Indiana City Getting New Earthquake Risk Maps

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Government officials and Evansville residents soon will have access to a new set of maps detailing areas at greatest risk of damage from strong earthquakes that periodically shake southwestern Indiana.

The maps will be unveiled at the Southern Indiana Career & Technology Center on Feb. 7, the same day Indiana and seven other states take part in an earthquake preparedness drill called the Great Central U.S. ShakeOut.

The Evansville area is at risk from quakes originating in both the New Madrid seismic zone that’s centered in Missouri and the Wabash Valley seismic zone that straddles the Illinois and Indiana state line.

Oliver Boyd, a geophysicist with the U.S. Geological Survey, told The Evansville Courier & Press the maps were developed based on two scenarios: a magnitude 7.7 earthquake from the New Madrid fault and a magnitude 6.8 earthquake from the Wabash Valley fault.

Those magnitudes were chosen based on historical data of earthquakes in the two seismic zones.

The seven-state New Madrid seismic zone generated a series of quakes as high as magnitude 7.7 in the winter of 1811-1812, causing geologic upheavals that stunned pioneer communities, including the sight of the Mississippi River temporarily running backward.

The New Madrid zone gets much of the public attention because of those quakes, but Boyd said the potential for liquefaction — when an earthquake makes the ground unstable, inflicting damage on buildings and bridges — is actually less in the Evansville area than the threat posed by the Wabash Valley seismic zone.

He said that’s because while there is a greater chance of a quake from the New Madrid zone, Evansville is at its northern reach, and the Wabash Valley zone is far closer to the city.

Boyd said there is about a 10 percent chance of a New Madrid quake in the next 10 years. And while earthquakes appear to only happen about every 12,000 years in the Wabash Valley zone, he said the damage from such a quake would be greater in Evansville because it is closer to that fault.

There is geologic evidence of Wabash Valley quakes occurring less than 25 miles northwest of Evansville, while the last major New Madrid temblor occurred 110 miles southwest of Evansville.

The mapping project was funded by the U.S. Geological Survey’s National Earthquake Hazards Reduction Program. It was developed with help from sources including Purdue University and The Center for Earthquake Research and Information at the University of Memphis, as well as the U.S., Indiana and Kentucky geological surveys.

The maps can be used by businesses, city planners, utilities and emergency managers to better prepare for future earthquakes. They also may interest property owners and the general public, said Christine Martin, director of the Southwest Indiana Disaster Resistant Community Corp.