Residents were woken early Saturday morning to shaking homes and falling bricks as another earthquake rattled the Upper Hunter about 6am.
A magnitude 4.5 earthquake appeared to have emanated from beneath the Mt Arthur coal mine at Edderton just outside Muswellbrook - the same epicentre as a series of quakes over the past few weeks.
In the past three weeks, Muswellbrook and its surrounding areas have been shaken by the biggest seismic activity in 20 years - a 4.8 magnitude tremor recorded on Friday, August 23 - setting off a litany of subsequent aftershocks in the following days and weeks.
Mining operations were stopped briefly at Mt Arthur as the operator BHP undertook inspections of the site, but there were no reports of injury.
"We continue to monitor all active work areas," a spokesperson said.
More on this issue: Geoscience Australia deploys earthquake monitoring devices in Muswellbrook
Geoscience Australia was inundated with just shy of 400 reports of people feeling the quake in the first 10 minutes after the event on Saturday morning that loosened and caused a few chimney bricks to fall around the centre of town. The region appears to have escaped any serious damage.
Reports of the earthquake being felt topped 1000 around four hours after the event.
About 2500 homes had the power cut in the early morning following the quake, registered at a depth of around five kilometres beneath the surface and described by Geoscience Australia as a "moderate" event.
The State Emergency Service was called twice for help clear fallen chimney bricks around the centre of Muswellbrook, but authorities say there were no injuries or serious infrastructure damage.
NSW Police urged residents to contact relevant local authorities as required. Electricity was restored to the area around 10.30am.
There have been around 30 registered seismic events in the area since January, according to Geoscience Australia records, and more than 150 in the past 20 years, many of which happen near or under coal mining operations because the mines are close to natural fault lines.
Earthquakes are typically caused when stress accumulated in the earth's crust is released in a sudden rupture, usually along cracks or faults where movement occurs.
Earthquake geologist Dee Ninis, who works at Melbourne's Seismology Research Centre, told the Newcastle Herald on Saturday human activity like mining, fracking or wastewater injection into the crust contributed to earthquakes. Still, seismic activity is induced or triggered by several factors.
The open-cut mining practices at Mt Arthur, essentially involving the removal of large amounts of rock from the earth's surface, contribute to changing stresses in the earth's crust and typically cause smaller magnitude earthquakes at closer proximity to the surface, usually below 2.5 magnitude and often a few hundred metres below the surface, Dr Ninis explained.
By contrast, mining or other human activity could exacerbate the "swarm" of deeper and more significant quakes seen over the past three weeks, but the activity may have also been caused by a build-up of stress along the nearby ancient Hunter-Mooki fault that runs through the state or other contributing factors.
The southern tip of the Hunter-Mooki is close to Newcastle, which memorably recorded a 5.42 magnitude earthquake on December 28, 1989, at a depth of around 11 kilometres.
Dr Ninis said the series of quakes rattling the region were likely connected, being of similar magnitude and depth, and originating from a similar epicentre. Still, there was unlikely to be one unequivocal cause.
"A recent study indicates that even large open pit mines do not appreciably change the stress along faults in the near-surface (within 5km depth) to trigger moderate-sized earthquakes," Dr Ninis wrote in The Conversation after the initial quake on August 23.
"Calculations show faults would need to be already tectonically stressed almost to the point of failure for mining to affect the timing of earthquakes."
Geophysicist Stuart Clarke of the University of NSW in Sydney said the geology around Muswellbrook acted as a funnel for the earthquake, channelling its power towards the town and explaining why locals felt the brunt of the shock.
"There is a long sinkline acting as a telegraph to transport the signal northward," Professor Clark said after the initial quake on August 23. "That's propagating [the shock] from the epicentre to that location along the geological structure and concentrating in that direction."
The recent seismic activity in the region has left climate lobby group Solutions for Climate Australia to raise "serious concerns" for the federal Opposition's plans to build a nuclear reactor in the Upper Hunter.
In recent months, Opposition Leader Peter Dutton said on a visit to Muswellbrook that if energy company AGL did not reverse its anti-nuclear stance, an elected Coalition government would seize the site of the former Liddell power station in the national interest to build a nuclear reactor on the land.
"The Liberal Nationals need to tell residents how an emergency at a nuclear reactor would be handled and who would handle it," Solutions for Climate Australia campaigner Elly Baxter said. "We need to have a mature conversation about the safety and security of nuclear reactors."
Geoscience Australia has indicated the most recent activity was felt as far south as Buff Point on the Central Coast and as far as Qurindi in the north-west.
