Eidsvold quake: so what really caused the earth to move?

SO what really made Queensland shake two days after Valentine's Day and confirmation of a change of government?

Some have blamed the collective stamping of the feet of bruised LNP supporters.

Others suggest coal seam gas mining is the culprit.

No doubt, it won't be long before vaccination and gay marriage are thrown into the mix.

Tthe township of Eidsvold was at the epicentre of the quake that would shake homes from the Sunshine Coast to Rockhampton.

The 5.2 earthquake was not the biggest in Queensland's history.

But it was undoubtedly the most talked about - given the proliferation of social media in 2015.

Queensland's largest known earthquake was in 1918 near Bundaberg.

This magnitude 6.0 earthquake at 4.15am local time on 7 June 1918.

The epicentre was probably about 100km off the coast between Rockhampton and Gladstone, according to Geoscience Australia's website.

The earthquake was felt from Mackay to Grafton (NSW) and west to Charleville.

It caused some damage in Rockhampton and Bundaberg, and stopped many clocks, including the one in the Pile Light in Brisbane, where it was felt in most suburbs.

Bedside lamps lit up as fast as social media this morning as Bundaberg residents across the region woke to an earthquake.
Bedside lamps lit up as fast as social media this morning as Bundaberg residents across the region woke to an earthquake. Geoscience Australia

So what really causes earthquakes in Australia?

Earthquakes are the vibrations caused by rocks breaking under stress. The underground surface along which the rock breaks and moves is called a fault plane.

Earthquakes in Australia are usually caused by movements along faults as a result of compression in the Earth's crust.

The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake.

These are put into a formula which converts them to a magnitude, which is a measure of the energy released by the earthquake.

For every unit increase in magnitude, there is roughly a thirty-fold increase in the energy released.

A magnitude 6.0 earthquake releases approximately 30 times more energy than a magnitude 5.0 earthquake, while a magnitude 7.0 earthquake releases approximately 900 times (30x30) more energy than a magnitude 5.0.

A magnitude 8.6 earthquake releases energy equivalent to about 10 000 atomic bombs of the type developed in World War II.

Fortunately, smaller earthquakes occur much more frequently than large ones and most cause little or no damage.

SHAKE IT OFF: A 5.2 magnitude earthquake near Fraser Island has been felt in the South Burnett.
SHAKE IT OFF: A 5.2 magnitude earthquake near Fraser Island has been felt in the South Burnett.

How earthquakes are measured

Earthquake magnitude was traditionally measured on the Richter scale.

It is often now calculated from seismic moment, which is proportional to the fault area multiplied by the average displacement on the fault.

The focus of an earthquake is the point where it originated within the Earth. The earthquake epicentre is the point on the Earth's surface directly above the focus.

Earthquake in Wellington, New Zealand.
Earthquake in Wellington, New Zealand. Mark Mitchell

The effects of an earthquake

The amplitude of the shaking caused by an earthquake depends on many factors, such as the magnitude, distance from the epicentre, depth of focus, topography, and the local ground conditions.

Earthquake effects, as noted by people, are rated using the Modified Mercalli (MM) intensity scale, which ranges from I (imperceptible) up to XII (total destruction).

If the people or buildings are on soft ground such as old river sediments, the MM intensity experienced may be one to two units higher; if on solid rock, it may be one unit lower. The intensity with which the earthquake is felt may also be higher on hilltops.

In Australia, the smallest known earthquake to cause fatalities was the 5.6 Newcastle earthquake in 1989.

However, magnitude 4.0 earthquakes occasionally topple chimneys or result in other damage which could potentially cause injuries or fatalities.

Earthquakes of magnitude 4.0 or greater may also trigger landslides which can cause casualties.

In areas underlain by water-saturated sediments, large earthquakes, usually magnitude 6.0 or greater, may cause liquefaction.

The shaking causes the wet sediment to become quicksand and flow. Subsidence from this may cause buildings to topple, and the sediment may erupt at the surface from craters and fountains.

Undersea earthquakes can cause a tsunami, or a series of waves which can cross an ocean and cause extensive damage to coastal regions.

The destruction from strong earthquake shaking can be worsened by fires caused by downed power lines and ruptured gas mains.

Interesting fact: Earthquake vibrations travel very fast, up to 14 kilometres per second. The fastest seismic waves take less than 20 minutes to reach the other side of the earth, a distance of almost 13 000 kilometres!

Source: Geoscience Australia



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