Geoscience Australia senior seismologist reveals WA town Gnowangerup in middle of earthquake swarm

The Great Southern town, which has been shaken by more than 100 earthquakes since the beginning of this year, is likely to continue experiencing tremors for several months, according to a senior seismologist.

Geoscience Australia’s Dr Jonathan Bathgate said Gnowangerup was in the middle of the earthquake swarm.

“We often encounter these types of foreshock, mainshock and aftershock patterns when there are smaller earthquakes that lead to a large mainshock, and then we see them taper off in this series of aftershocks,” he said.

“So, over a certain period of time, its magnitude and frequency decrease, then it fades and stops, but the situation is different in the earthquake swarm.

“There are a lot of moderate or small to medium earthquakes in between that don’t have a true mainshock, and that’s what we’re seeing here.”

There have been more than 100 low-level earthquakes in Gnowangerup since January, most of which went unnoticed by residents.

But during the 3.8 magnitude earthquake on March 2, more than 60 people (some more than 50 km away in Katanning) filed “felt” reports.

Dr Bathgate said the largest earthquake ever occurred was within a cluster radius of about 10 kilometres.

He said cluster swarms were not “unusual” in the area known as the South West seismic zone, and swarms had previously been seen at Burakin between 2000-2001 and Beacon between 2009-2011. and Arthur River from 2022-2023.

“The Australian continent is moving northwards at a rate of about 7cm every year, and with this tectonic movement northwards and eastwards, we are colliding with plates to our north, through Papua New Guinea and Indonesia, and to our east, along New Zealand, Tonga and the South Pacific islands,” Dr Bathgate said.

“So some of this stress is being directed to the crustal rocks within the plate in Australia.

This isn’t something we actually understand enough to say how long it will last, but we’ve seen them last a few years and then slowly fade away.

Doctor Jonathan Bathgate

“That stress builds up and eventually releases along these … lines of weakness in the rocks, so these little swarms of earthquakes form where there are pre-existing fault lines.”

Dr Bathgate said the swarm was unlikely to cause a damaging earthquake, but it was impossible to rule out the possibility.

“There was a 4.5-5 magnitude earthquake in this region in 2023, so we know that there is the potential for larger earthquakes to occur here,” he said.

“But right now the behavior seems to be that we’re getting lots of these two- and three-order magnitudes, and the more energy that’s released through these smaller events, the less energy or less stress is stored in these crustal rocks.”

Gnowangerup residents should be prepared for the swarm to continue for several more months, he said.

“This isn’t something we actually understand enough to say how long it will last, but we’ve seen them last a few years and then slowly fade away.” Doctor Jonathan Bathgate

“When we encounter these types of activities, unless we are lucky, they may not occur very close to our permanent stations,” Dr Bathgate said.

“Geoscience WA placed a number of recording stations very close to the shoal to get really accurate locations and learn a little more about where they occur and how deep they are.

“We want to try to tie this back to the local geology and understand a little more what the mechanisms might be and what the cause or causes of this are.”

A 2.5 magnitude earthquake was reported near Gnowangerup at around 4.30am on Monday, continuing the recent trend.