No link between large quakes: study

Monday, 28 March 2011 Carl Holm
ABC

The researchers found no link between major earthquakes such as the recent quakes in Japan, Christchurch and Chile (Source: Martin Bernetti/AFP)

Remote control Large earthquakes usually trigger a series of severe local aftershocks but they don't cause major quakes further away, according to a new study.

The finding, in this week's online edition of Nature Geoscience, come an analysis of recent earthquakes greater than magnitude 7.

Dr Tom Parsons of the United States Geological Survey, and co-author Aaron Velasco of the University of Texas, assembled the 30-year catalogue and then measured relative timing to establish if they could have triggered other major quakes.

They found on a fault line close to the epicentre of an earthquake, the risk of strong subsequent shaking rises after the event, typically referred to as aftershocks.

But beyond a radius two to three times the length of its rupture, only small follow-up earthquakes seem to be triggered.

"There's talk of linkages between the Christchurch earthquake and Japan and Chile and I think it's very tempting to say 'well this happened and this must be connected'," Parsons says.

"If you just look at a handful of things it's very easy to draw lines between events but it really requires looking at many different main shocks and a lot of different aftershocks before you can come up with a consistent answer. That's what we've tried to do here."

Bouncing shock waves

"When you have a big earthquake like the [recent] one in Japan," Parsons says, "it sends out seismic waves, some of which get trapped in the Earth's crust."

"These waves bounce around in the upper 20 or 30 kilometers and they travel at about 4 to 5 kilometres a second," he says.

"At long distances away from the mainshock, they are the highest amplitude waves seen on a seismogram, and they are primarily responsible for triggering other earthquakes."

Parsons says this study stems from work published in Nature Geoscience in 2008, which identified triggered earthquakes around the world. That study indicated that deformation of the Earth's crust as a result of surface seismic waves following major earthquakes commonly triggered other quakes.

"We looked at the seismic arrivals at different stations from a dozen large earthquakes and we saw all these little earthquakes that were associated with them," Parsons says.

"When we put it all together, we found that there was a surprisingly large number of earthquakes triggered all over the world in all kinds of places, including Australia. It became clear to us that the whole planet is basically an aftershock zone after big earthquakes."

Parsons says that in many cases it's difficult to know much about these triggered quakes, as often they are picked up on a lone seismic trace.

But he says they were usually small, less than magnitude 3.

"The question then became 'If that can happen, should we be worried about the overall hazard of large earthquakes arising after every big earthquake?'"

Surprising findings

That prompted Parsons and Velasco to look back at the earthquakes greater than magnitude 7. They found around 200 quakes and checked seismic records for possible relationships with other 'large' quakes, of magnitude 5 or more.

"Events greater than [magnitude 5] can be heard anywhere on the planet because they're picked up on all the global stations," Parsons says. "So we have a complete record of everything that's happened in the last thirty years. We looked for associations but didn't find them."

"We were surprised to find out that that doesn't seem to happen," he says. "So at this point we're working on trying to understand the physics of it."

Source

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