Boom! Why we felt tremors
WAS it an earthquake? Was it a ghost? No, it was a sonic boom.
North Coast residents from Terranora to Coffs Harbour were baffled yesterday as to what caused the earth to move on Thursday night.
A large number of people reported windows violently rattling, and even items falling from shelves, from what they thought was an earthquake, about 10pm.
An Australian Defence spokesperson yesterday said the tremors may have been caused by the supersonic flight of one of their aircraft, which was training off the coast.
“The possible source was supersonic activity as part of ECADEX (East Coast Air Defence Exercise),” the spokesperson said.
“The aircraft were operating in accordance with flight rules and restrictions for supersonic flight, but at times atmospheric conditions can propagate the sound waves further than expected.
“There have been recorded events of sound waves being heard from over 100 kilometres away.”
The spokesperson said FA-18s, F1-11s and Hawks were involved in the flying exercise, which concluded yesterday.
“We cannot specify the aircraft that may have produced the sound.”
The majority of the aircraft assets and personnel operated out of their home bases at Williamtown (NSW) and Amberley (Qld) and exercise flying was conducted over water from Newcastle to Evans Head.
Hastings Point resident Bruce Melville said he had just gone to bed when he felt the sonic boom.
“We were laying there and the window started shaking and it shook for about 20 seconds,” said Mr Melville from the Professionals at Pottsville.
“It was vibrating quite severe; like someone was hitting the window. I thought it was a ghost,” he laughed.
Mr Melville said he thought someone may have been outside, but realised the sensor light had not turned on.
“I was wondering what it was all night.”
Popular opinion yesterday was that the tremor may have been caused by a small earthquake, however, seismologist Claire Payne confirmed it was not.
“There was no seismic activity in the area,” said Mrs Payne, from the Seismology Research Centre.
“We would have picked it up if it registered on the Richter scale.”What is sonic boom?
The term sonic boom is commonly used to refer to the shocks caused by the supersonic flight of an aircraft.
Sonic boom is an impulsive noise similar to thunder.
It is caused by an object moving faster than sound, about 1,225 kilometres per hour at sea level.
An aircraft travelling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship’s bow.
When the aircraft exceeds the speed of sound, these pressure waves combine and form shock waves which travel forward from the generation or “release” point.