’s local Richter magnitude calculations were typically higher than the SRC’s, but in recent years our magnitudes tend to average out a few points higher than GA’s. Local site effects and tweaks to the seismograph response can affect this calculation. At various times the final magnitude is closer to our initial estimate, and at other times GA’s first calculation ends up closer.
So, who’s right? It depends on where the earthquake occurs, how big it is, and how many seismograph stations are nearby. Sometimes the SRC’s initial solutions are closer to the final solution, and sometimes GA’s end up closer.