FELT IT: A 5 magnitude earthquake shook North Queensland, with Geoscience Australia saying people as far north of Cairns felt the quake.
FELT IT: A 5 magnitude earthquake shook North Queensland, with Geoscience Australia saying people as far north of Cairns felt the quake.

Whitsunday earthquake felt in CQ upgraded in size

AN EARTHQUAKE that shook the Whitsundays last week has been upgraded from its initial magnitude rating.

A 4.8 magnitude earthquake which struck the Whitsundays last week has been upgraded to a 5 magnitude by Geoscience Australia.

The magnitude upgrade means the earthquake experienced 1.6 times more movement at its epicentre.

A spokesman for Geoscience Australia said reassessing earthquake magnitude was a common occurrence after an initial tremor, with a goal to release the breaking information within 15 minutes.

TREMOR: A 5 magnitude earthquake has been recorded off the coast of the Whitsundays. Picture: Geoscience Australia
TREMOR: A 5 magnitude earthquake has been recorded off the coast of the Whitsundays. Picture: Geoscience Australia

"Following significant earthquakes, it is important for earthquake alert centres around the world … to issue rapid notifications of an earthquake's occurrence, its location and magnitude to emergency management authorities, the media and general public," the spokesman said.

"It is common for earthquake locations and magnitudes to be reassessed over the coming hours or days as more data becomes available, as well as more thorough analysis.

"In the case of the recent Whitsunday earthquake, this more thorough analysis led to a slight change in the earthquake's location and an increase in the magnitude from 4.8 to 5.0"

The spokesman said an earthquake's magnitude was calculated based on the average magnitude determined for each seismic recording site.

The 0.2 magnitude upgrade meant the Whitsundays earthquake caused 1.6 times more ground movement than initially recorded.

The spokesman said while it was a significant change in terms of the damage potential for the earthquake, it was a relatively small change in shaking amplitude.

"An earthquake's magnitude is represented using a logarithmic scale. For example, this means the increase in ground movement from a magnitude 3 to magnitude 4 earthquake would be a factor of 10 at the earthquake's epicentre," he said.

"A magnitude 5 earthquake would result in 100 times more movement than a magnitude 3.

"Therefore, the difference in the ground shaking we experience from earthquakes increases very rapidly with the increase of earthquake magnitude.

"While (a 0.2 magnitude change) is significant in terms of the damage potential for earthquakes, it is a relatively small change in shaking amplitude when we consider the full spectrum of ground shaking possible from earthquakes over a range of magnitudes."