Karela Fry

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Do we live in an age of megaquakes?

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Have you also been having conversations about megaquakes recently? Has someone pointed out that 2012 is coming? It turns out that similar questions have been exercising geo-scientists. Nature reports:

Beginning in late 2004, a flurry of massive, tsunami-spawning earthquakes have rocked the world, first slamming Indonesia, then Chile and most recently Japan. Temblors that size are rare indeed: only 7 quakes as large or larger than 8.8 — the magnitude of last February’s Chilean event — have occurred since 1900.

So what does it mean that three of those seven shocks have happened almost within the span of six years?

That’s a beautiful piece of writing: putting the problem in terms that everyone recognizes in a few short opening sentences. The article goes on to report a headlines-grabbing scenario:

The recent spate of far-flung quakes is remarkably similar to a cluster that occurred in the middle of the last century, says Charles Bufe, a seismologist retired from the US Geological Survey (USGS) in Denver, Colorado. The seismic events in that supposed grouping, consisting of 3 magnitude 9 or higher temblors, struck Kamchatka, then Chile and then Alaska within a 12-year interval. The odds of quakes that large occurring randomly within such a short time span is only four per cent, Bufe noted today at the annual meeting of the Seismological Society of America in Memphis, Tennessee. … According to their model, Bufe says, the probability of another quake of magnitude 9 or larger striking in the next 6 years is about 63 per cent.

The simpler alternative, and perhaps more mainstream, view is:

[T]he apparent clustering of such megaquakes, including the recent Indonesian, Chilean and Japanese events can be accounted for without a direct link, several scientists say. “When you run statistical tests, you can often get numbers that sound interesting,” says Richard Aster, a geophysicist at the New Mexico Institute of Mining and Technology in Socorro. In this case, he suggests, the clumping could come down to the statistics of small sample sizes. Since 1900, there have been only 14 quakes larger than magnitude 8.5. And whereas modern seismology goes back only a little more than a century, the tectonic processes that generate major earthquakes unfold over hundreds or thousands of years, he adds.

More detailed studies seem to bear out the conventional view, as the report goes on to say:

Tom Parsons, a seismologist also with the USGS in Menlo Park, and colleague Aaron Velasco, of the University of Texas at El Paso, analyzed the USGS earthquake database to see if temblors of magnitude 7 and higher might have triggered midsized quakes elsewhere in the world. Between 1979 and 2009, seismometers recorded 205 quakes with magnitudes above 7, Parsons notes. Although many of those quakes triggered local aftershocks in the day or so after the initial event, Parsons and Velasco found no corresponding increase in the frequency of distant quakes with magnitudes ranging between 5 and 7.

The team’s analysis also suggests that stress redistribution to nearby faults after a major quake is limited to distances from the epicentre no more than two or three times the length ruptured by the original quake. That, says Parsons, means that even megaquakes shouldn’t trigger large quakes more than a couple of thousand kilometres away.

So, the answer to the question posed in the title is likely to be “No”.


Written by Arhopala Bazaloides

April 16, 2011 at 3:31 am

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