Additional seismic activity from Friday’s quake cannot be determined

Friday, April 18, 2008 | 6:52 p.m. CDT; updated 11:35 a.m. CDT, Sunday, July 20, 2008

COLUMBIA — Scientists say there’s no telling whether the earthquake that was felt Friday morning across a wide part of the central U.S. signals additional seismic activity.

“How likely it is for an earthquake to happen again is highly debated because of contradictory information,” said Eric Sandoval, professor of geophysics and geodynamics at MU. “The answer always has to be: ‘We don’t know.’”

The quake, which took place at 4:36 a.m., caused minor shaking in Columbia. The epicenter near Bellmont, Ill., in the Wabash Valley seismic zone measured a magnitude 5.2 on the Richter scale. An aftershock at 10:15 a.m. Friday was listed at magnitude 4.5.

The National Weather Service in St. Louis reported minor damage, according to earlier Missourian reports.

Dave Overhoff, a geo-hazards geologist for the Missouri Department of Natural Resources, characterized the Wabash Valley seismic zone as independent of the New Madrid seismic zone that caused the historic magnitude 8.0 earthquake of 1811-1812.

Gary Patterson of the Center for Earthquake Research and Information at the University of Memphis said there are numerous faults where the quake occurred, making it difficult to pinpoint which one was responsible.

Patterson said there’s no way to know whether this morning’s quakes would trigger activity along the New Madrid seismic zone, which includes parts of southeastern Missouri, Kentucky, Tennessee and Arkansas.

“It could, but we just don’t know,” he said Friday afternoon. “We haven’t seen any.”

Sandoval said he thinks it is unlikely there will be any more aftershocks.

“You don’t get many for 5.2 magnitude (earthquakes),” he said.

Michelle Dry of the Center for Earthquake Research and Information at the University of Memphis said that a magnitude 5.2 earthquake isn’t that large, even though it may be felt by many people.

Patterson said most of the quakes in the area are below a magnitude 5.0, and the last quake above that number was in 2002.

The predawn quake Friday morning was felt from Georgia to Michigan and westward into Kansas, according to state geologists.

The extent to which an earthquake may be felt depends on local conditions, said Francisco “Paco” Gomez, assistant professor of paleoseismology and neotectonics at MU.

“Where a person is standing can be affected if the energy flowing from an earthquake is amplified by local ground conditions,” he said.

If someone is standing on thick unconsolidated sediment, the earthquake or its aftershock may not be felt as much as if the person is standing on firmer ground, which may partially account for why some Columbia residents felt the early-morning tremors while others did not.

Sandoval said some data, although controversial, have shown that the earthquake early Friday was an aftershock from the 1811-1812 New Madrid earthquake that caused the Mississippi River to flow north.

The New Madrid seismic zone produces more than 200 small earthquakes each year, making it one of the country’s most active seismic zones, Overhoff said.

Other geological data suggest that large earthquakes occur in the Wabash Valley seismic zone every few thousand years.

“Whether it was triggered by earthquakes 200 years ago or not, it’s hard to tell,” Gomez said.

MU’s geology building houses a seismograph, although it is used for display purposes. However, it was on in time to measure the movement of the ground in Columbia during the aftershock.

The reverberations recorded by the seismograph can be the result of an earthquake that happens on the other side of the Earth.

“It’s like when you throw a rock in a pond and it creates ripples,” Gomez said. “The waves may wash up on many places and it may depend on the size of the rock.”

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