A composite image showing the location of recent tsunami warnings in the Pacific Ocean. The orange star marked '1' represents the epicentre of this morning's earthquake.

Credit: NOAA

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SYDNEY: An earthquake of magnitude 8.1 rocked the Solomon Islands this morning, threatening tsunamis in Australia and the South Pacific.

The Australian Bureau of Meteorology issued a tsunami warning for the east coast of Australia from Cooktown, in north Queensland, to Hobart, in Tasmania.

"This is the first such warning issued from the [Australia] Tsunami Warning System," said earth scientist Dion Weatherley of the University of Queensland in Brisbane.

However, "there is no cause for immediate alarm," she said. "An earthquake of this size is capable of generating a tsunami of up to two metres, or not generating a tsunami at all."

Media reports suggest that the earthquake has already created a several-metre-high tsunami in the western part of Solomon Islands, possibly washing away as many as 20 homes in the town of Gizo.

The Bureau of Meteorology tsunami warnings prompted Surf Life Saving Australia to close all beaches in Queensland and New South Wales, and Sydney's Manly and eastern suburb ferries have been suspended as a precaution.

Tsunamis, from the Japanese words meaning harbour wave, are among the most destructive forces in nature. While tsunamis can be caused by volcanic eruptions, meteorite impacts, and undersea landslides, most are caused by undersea earthquakes.

The infamous magnitude 9.0 earthquake off the coast of the Indonesian island of Sumatra on Boxing Day, 2004 caused a tsunami that wrought destruction across Southeast Asia and left more than 225,000 people dead.

Like the decibel, the unit used to describe the intensity of sound, the Richter Scale used to describe the size of earthquakes is an exponential, rather than a linear scale.

This means that, while this morning's Solomon Island quake measured 8.1 and the Sumatra quake measured 9.0 - "it is [actually] 30 times smaller than the Sumatra earthquake," said Weatherley.

The Earth's surface is made up of tectonic plates of varying densities. Relatively light regions of these plates float higher on the Earth's liquid mantle and form continents. Heavier, denser plates float lower, and make up the seafloor. When two plates of differing densities collide, often the heavier one slides below, or 'subducts', beneath the lighter one.

The quake responsible for the 2004 Boxing Day disaster was caused when a 1,200 km stretch of the plate that makes up the floor of the Indian Ocean dragged against the Burma plate as it slipped under it, 250 km off the coast of Sumatra. The drag of the Indian plate caused the Burma plate to flex upward by several metres and then snap back. The flexing released energy equivalent to about 480 million tonnes of TNT, and sent powerful tremors through the water column, travelling outward at 800 km/sec.

Not all undersea quakes produce this up and down movement; some produce side-to-side slips in plates that do not displace the water column. Because no water is displaced, these quakes, even if they measure the same intensity, do not produce tsunamis.

Tsunamis remain small while in the open ocean; the 2004 Boxing Day was only 50 cm high until it entered shallow water. But when tsunami waves near the coast and enter shallow water, they rear up and can cause breakers up to 30 m in height or more, according to Geoscience Australia, the national agency for geoscience research headquartered in Canberra.

This morning's quake hit the Solomon Islands at 6.40 am Australia Eastern Time, and was centred 10 km beneath the surface about 350 km west-northwest of the capital Honiara.

More information:

Tsunami, Wikipedia