Aftershocks of Midwest Earthquake Expected for Weeks to Come

Geologists who gathered a week after a 5.2-magnitude earthquake shook the Midwest said they expect mild aftershocks to rattle the region for several weeks.

The latest aftershock, a 3.7 magnitude temblor, was felt in Evansville on Friday, April 25, 2008, as geologists from nine states met in the southwestern Indiana city to discuss the recent quakes.

“We will probably be seeing these for weeks,” said Bob Bauer of the Illinois State Geological Survey.

The U.S. Geological Survey says 26 aftershocks have been centered in Illinois since a 5.2-magnitude quake centered in an area below West Salem, Ill., caused minor damage.

Two of the aftershocks have measured at least a 4.0 magnitude, but most have been about 2.0.

The epicenter of Friday’s aftershock was below Bellmont, Ill., about 35 miles from Evansville. There were no reports of damage or injuries, but some Evansville residents felt it.

The geologists said such quakes may unsettle the public but they help geologists better understand the Wabash Valley Seismic Zone, of which relatively little is known.

Since the April 18 quake, geologists have installed additional seismic recording devices in the area to track the aftershocks. The data gathered by those instruments not only shed light on current earthquakes but provide a better understanding of past quakes, panel members said.

“Anytime we have a moderate event, a smaller event, we learn things from an engineering perspective,” said Ed Woolery, a professor of geophysics at the University of Kentucky.

William Andrews of the Kentucky Geological Survey said he and other geologists “are finding faults as the quakes happen.”

The panel was part of the annual meeting of the North-Central Section of the Geological Society of America on Thursday and Friday.

Geologists at Friday’s meeting said they are worried about the damage much larger quakes might have on the region, such as the magnitude 6 or 7 quakes the New Madrid and Wabash Valley seismic zones are known to have produced in the past.

John Nelson of the Illinois Geological Survey said the April 18 earthquake was likely produced by a known fault in the Wabash Valley Seismic Zone system.

Yoon Seok Choi, a geotechnology graduate student at Purdue University, gave the panel an update Friday on a project to map the earthquake hazards of the greater Evansville area.

Using information gathered by the U.S. Geological Survey, Purdue is creating a map that will show the varying levels of damage susceptibility throughout the region.

That map is based on assessments of how likely the soil is to liquefy or become unstable and less able to support bridges or buildings during a projected 6.5 magnitude earthquake from the Wabash Valley system, or a 7.7 quake from the New Madrid system.

Choi said the maps will help developers plan their buildings and help emergency officials plan their earthquake responses.

For more information on the latest U.S. earthquakes:
http://earthquake.usgs.gov/eqcenter/recenteqsus/