few notes on the earthquake that shook my sister awake the other night.The U.S. Geological Service page on the quake. Northern Illinois earthquake: ‘This is a normal, natural phenomenon’
By Jeff Kolkey
Posted Feb 10, 2010 @ 10:33 PM
Last update Feb 11, 2010 @ 06:19 PM
ROCKFORD — An unmapped fault 8.6 miles beneath the surface of the Earth shifted just enough to give northern Illinois a 4 a.m. wake-up call Wednesday.
The seismic activity measured a magnitude 3.8, a minor tremor by earthquake standards. It was enough to rattle the dishes and shake up houses, but not enough to cause significant damage.
In contrast, the 7.0-magnitude quake that devastated Haiti was about 30,000 times stronger, Northern Illinois University geologist Philip Carpenter said.
It was another of the minor earthquakes that occur sporadically in northern Illinois as a result of geologic pressure.
“If this holds to pattern, it was an isolated earthquake and we won’t see any activity in that area again in our lifetimes,” Carpenter said.
“And in another place in northern Illinois in about six years, we’ll see another small quake.”Quake is ‘normal’
Earthquakes are relatively infrequent in northern Illinois, but that doesn’t mean Wednesday’s quake was unusual, Rock Valley College geology professor Tom Guensburg said.
The Earth’s crust is in constant motion, putting pressure on local bedrock even in stable parts of the continent. Earthquakes occur everywhere, Guensburg said, and there is no indication that a severe earthquake would follow a minor one.
“It’s not mystical or anything,” Guensburg said. “This is a normal, natural phenomenon for this planet.”How they happen
The surface of the Earth, called the crust, is made up of tectonic plates that resemble gigantic puzzle pieces. These pieces move — usually imperceptibly — on the top layer of the mantle, which is below the crust.
Earthquakes occur most often at the plate boundary, where large and well-known fractures, called faults, rub against each other. As these blocks of rock slip past one another beneath the Earth’s surface, shock waves can cause tremendous damage.
Northern Illinois is far from any plate boundary and free of significant active faults — just the opposite of southern Illinois. “They have active faults, and the potential for destructive earthquakes” is much greater, Carpenter said.
But seismic activity at the plate boundaries places pressure on the crust even in this area.
That pressure can create fractures in the Earth’s crust; many of them aren’t mapped and remain unknown. Experts put the epicenter of Wednesday’s quake six miles west of Elgin. Until now, no fault was known to exist there.
But the fact that the earthquake occurred means there is one, scientists said.
Another theory is also at play: Northern Illinois was buried beneath a glacier 18,000 years ago, so the bedrock continues to rebound.Unmapped faults
Definitive answers about Illinois geology will remain a mystery unless the granite deep below the surface is mapped. Because earthquakes in northern Illinois are rare and typically minor, few resources have been dedicated to mapping that geology.
Data collected at seismograph stations in the region suggest that the quake was caused by a strike-slip fault — a type of fracture in blocks of rock beneath the Earth’s surface that slide past each other horizontally.
“We know the cause is regional stresses and strains,” said Julie Dutton, a geophysicist with the U.S. Geological Survey. “There are a lot of faults that aren’t mapped or understood. It’s probably not something that we will have a definitive answer on.”No aftershock expected
Carpenter is seeking funding to conduct studies that could map previously unknown faults in northern Illinois.
Large quakes are often preceded and followed by shocks, but minor quakes typically have no aftershock — at least none that is easily perceived.
“With the small quakes, there is a small amount of slip, and one section of the fault fractures, compared with Haiti, which is a massive fracture,” Carpenter said. In Haiti, the fault line “doesn’t fracture all at once, and then the rest of the fault starts popping and snapping and causes the aftershocks. Small ones have one event and that’s it and it’s over with.”Quake felt for miles
Vibrations from Wednesday’s quake were felt for miles around the epicenter. Weak vibrations were detected northwest of Beloit, Wis., more than 50 miles from the epicenter, according to a “shake map” from the U.S. Geological Survey.
Guensburg, the Rock Valley College professor, said that’s because the quake was a relatively shallow 8.6 miles deep. And it occurred in 1.5 billion-year-old granite that’s “ancient beyond comprehension.”
“It was a small earthquake, but it was felt for miles around because the rocks involved are brittle and they transmit seismic energy efficiently. And the fact is, it wasn’t very deep, so the energy where it moved was relatively close to the surface.”
Reach staff writer Jeff Kolkey at email@example.com or 815-987-1374.