Midwest to face Bigger Quake?

Posted by Joshua S. Hill

April 23, 2008

That I live in Australia is – by now – not a mystery. What may be a surprise though is that we very rarely suffer earthquakes. Every now and again one will pop up somewhere, but they are always relatively small in size.

I tackle this story with that lack of firsthand knowledge, so bear with me.

Most will already be aware of the 5.2 magnitude earthquake that hit the Midwest of America on Friday. It was felt from Kansas to Georgia, and aftershocks have continued over the weekend, including a 4.0 magnitude quake that hit just before 12:40 am, Monday, just northwest of Mount Carmel.

However, according to geophysicists, the aftershocks could continue for months, emanating from the nation’s center, known to be a weird seismic locale.

Residents of Mount Carmel, Illinois, are not surprisingly, a little jittery. The epicenter focuses on the northern extension of the New Madrid fault line, 6 miles north of Mt. Carmel, and is the same fault responsible for the devastating quakes of 1811 and 1812 in the Mississippi Valley.

For a long time now, scientists and locals have wondered when the fault line would strike again.

“I think we saw a window to this possibility today in the Wabash Valley,” said geophysicist Allessandro Forte of the Université du Québec à Montréal, who has studied the region’s seismicity. “It’s to the north of the New Madrid seismic zone, but given the strength of crust, the stress can be distributed great distances. It’s not clear if we could see something in the next few years or even next few months, I would say.”

Measuring earthquakes is a tricky business. The last earthquake in the region close to Friday’s, measured in at 5.0 back in 2002. However back in 1968 a quake with a magnitude of 5.3 struck, and though there was only a .1 difference, this quake was felt in 23 states, including Ontario and Boston.

This most recent quake portends two different scenarios; one, that the small quake has relieved some of the geological stress and will see a cool down for the next few decades, or two…

“There is the possibility, and we can only see over next few months what will happen, that the redistribution of stress on neighboring faults might trigger further earthquakes, and we can only guess as to whether they’ll be equally large as today’s earthquake,” Forte said.

“If we are seeing a propagation outward of stress changes after today’s 5.2, which was a big one, and those stress changes finally come up on a fault which is on a hair trigger and ready to go, those small changes are sufficient to generate another big one on a fault which is locked and ready to go,” Forte added.

As for looking for a figure, a percentage, on the chances of a large quake hitting the region over the next few decades? Good luck! Expert opinions range from a 90 percent chance, to a 25 – 40% chance, to between 7 and 10%.

Needless to say, Forte mentions that “This is not exactly a well-defined science as yet.”

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