How small earthquakes affect Seismic Hazard

SRC seismologists explain the importance of having a complete earthquake catalogue for hazard studies for low seismicity areas.


In intraplate areas of low seismicity, such as Australia, the magnitude completeness of an earthquake catalogue is particularly important in Probabilistic Seismic Hazard Analysis (PSHA) studies.

The completeness magnitude, Mc, refers to the magnitude above which all events are assumed to be reliably recorded and known for inclusion within a PSHA study. When using the Gutenberg-Richter (G-R) relationship in earthquake recurrence estimates, having a complete catalogue is important as it defines the magnitude at which the lower end of the cumulative magnitude recurrence should be incorporated.

If the Mc value is too high it means that valuable data could be discarded leading to under-sampling. If an Mc value is selected that is below the level of completeness of the catalogue, using the dataset will lead to errors in estimates of the seismicity parameters.


It is therefore important to accurately select the Mc magnitude threshold based on a comprehensive understanding of the earthquake catalogue and the seismic network coverage in the area of interest.

In Australia, where the occurrence of higher magnitude earthquakes is much less frequent than more active regions around the world (such as California and subduction zones), we must rely on a lower Mc in order to adequately define the b-value (the relationship of small to large earthquakes) for the G-R relationship to apply. It is also assumed that extrapolation of smaller earthquakes gives a better long-term estimate of earthquake activity than just considering a very small number of larger earthquakes.

Victorian Earthquakes 1970-2014

A comparison of two earthquake datasets for the same region in Victoria for the same time period shows the distinct difference in total number of collected earthquake data.

The SRC has four times more earthquake events in their database (compared to the public national database) and is therefore a superior source of data for the preparation of earthquake magnitude recurrence estimates in PSHA studies. Using the SRC earthquake catalogue for studies in eastern Australia will give better and more reliable results on the hazard estimates provided in a PSHA study.

For any assistance or requests to prepare PSHA studies for dam safety reviews and dam designs, or any other significant engineered structure, please contact us using the form on this page or call us on +61 3 8420 8940.