Underground coal mines may be having a profound affect on earthquakes in the Appin region, with mining recognised worldwide as a potential trigger of seismic activity.
South32 said there was "no evidence" linking the recent tremors to its mining activities and there had not been blasting immediately preceding the quakes.
But the tremors follow a trend of most quakes detected across the region since 2010 being centred near that location.
Despite seismic reporting being part of South32's environmental approval conditions, the company has not published any of the event reports it was obliged to produce.
There is a "close association" between underground mining and seismic activity in the area, Geoscience Australia senior seismologist Trevor Allen told the Mercury.
"There's certainly a close association there between the mining activities and the seismicity that has been observed over time," he said.
"It's recognised globally that mining activity can be the cause of seismic activity.
"In South Africa they have some of the deepest gold mines in particular in the world, down to, say, 4km.
"And so most of the seismicity in those mines is, is really associated with those, those deep underground gold mines.
"There's also evidence for earthquakes being triggered by coal mining as well, which tends to occur at shallower depths."
Stress changes to the Earth's surface caused by the removal of large amounts of rock, as well as blasting, can each be a trigger, he said.
The recent quakes, all minor, were centred west of Appin near where longwall mining is taking place in South32's Bulli Seam Operations project (BSO). Their epicentres ranged in depth from 1-4km.
The area directly west of Appin has become an earthquake hotspot in recent years.
In the 50 years from 1960 to 2010 there were 44 earthquakes within 40km of Monday's magnitude 2 quake at Appin. These were spread fairly evenly around the region.
In the 13 years since 2010, there have been 21 in the same radius - the vast majority of them concentrated in the area north and northwest of Appin towards Douglas Park and Menangle.
A South32 spokesman said there had not been any blasting undertaken as part of the Ventilation and Access Project near Douglas Park, where work was still in its early stages.
"No blasting has taken place on or around the dates when the seismic activity was recorded and there is no evidence to suggest the seismic activity in June was linked to our operations," he said.
"South32 Illawarra Metallurgical Coal monitors seismic activity in the local area in collaboration with the Seismology Research Centre."
Dr Allen, the leader of GA's National Seismic Hazard Assessment project, said mining can trigger earthquakes because large amounts of material are moved from under-pressure rock mass.
"The earth is constantly stressed by its movement due to plate tectonic forces," he said.
"The Australian continent is moving at a rate of around about 7cm per year in a northeast direction and that imposes stresses within the rocks in Australia.
"Those stresses have had millions of years to build up and impose themselves within the rock mass beneath our feet.
"When we change those stresses by removing large volumes of rock mass, that can change the stress within the rocks and it can trigger earthquakes on pre-existing faults or fractures within the rocks in the vicinity of the mine.
"There's also several examples in Australia where earthquakes have been triggered by mining [and] they've led to temporary or permanent closure."
Three years ago the Raleigh gold mine in Kalgoorlie was forced to close because of a "significant seismic response" caused by underground blasting.
Dr Allen said most miners take the issue "very seriously".
"Most underground mines in Australia do actually have sophisticated in mine seismic monitoring systems and they can use that data to mitigate the risk of earthquakes actually occurring by adjusting their mining practices in the event of any changes in the seismicity in the rock mass," he said.
"They're very sophisticated operations and, and they do take this stuff very seriously."
South32 has a regional seismic event monitoring program (RSEMP), last updated in January, and is supposed to produce annual reports showing the results and assessment of seismic events that occur.
It is to "improve the understanding of the relationship between underground mining and seismic events".
But these have not been published, an independent environmental auditor, ERM, noted in 2022.
"Although ERM understands the Seismic Monitoring Reports have been prepared, no Seismic Event Monitoring reports are included in the Annual Reviews and none have been able to be reviewed by auditor as required by this condition, therefore BSO is noncompliant," its audit stated.
Illawarra Metallurgical Coal's response to this, recorded in the audit report, was that it will either include them in next time, or scrap the obligation altogether.
"Seismic Event Monitoring Reports will be attached to future Annual Reviews or the Seismic Event Monitoring Program will be revised to remove this requirement," IMC's response stated in December 2022.
The South32 spokesman said the seismic event reports would be published in September.
"Earlier this year, we made a request to the NSW Department of Planning and Environment to publish seismic data recorded in the Bulli Seam Operation region in our annual review document," he said.
"This request was accepted and the data will be published in September. The report will summarise the seismic events located in the BSO region during the period."
He also quoted from the RSEMP: "The incidence of seismic activity and the relationship to mining is dependent on the analysis of events over a period of time. It is not expected that a relationship can be determined based on a single event."
The company's reporting draws on an earthquake monitoring network established by South32 around Appin which includes 11 seismic monitoring stations, some of them named after the longwall panel they sit above.
The newer monitors, installed in 2018, are above the Douglas Park area where recent earthquakes have been centred.
But this conclusion was doubted by an Australian expert, who said that at 12km underground, that quake was too deep for mining to be the cause.
"It may have, but it is unlikely," Professor Stephen Cox from the Australian National University said in 2007.
"The stress change that's induced by mining commonly triggers very small earthquakes, fairly close to mining, that is within 2-3km of mining," he said.