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Quakes away from plate boundaries may be triggered by mantle flow

Earthquakes Earthquakes occurring away from tectonic plate boundaries can be triggered by the rise and fall of hot material through the Earth's mantle, according to a new study.

The findings reported in the journal Nature are based on surveys of an earthquake-prone region of the western United States called the intermountain seismic belt, and may also help explain intraplate earthquakes in other locations, including Australia.

"We found that the location of intraplate seismicity in the western United States appears to be predominately controlled by deep mantle convection processes, pushing and pulling from underneath the plate," says the study's lead author, Professor Thorsten Becker of the University of Southern California.

Earthquakes occur along tectonic plate boundaries as the plates move past each other, or at geologic hotspots such as Hawaii, where plates move across deep mantle plumes.

Plate movement is powered by the rise and fall of hot and cold mantle material over geologic timescales of millions of years.

"The convection is caused in the mantle by internal heat production by decay of elements such as uranium and thorium," says Becker.

"It's sort of like looking at a boiling pot of honey, with the mantle convecting in this sluggish fashion."

However, what causes earthquakes in the middle of tectonic plates away from boundaries and plumes, is poorly understood.

In the western United States, intraplate earthquake activity is concentrated in a region known as the intermountain belt that stretches from south-western Utah up through Idaho and Wyoming to Yellowstone National Park and north along the Snake River through western Montana and into Canada.

Becker and colleagues used new detailed seismic, GPS and gravity data provided by the Earthscope array, a network of seismic monitors across the western United States.

"The Earthscope experiment has allowed my collaborators and others to develop really detailed models of the Earth," says Becker.

The authors developed a computer model showing how the flow of deep mantle material, 100 to 160 kilometres below the surface, pulls and pushes on the overlying crust, triggering seismic activity.

However Becker says this work won't be the last word on the origin of intraplate earthquakes.

Other contributing factors include the history of a region, such as changes in the thickness of the crust, and in the strength of the lithosphere, which comprises the planet's crust and upper mantle, as well as variations in the gravitational potential energy of a region.

"Gravitational potential energy is the tendency of thick crust to flow apart over long time scales, it's bit like a ball of silly putty left sitting on a table, which after a while will slowly flatten out into a pancake," says Becker.

The authors found preliminary studies in the Mediterranean-Middle East region also showed seismic activity may be influenced by deep mantle flow.

And according to Becker, the mantle flow model could also apply to other continental plates including the Australian plate, because small scale convection underneath Australia may still cause stressing.

"The difference in the geological setting is that we're talking about a very old continental plate in the case of Australia, where the western United States are tectonically much more active," says Becker.

As the Australian plate is very old it is much thicker than the western part of the North American plate, so it is possible that weaknesses caused by changes in the continental crust may be also be a factor, he says.

"I am not sure if the mantle models we have for Australia are at the level we'd need them to be for our analysis. Initial tests elsewhere (we're looking at the Mediterranean right now) are promising, but we haven't look at Australia closely yet."