Two east coast communities — 2,000 kilometres apart — woke to unusually similar news on Thursday morning.
Two separate earthquakes had occurred within hours of each other.
A magnitude-4.2 earthquake had struck 100 kilometres off the coast of Cairns and was felt by residents in the Far North Queensland city the night before.
A few hours later the northern New South Wales town of Grafton also started to shake — the result of a magnitude-3 earthquake.
To those who don’t keep across the nation’s quakes with a close eye it feels like a significant event, but can the two events tell us anything about seismic activity in Australia?
Were the two earthquakes linked?
The two earthquakes were on different fault lines, so despite being on the same tectonic plate and happening mere hours apart, they are separate events.
The Australian tectonic plate has small cracks and fault lines right across the country and in surrounding ocean beds.
The earthquakes are the result of a release of stress when local fault lines are activated by the movement of the plate.
“The big faults are where the plate boundary is, where the Australian plate is colliding with the Pacific plate through Papua New Guinea and New Zealand,” Geoscience Australia’s senior seismologist Jonathan Bathgate said.
That’s why those regions have heightened seismic activity compared to Australia.
“We record more than 100 magnitude-3 earthquakes every year,” Mr Bathgate said.
“The Australian tectonic plate is moving constantly at about 7 centimetres a year to the north-east and those stressors just activate these local fault lines that we have in our landscapes.”
Is Australia seeing an increase in seismic activity?
No. Despite Melbourne experiencing a magnitude-4 earthquake late last month, Mr Bathgate believes that Australians are noticing more seismic activity due to increasing accessibility to earthquake data on websites like Geoscience Australia.
“Australia gets more earthquakes than people appreciate,” Mr Bathgate said.
He said that Australia records many events that generally go unnoticed by the public.
“The earthquakes we’ve seen in Melbourne aren’t that unusual, we get a number in that region and further to the east in the Gippsland area,” Mr Bathgate said.
There is also a history of seismic events occurring offshore near Cairns, with a magnitude-4 quake rocking the region about every 10 years.
Although Australia tends to experience earthquakes in “random locations”, the Grafton quake was not typical.
“It is quite unusual, we haven’t got many earthquakes on record from the Grafton region,” the seismologist said.
How are earthquakes measured?
Earthquake recording instruments are called seismometers.
They were first placed in Sydney, Melbourne and Perth in the early 1900s and at the time, could only measure large earthquakes.
There are now more than 100 seismometer stations across Australia.
