Tremors from a moderate earthquake centered in downstate Illinois rattled people awake across the Chicago area Friday morning, but there were no immediate reports of injuries or significant damage, authorities said.

The earthquake, which measured preliminarily 5.4 on the Richter Scale, struck at 4:36 a.m. near the town of West Salem, Ill., about 60 miles northwest of Evansville, Ind., according to the U.S. Geological Survey.

If the strength is confirmed, it would tie the 1968 earthquake as the strongest to ever be recorded in the region.

The tremors could be felt in downtown Chicago at the Tribune Tower, 435 N. Michigan Ave., for about 20 seconds beginning around 4:40 a.m. Numerous people from across the Chicago area reported being awakened by rattling windows and shifting furniture.

Illinois State Police were investigating whether the earthquake was responsible for leaving a "metal rod" sticking up out of the pavement on the Edens Expressway near Dempster Street. At least seven cars were disabled after striking the rod this morning, a state trooper said. Crews had shut down the far left southbound lane of the expressway indefinitely.

"We don't know what caused it yet," the trooper said. "Right now at least six or seven cars have been disabled, mostly flat tires."

Chicago Fire Department Chief Joe Roccasalva said the tremors caused a "huge spike in calls" to the city's 911 center, but fire crews had not responded to any emergencies related to the incident. "It's mostly people calling to say things were moving on their shelves, that the Earth was moving," he said.

The tremors were also felt at Police Headquarters, 3510 S. Michigan Ave., Officer David Banks said. He said police had not received any calls requesting service in connection with the tremors.

Authorities at the National Weather Service in Romeoville began fielding numerous calls from across the Chicago region about the ground shaking.

"We're getting calls from as far away as Valparaiso," meteorologist Bill Wilson said.

The quake shook tall buildings in the Loop, 240 miles north of the epicenter, and in downtown Indianapolis, about 160 miles northeast of the epicenter, the Associated Press reported.

Residents in Cincinnati and St. Louis also reported feeling the earth shake.

"It shook our house where it woke me up," said David Behm of Philo, 10 miles south of Champaign. "Windows were rattling, and you could hear it. The house was shaking inches. For people in central Illinois, this is a big deal. It's not like California."

In Mount Carmel, 15 southeast of the epicenter, a woman was trapped in her home by a collapsed porch but was quickly freed and wasn't hurt, said Mickie Smith, a dispatcher at the police department.

The department took numerous other calls, though none reported anything more serious than objects knocked off walls and out of shelves, she said.

According to the U.S.G.S., Friday's earthquake occurred in the Ozark dome region, a seismically active New Madrid fault that stretches from Indianapolis and St. Louis to Memphis.

"Moderately frequent earthquakes occur at irregular intervals throughout the region," the agency said on its Web site. "The largest historical earthquake in the region (magnitude 5.4) damaged southern Illinois in 1968."

Damaging earthquakes strike about once every decade or two, and smaller earthquakes are felt about once or twice a year, the U.S.G.S. said.

In a 2005 Tribune article, earthquake experts reported an increase in seismic activity in the New Madrid region, and some said it was a harbinger for a larger quake to come.

The article noted a study published in the journal Nature from the University of Memphis showing minute shifts in recent years in the Midwest terrain. It's a possible indicator of underground pressure buildup that triggers earthquakes, researchers said.

Using history and emerging data, some geologists estimated there is a 25 to 40 percent chance of a magnitude 6.0 earthquake striking in the next 50 years--enough to do significant damage from St. Louis to Memphis, according to the article.

Until recently, the history of New Madrid earthquakes has been shrouded in legend, making any risk assessment questionable.

A series of huge quakes hit the region from 1811 to 1812--though exactly how large is uncertain because the area was sparsely settled. Some accounts described a cataclysm so intense it caused the Mississippi River to flow backward. Although the zone has regularly produced small- to medium-size quakes since then, geologists had little basis for saying when the next devastating quake--of magnitude 7.5 or larger--might happen.

But since the early 1990s, researchers have found increasing evidence in the geological record that the 19th Century quakes were part of a larger pattern stretching into prehistory.

Teams of researchers began studying ancient features in the region called sandblasts--eruptions of sand and water that happen when an intense earthquake liquefies underground sediment. Many researchers had assumed that the largest sandblasts, which can be hundreds of feet across, stemmed from the 1811-1812 quakes.

(Chicago Tribune, 18/04/2008)