|Earthquake activity in our region since 1973, showing small clusters of seismically active regions near Perth, Adelaide and the east coast. Credit: Solomon Buckman, Author provided|
Small earthquakes can point to a region where larger, destructive and potentially deadly quakes may occur.
Australians rely on government organisations to protect us from and advise us of natural dangers that are reasonably foreseeable. We expect that authorities will respond in a timely fashion if our house is threatened by bush fire.
If a cyclone is looming then we expect to get several days warning, if it’s a severe thunderstorm then a few hours warning at least. But can we ever expect the same degree of forewarning for large and potentially damaging earthquakes?
Do all quakes matter?
Earthquakes occur every day throughout Australia and most of them are so small that they’re not felt. Geoscience Australia (GA), Australia’s federal government geophysical authority, is responsible for monitoring and analysing most earthquakes that occur in Australia.
The only requirement of GA is that it advise any emergency management stakeholders if a significant earthquake of magnitude 3.5 or greater occurs within Australia, and it does this quite diligently. But what about the hundreds of smaller earthquakes that are occurring but go unnoticed? Are they so insignificant that they can be ignored?
Big quakes can kill and cause damage
On February 15, 2015, a magnitude 5.2 earthquake erupted just a few kilometres north west of Mt Perry, inland from Bundaberg. This was strongly felt throughout Central Queensland, up to Mackay and down to the northern Brisbane suburbs. For comparison, the 1989 Newcastle earthquake was magnitude 5.6, and resulted in 13 deaths. The Mt Perry event may have been smaller, but it was also potentially damaging.
The only reason it caused no significant damage on this occasion was possibly due to its location in remote cattle grazing country, far from any densely populated centre. Subsequent to the Mt Perry earthquake, GA deployed a seismic monitoring network of four stations, to record aftershocks. This network was in place for three weeks, and was able to capture many aftershocks to augment the organisation’s database.
CQSRG also recorded a similar number during that period, but has continued monitoring up to the present time, and recorded more than 200 earthquakes down to magnitude 0.1. Comparison of the GA and CQSRG catalogues in the decade prior to the Mt Perry event suggests that low magnitude events in the immediate area were precursor indicators of the subsequent larger event.
Small quakes as a warning
Although it is not possible from the pattern of the precursor events to accurately predict when and where a larger event may happen, there was sufficient low level activity to warrant closer monitoring and possibly to advise governments that a “watch and wait” situation was in progress.
In July/August 2015, three earthquakes of magnitudes 5.4, 5.3 and 5.1 occurred out to sea, off Fraser Island. The aftershocks of these events are shown in the images above, the GA data is first followed by our CQSRG data.
The events in the GA data are more closely clustered than the CQSRG data suggesting that the locational uncertainties are smaller in the GA data. But the CQSRG data shows many more low magnitude events, and even hints at a third cluster just north west of Gin Gin, between Mt Perry and Bundaberg, possibly another area to keep a watch on.
Missing precursor warning?
No precursor earthquakes were recorded in the decade prior to the three larger Fraser Island earthquakes. Why were similar low level events not recorded offshore from Fraser Island in the years leading up to August 2015, as happened for the Mt Perry events?
The answer may be that the nearest earthquake monitoring station to the Fraser Island earthquakes is the CQSRG station more than 250km away, just south west of Gin Gin. At that distance it is not capable of detecting low magnitude earthquakes that may have been occurring out to sea.
GA does have a monitoring station at Eidsvold, 320km from the Fraser Island earthquakes but, like the CQSRG station, this was too far away to detect low magnitude quakes. If there had been stations at Bundaberg and Maryborough, and if someone was actively monitoring, there is a possibility that precursor earthquakes may have been detected, and acted as a warning that something bigger was coming.
Mt Perry and Fraser Island are not unique. The same can be said for the rest of Queensland and the rest of Australia. For example, prior to August 2016 no earthquakes are noted in the GA catalogue for the area to the east of Bowen and north of Airlie Beach.
Then in August 2016, one of the largest earthquakes to be recorded on the east coast of mainland Australia erupted there. The magnitude 5.8 quake was followed by hundreds of significant aftershocks.
Establishing infrastructure to monitor for low magnitude earthquakes is a costly exercise. It would require monitoring stations every 50 km or so. At a cost of A$25,000 or more per station we are talking millions of dollars of investment.
But this needs to be considered against the social and economic damage bill if a big earthquake strikes near to an urbanised centre or a critical industry hub such as Gladstone for example.
We can’t prevent large and damaging earthquakes from happening, but with proper monitoring of low level earthquakes we may get a better idea where they will occur in the future.
Small earthquakes do count, so we do need to start counting them sooner rather than later.
This article was originally published on The Conversation.