WA Town Gnowangerup in the Midst of Ongoing Earthquake Swarm
A Great Southern town in Western Australia has been rattled by more than one hundred earthquakes since the beginning of this year, with a senior seismologist indicating the seismic activity is likely to persist for several additional months. Geoscience Australia's Dr Jonathan Bathgate has confirmed that Gnowangerup is currently experiencing what is known as an earthquake swarm, a phenomenon distinct from typical foreshock and aftershock patterns.
Understanding the Earthquake Swarm Phenomenon
Dr Bathgate explained that unlike standard earthquake sequences, which involve smaller foreshocks leading up to a major main shock followed by diminishing aftershocks, an earthquake swarm consists of numerous moderate or small to moderate-sized events without a dominant main shock. "With an earthquake swarm, it's different," he stated. "It's just a lot of moderate or small to moderate-sized earthquakes with no real main shock in amongst that, and that's kind of what we're seeing here."
Since January, Gnowangerup has recorded over one hundred low-level earthquakes, most of which went unnoticed by local residents. However, a magnitude 3.8 quake on March 2 prompted more than sixty individuals, including some located over fifty kilometres away in Katanning, to submit reports indicating they felt the tremor. This event, the largest in the current sequence, occurred within an approximate ten-kilometre cluster radius.
Historical Context and Geological Causes
Dr Bathgate noted that such cluster swarms are "not uncommon" in the South West seismic zone, with previous instances documented in Burakin from 2000 to 2001, Beacon from 2009 to 2011, and Arthur River from 2022 to 2023. He attributed the seismic activity to tectonic forces, explaining that the Australian continent is moving northward at a rate of about seven centimetres annually.
"With that tectonic motion north and east, we're colliding with the plates to our north through Papua New Guinea and Indonesia, and to our east through New Zealand and Tonga and those South Pacific islands," Dr Bathgate said. "Part of that stress is redirected to within the crustal rocks within the plate, which is within Australia. That stress builds up and then ultimately is released along lines of weakness in those rocks, so where we have these pre-existing fault lines, you get these small earthquake swarms."
Potential Risks and Monitoring Efforts
While Dr Bathgate suggested it is unlikely the swarm will result in a damaging earthquake, he acknowledged the possibility cannot be entirely ruled out. "There was a magnitude 4.5-5 earthquake in this area in 2023 so we do know that there is potential for larger earthquakes to occur here," he cautioned. However, he added that the current behaviour, characterised by numerous magnitude two and three events, may help release stored energy, potentially reducing the risk of a larger quake.
To better understand the swarm, Geoscience WA has deployed additional recording stations near the activity to obtain precise data on locations and depths. "We want to kind of try and tie it to the local geology and understand a bit more what the mechanisms might be and what the cause or causes for it are," Dr Bathgate explained.
Duration and Community Preparedness
Residents of Gnowangerup should prepare for the swarm to continue for another few months, according to Dr Bathgate. "It's really not something we understand enough to say how long it will last for, but we have seen them last for a couple of years and then sort of just die off slowly," he said. The ongoing nature of the swarm was underscored by a magnitude 2.5 earthquake reported near Gnowangerup around 4:30 am on Monday, continuing the recent trend of seismic activity confined within a ten-kilometre radius.
