Tunnel under HMAS Watson, Watsons Bay NSW
From ABC and Cairns News
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?
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.
A Cairns News contributor four decades ago received a report of a substantial, concealed, man-made tunnel system discovered by a bushwalker in the Pilliga scrub in NSW.
Other mysteries have plagued the Pilliga area for generations which have developed into folk lore and passed down by local families.
Some of these are the numerous anecdotal reports over many years by truck drivers who had seen Yeti-type creatures on the Newell Highway which intersects the Pilliga Scrub.
In the late 70’s one terrified truck driver who had made a rest stop in the scrub tore into a road house in Coonabarabran alleging an encounter with a Yeti that tried to break into his truck cabin in the middle of the night.
There have been reports of extensive tunnel systems across Australia over many years. More recently articles on alternative media have reported the deliberate destruction of deep underground military tunnels worldwide with high explosives placed by entities unknown.
These explosions reportedly have been recorded on seismographs but officially passed off as earthquakes.
The American super secret Pine Gap military base in Central Australia reputedly has an extensive tunnel system. Sherga air force base near Weipa in far northern Queensland also has underground facilities.
Forestry roads through the Pilliga Scrub are collapsing, a rare natural phenomenon usually seen in central Queensland’s coalfields where underground mining has caused massive land slippage across cattle property pastures.