Wollongong earth shake was a quake: GA
Was it a continuous clap of thunder or a small earthquake that rattled the Illawarra on Monday night?
That’s what many people wanted to know after a large swathe of the region reported feeling the earth move.
The answer – an earthquake, albeit a very small one.
Geoscience Australia (GA), the government agency that deals with earthquakes, fielded 15 “felt reports” from the public via its website about the quake.
The first report came from Tarrawanna at 8.54pm, when a resident reported hearing a sound that travelled from the north-east and headed in a southerly direction, GA duty seismologist Dan Jaksa said.
However, the distance of the reported quake from GA’s seismic monitoring stations meant there was insufficient data to pinpoint a location or determine its magnitude.
“It wasn’t big enough to reach enough stations, you need basically three [stations] to be able to triangulate back to get an epicentre. You need an epicentre before you can get a magnitude,” Mr Jaksa told the Mercury.
“Magnitudes are related to the amplitude of the seismic wave and the amplitude of the seismic wave diminishes over distance.”
With rain and possible isolated storms lingering off the Illlawarra coast at the time of the quake reports, many were quick to suggest the noise was thunder.
Mr Jaksa said it could be difficult to determine the difference between thunder and an earthquake in the short term, but ruled out suggestion it was a rumble from above.
“As we did see a seismic signal in our Canberra station, we’re pretty confident it was an earthquake, but very, very small – probably less than magnitude two,” he said.
“Thunder doesn’t enter into the ground, a seismic wave doesn’t go into the ground from thunder so you wouldn’t see it on a seismometer.”
A spokeswoman for the Bureau of Meteorology said a check of Monday night’s satellite and radar imagery suggested there was no thunderstorm activity in the area at the time.
Weatherzone radar images captured at 8.50pm suggested there may have been some lightning embedded in heavier showers off the coast.
The Illawarra's most recently recorded seismic activity was a 2.1-magnitude earthquake between Wollongong and Wilton on January 10, 2015.
“We can detect a [magnitude] 2.1 in that region, so this [latest seismic activity] would have been 1.5 or something like that,” Mr Jaksa said.
The difference between a magnitude 1 and a magnitude 2 is 32 times – i.e. a magnitude 2 earthquake is 32 times bigger than a magnitude 1.
HOW THE QUAKE WAS REPORTED TO GEOSCIENCE AUSTRALIA
- “Heard a very loud rumbling noise; what sounded like a massive wave hitting the beach and my place shook.”
- “Sounded like a low-flying jet.”
- “A few seconds rumble with ground vibrating.”
- “Was a large rumbling sound unlike thunder or a truck. One of my non-sensitive dogs started crying, the windows were rattling a little.”
- “The sound travelled from north-east and headed towards the south.”
WHAT TO DO IN A LARGE EARTHQUAKE
In Australia, earthquakes with magnitudes of less than 3.5 seldom cause damage, Geoscience Australia says.
The smallest magnitude earthquake known to have caused fatalities was the magnitude 5.4 Newcastle earthquake in 1989
Dan Jaksa, a duty seismologist at Geoscience Australia, provides the following advice:
- If you’re inside, remain inside and get underneath some strong furniture.
- Stay there until the shaking finishes and then go outside at least five metres away from the building.
“Most deaths from earthquakes occur in the zero to five metres outside a building. Inside buildings you’re a lot safer,” Mr Jaksa said.
“If you think about it, the building is not going to fall in on itself, it generally falls out.”