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Experts 'Can't Explain' The Cluster Of Earthquakes On Australia's Edge

A number of large earthquakes around Australia's north-west are purely "coincidental", experts claim.

An undersea earthquake, initially measured as 6.2 magnitude, hit the Indonesian island of Bali on Tuesday morning, according to the European monitoring agency EMSC.

No major damage has been reported, though as many as eight million people may have felt the quake, the EMSC said, with people on the ground reporting strong and "frightening" shaking.

It comes less than 24 hours after a magnitude 6.4 earthquake struck off Papua New Guinea, the U.S. Geological Survey reported.

Nearby, the Indonesian Moluccas islands were hit by scores of aftershocks after another quake, initially recorded as magnitude of 7.2, killed at least two people and prompted hundreds of people to flee their homes.

These quakes have all hit in the days since Western Australia's north was rocked by the country's equal-largest earthquake on Sunday.

READ MORE: Bali Hit By Huge 6.2 Magnitude Earthquake, Hotels Evacuated

READ MORE: 'Never Experienced Anything Like It': Earthquake Shakes West Australian Coast 

If this feels like a lot, you're not wrong. According to experts, such frequent earthquake activity, both in Australia and in Indonesia, is not unusual -- and the fact so many have occurred in the same time period is just coincidental.

"It would be fair to say that there has been a cluster of larger earthquakes, although it's part of a regular pattern of seismic activity," Chris Elders, Curtin University's Professor of Petroleum Geology, told 10 daily.

"Earthquakes occur very frequently, both in Australia and in Indonesia. On this occasion, that frequency has happened to coincide ... we can't really explain why."

Why Are Earthquakes Happening In Indonesia And Australia?

Indonesia is a meeting point of several tectonic plates, and is prone to earthquakes. But Australia's tectonics are lesser-known and -- usually -- less volatile.

"Although we think of Australia as being an old and stable continent, in fact it is quite seismically active," Elders said.

"The reality is earthquakes -- much smaller ones -- are occurring on a daily basis." 

Australia sits on one large tectonic plate that is moving northwards towards Indonesia at a rate of seven or eight centimetres per year. That's about the same rate that your fingernails grow.

PHOTO: USGS

Australia is being pushed northwards, away from Antarctica, where it is colliding with the plate upon which Indonesia rests.

"In the region between PNG and Timor, the continent has already collided with that part of the Eurasian plate," Elders explained.

"As you come further to the west, around Lombok and Bali, the ocean crust that forms part of the Australian plate is moving down underneath the Indonesian plate."

READ MORE: Massive 7.5 Earthquake Rocks Darwin, Buildings Evacuated

This explains why larger quakes are generated in Indonesia than those in Australia.

"On this occasion, they happen to have occurred at the same time," Elders said. 

Shakes Felt Across Western Australia

According to government agency GeoScience Australia, the magnitude 6.6 earthquake off the coast of Broome on Sunday was on par with the country's largest quake on modern record.

Data shows it matched the 1988 quake in Tennant Creek in the Northern Territory, and the 1968 Meckering earthquake in Western Australia.

"It was probably one of the quakes that was most widely felt across Australia. Pretty much the whole state of WA felt this particular earthquake, so it was quite significant," GeoScience Senior Seismologist Dr Trevor Allen told 10 daily.

"We do know the region off the northwest coast of Australia is one of our most seismically active regions, so it's not surprising that a large earthquake has occurred off the coast." 

According to Allen, the amount of energy released in the Broome earthquake was roughly 100 times more than what was released in the 1989 quake in Newcastle.

"The main differences between the impacts that we saw is that the major area where the energy was released, and as a consequence, the strongest ground shaking, was well away from any major population centres," he said. 

"We were  very fortunate in this case the earthquake occurred a long way off the coast."

Is Australia Prepared To Withstand A Large Quake?

Measuring preparedness for an earthquake is the role of state emergency management services, according to GeoScience Australia Structural Engineer Martin Wehner.

The government agency, which provides real-time assessment of earthquakes of magnitude 3.5 and above within Australia and adjacent regions, also works with state partners to research and model impact.

"Are we prepared? It would depend on where the earthquake occurs --  how close it was to a town or city -- how big it was and the make up of the  affected settlement's building stock," Wehner said.

"If we had the weekend event in Broome under settlement, the damage would have been worse than the Newcastle earthquake in 1989, which was directly located underneath the city's centre."

In the aftermath of the Newcastle earthquake, the earthquake loading standard was incorporated into the National Construction Code 1990.

"Prior to that, there was very little in terms of requirements to design buildings to withstand expected ground motions from earthquakes," Wehner said.

"We have this legacy in the Australian building stock of building older buildings that weren't specifically designed in this way -- they are most at risk."

Photo: GeoScience Australia

Allen said a strong building code is one of the most important ways to ensure preparedness.

"We shouldn't get too carried away about being worried, but it is important to be prepared," he said.

"Having a strong building code and good compliance with that code does affect life."

Featured image: GeoScience Australia