Earthquake

[rew]

The earthquake made the day interesting and provided an opportunity to learn something. The New Madrid fault line extends into southern Illinois. It is the most active seismic zone east of the Rockies and the probabilityof a major seismic event (magnitude 6.0 or greater) occuring within the next thirty years is 90%.


ETA: maybe we should all be calling our insurance agents.

[d204mom]

Quake aftershock felt early Monday morning


April 21, 2008

FROM STNG WIRE REPORTS
MOUNT CARMEL -- An aftershock of 4.5 magnitude from Friday morning’s earthquake in Southern Illinois has been reported early Monday.

The aftershock, which had a preliminary reading of 4.5 on the Richter scale, was reported at 12:38 a.m. Monday, according to U.S. Geological Survey geophysicist Amy Vaughan.

“It’s right in the same area from the main shock on Friday morning,’’ Vaughan said.

It occurred about five miles northwest of Mount Carmel and about 11 miles southeast of West Salem, she said.

A police dispatcher in West Salem said the aftershock was definitely felt Monday morning, but no reports of damages or injuries had been reported as of 5:15 a.m. There was, however, a spike in 911 calls immediately after the tremor.

Monday’s aftershock was the second police had received calls on since the earthquake on Friday. The first aftershock occurred about 10:15 a.m. Friday and was recorded as a 4.5 magnitude on the Richter Scale, the dispatcher said.

The earthquake itself occurred about 4:36 a.m. Friday in West Salem and was recorded as a magnitude of 5.2 on the Richter Scale, according to the U.S. Geological Survey.

[d204mom]

Scientists: Even Bigger Quake Could Hit Midwest Robin Lloyd
LiveScience Senior Editor
LiveScience.com
Fri Apr 18, 4:32 PM ET

The magnitude 5.2 earthquake that rocked the Midwest on Friday was felt from Kansas to Georgia, and aftershocks could continue for months at this strange seismic zone at the nation's center and even trigger another big quake, a geophysicist said.

The quake occurred on a northern extension of the New Madrid fault, about 6 miles north of Mt. Carmel, Ill. The New Madrid fault was responsible for devastating quakes in the Mississippi Valley in 1811 and 1812. So the Friday quake and its aftershocks likely are raising the blood pressure of some residents and scientists.

For decades, scientists have debated whether and when the underlying fault could generate another temblor of similar and deadly strength.

I think we saw a window to this possibility today in the Wabash Valley," said geophysicist Allessandro Forte of the Universite du Quebec à Montreal, 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."

The last earthquake in the region to approach the severity of Friday's temblor was a 5.0 magnitude quake that shook a nearby area in 2002, according to the U.S. Geological Survey.

An event actually surpassing today's magnitude last occurred in 1968, a magnitude 5.3 quake that was felt in 23 states, said Forte. The magnitude scale is logarithmic so a change of 0.1 or 0.2 makes a big difference in terms of energy output. The 1968 event was felt in Ontario and Boston.

"The $64,000 question is what this earthquake portends for the future," Forte said. "The answer is I'm afraid it can go either way."

Stress relief or hair trigger?

One scenario predicts that some stress is relieved on the local faults where this earthquake occurred and will cool things down for a few decades. The other scenario is not so happy.

"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.

Aftershocks from the Friday quake will continue for several weeks, maybe months, he said. Already, there have been many, of magnitudes in the range of 2 and 3, radiating outward from the epicenter.

"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 said.

How much risk?

Recent estimates have downgraded the risk of a large earthquake on the New Madrid fault.

In the 1980s, scientists said there was a 90 percent chance of a magnitude 6 or 7 temblor occurring in this area within the next 50 years.

A 2007 USGS fact sheet, however, said there is only a 25 percent to 40 percent chance of a magnitude 6 or larger there in the next 50 years.

However, a team that includes Michael Ellis of the University of Memphis estimated in 2005 that the odds of another 8.0 event in the region within 50 years are between 7 and 10 percent.

These debates about the New Madrid fault are far from resolved, Forte said, with some saying the accumulated stress in area faults is weakening while others say it is not going to dissipate any time soon. "This is not exactly a well-defined science as yet," he said.


Forte is part of the latter camp, based on his research on an ancient, giant slab of Earth called the Farallon slab that started its descent under the West Coast 70 million years ago and now is causing mayhem and deep mantle flow 360 miles beneath the Mississippi Valley, where it effectively pulls the crust down an entire kilometer (.62 miles).

"The stresses from the sinking Farallon slab are not going to disappear any time soon," he said.

So, apparently, is J. David Rogers in the latter camp. The geological engineer at Missouri University of Science and Technology says Midwestern earthquakes are potentially more powerful than California quakes.

Shakier situation

Unique geology in the Midwest increases the shaking intensity of earthquakes because seismic energy moves through the dense bedrock at very high speeds, then becomes trapped in soft sediments filling river channels and valleys, Rogers said.

Rogers and some of his graduate students have been modeling synthetic seismic events in the New Madrid region. Most of their scenarios are modeled after an 1895 earthquake with a magnitude of 6.4 that was centered in Charleston, Mo.

The preliminary results are sobering, said Rogers. Data indicates ground shaking would be magnified about 600 percent within the flood plain of the Missouri River, a development that would cause most of Missouri's existing long-span bridges to collapse.

"You don't even need a really big earthquake to do significant damage in Missouri," Rogers says. "It could happen tomorrow."

The relative quake risk of the New Madrid seismic zone is a great debate that might be driven in part by competition for grant money, Forte said. Those scientists who work on West Coast quakes have an incentive to claim that the research money should be spent on that region, while the central continent-focused researchers obviously are more invested in funds coming their way.