(That was an earthquake.) Downstate tremor a reminder of seismic hot zone
A massive rock formation 7 miles below West Salem, Ill., moved just an inch or so at 4:36 a.m. Friday, but that was enough to cause the 5.2 magnitude earthquake that broke windows and cracked walls near the epicenter, shook Chicagoans in their beds and sent shock waves that people felt as far away as Kansas and West Virginia.
Neither the main quake nor the half-dozen aftershocks caused any major injuries or property damage. But the rattling was the worst to hit Illinois since 1968, when a 5.3 quake caused more damage to buildings but no injuries.
The quake is a sharp reminder of the hidden hot zone beneath Illinois, which in some ways rivals the New Madrid fault zone that caused massive quakes in Missouri early in the 19th Century.
It's still modest compared with quake-riddled California, but the Midwestern event stood out in how far away its rumbling was felt. Experts said the deep rocks in this part of the country are unusually solid and cool, allowing them to conduct shock waves up to 10 times farther away than is possible with the jumbled, warm rocks out West.
"You don't feel a magnitude 5 quake in California in nearly as big a region as you do in Illinois," said Stuart Sipkin, a geophysicist with the U.S. Geological Survey.
The shaking was light enough in Chicago that some people thought it was just a bad dream. But in Downstate Mt. Carmel, the earthquake jarred awake 20-year-old Whitney Schulte and knocked her alarm clock off her nightstand.
"It felt like there was thunder, how thunder just rattles your house, but it just kept going for about 35 or 40 seconds," said Schulte, who was at her parents' house. "We got up and thought, 'What was that?' "
Comparable quakes hit the fault system in the Wabash Valley seismic zone about once every 15 or 20 years, experts believe. Compared with California, where most faults are close to the surface and bear clear marks of past activity, the fault systems in this region are difficult to study, making predictions much more fuzzy.
This is an older hunk of geology than the western U.S., and the faults here are probably the remnants of scars in the Earth that may have come from the ancient splitting of continental plates or mountain-building processes.
Just as the scar on a broken bone is more vulnerable to breaking later on, the faults in this region bear the brunt of the ordinary stress caused by the movement of continental plates on the earth's surface.
"As the whole plate moves, there are random, slow motions within it which are tiny compared to the movement of the plate as a whole," said Seth Stein, a professor of geologic sciences at Northwestern University.
The biggest aftershock on Friday measured 4.6, most likely caused by excess strain that the original, bigger quake did not relieve, Stein said.
Some geologists describe the Wabash Valley zone in southern Illinois and Indiana as separate from the New Madrid formation, while others say it's part of the same system. What's clear is that the Illinois and Indiana faults can pack as much energy as the better-known New Madrid, which is centered in Tennessee, Missouri, Arkansas, Kentucky and the southern tip of Illinois.
The worst quakes ever to hit the Wabash Valley region may have matched the huge New Madrid events of 1811 and 1812, which had an estimated magnitude of up to 8 on the Richter scale. Those quakes made church bells ring in Boston and caused damage as far away as South Carolina and Washington, D.C., historians believe.
A similarly violent quake hit the Wabash River area thousands of years ago.
That quake would have registered as at least a magnitude 7, experts believe — slightly stronger than the Northridge, Calif., quake of 1994 that killed 57 people.
"You could probably expect a magnitude 7 quake in this region every 1,500 years or so," said Stein.
Quakes from the New Madrid system are more frequent than in the Wabash Valley, but even New Madrid quakes have rarely reached a magnitude of 5 in recent history. (On the Richter scale, a magnitude 5 quake is 32 times more intense than a magnitude 4.) The last major New Madrid event occurred in 1895.
Friday's quake was a novelty in Chicago as it swayed buildings, startled pets and bemused insomniacs and light sleepers. Thoughts then quickly turned to possible damage in a city designed for tornadoes rather than temblors.
But the quake seemed to have left no mark on the skyline. Heavier, lower brick buildings with un-reinforced skeletons might have been vulnerable, but would have shown signs of damage quickly through cracks and crumbling walls. No such damage was immediately discovered, city inspectors said.
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