Wabash Temblor a First, Says Panel

By MARK WILSON, Courier & Press staff writer 464-7417 or wilsonm@courierpress.com

Tri-State residents can expect more mild earthquakes, but at least the big one last week was generated locally.

That was the word Friday from a panel of geologists from nine states gathered in Downtown Evansville to discuss topics that included a project to map the city’s susceptibility to damage from quakes.

“This is.. the first earthquake in the Wabash Valley fault system that we can blame on the Wabash Valley fault system,” said John

Nelson, of the Illinois Geological Survey

He said that the 5.2 magnitude quake on April 18 was likely produced by a known fault in the system, while other recent quakes of magnitude 5 or greater seemed to originate in areas where faults were not known to have existed.

That kind of information was why geologists at the conference said that such quakes are actually helpful in better understanding the Wabash Valley Seismic Zone, of which relatively little is known.

“The mapping is ongoing, is the point. You are finding faults as the quakes happen,” said William Andrews of the Kentucky Geological Survey.

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

Ironically, the geologists were meeting as a 3.7 magnitude temblor rattled the city shortly after noon, the latest in the series.

The epicenter of Friday’s aftershock was eight miles east of West Salem, Ill.

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

Bauer was one of the panel of earthquake experts who convened Friday to answer questions about the recent earthquakes. Panel members were more concerned with the effects of much larger quakes such as the 6 or 7 magnitude quakes known to have been produced by the New Madrid and Wabash Valley seismic zones in the past.

The panel was part of the annual meeting of the North-Central Section of the Geological Society of America at the Casino Aztar Conference Center on Thursday and Friday. Playing host was the University of Southern Indiana’s Department of Geology and Physics.

Earlier in the afternoon, Yoon Seok Choi, a geotechnology graduate student at Purdue University, provided an update on a project to map the earthquake hazards of the greater Evansville area.

Using information gathered by the U.S. Geological Survey, the university is creating a map that will show the varying levels of damage susceptibility throughout the area based on assessments of how likely the soil is to liquefy during a projected 6.5 magnitude earthquake from the Wabash Valley system or a 7.7 quake from the New Madrid system.

The shaking of earthquakes reduces the stiffness of soil. It can support less weight and foundations for buildings and bridges subside, causing damage. Much of the Evansville area is built on areas with a high risk for liquefaction, Choi said.

When they are completed later this year, the maps will help developers plan their buildings and help emergency officials plan their earthquake responses, Choi said.

Earthquakes can’t be predicted, panel members said, but the way they may affect us can be better understood and that information put to use.

The variety of soil types throughout the Tri-State also controlled how strongly people reported feeling the earthquake and why some people closer to its epicenter may not have felt it as strongly as some farther away, Bauer said.

The quakes also call attention to the need for more study of the area. The information gathered by those instruments not only gives insights into current earthquakes but provides a better understanding of past quakes, panel members said.

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