Earthquakes have happened in the Midwest for the past 20,000 years, and they have left signposts for seismologists to study.
The Wabash Valley Seismic Zone has produced earthquakes for the past 20,000 years, with some registering 7.5 on the Richter magnitude scale. Seismologists believe that the Wabash Valley fault dates from Precambrian times, the oldest era of the Earth's history, and that the fault has been reactivated.
Two Seismic Zones Connect
Centered in the Wabash River Valley, the Wabash Valley Seismic Zone straddles the state line between southeastern Illinois and southwestern Indiana and spreads into part of western Kentucky. Scientists believe that it is a branch of the New Madrid system, which extends south from Cairo, Illinois, through Missouri, Arkansas, Mississippi and parts of western Tennessee, according to the Arkansas Center for Earthquake Education and Technology Transfer.
Scientists Discover Another Fault
The 1968 Illinois earthquake or New Madrid event, hit Illinois on November 9, 1968, and measured 5.4 on the Richter scale. It affected 23 states over an area of 580,000 square miles, causing much structural damage to buildings but no fatalities. In researching its cause, scientists discovered the Cottage Grove Fault in the Southern Illinois Basin, which is a small tear in the Earth's rock running west to east under Saline County, near Harrisburg, Illinois. It connects to the north-south running Wabash Valley Fault System at its eastern end.
Warning Quake
On June 18, 2002, an earthquake measuring 5.0 on the Richter scale struck Evansville, Indiana. According to the Indiana University Geological Survey, the earthquake was a warning to residents of the Wabash Valley Seismic Zone that earthquakes can, and do, strike close to home. Dr. Won-Young Kim, a seismologist at the Lamont-Doherty Earth Observatory at Columbia University, investigated the probabilities of future earthquakes in the Wabash Valley Seismic Zone.
Ancient Fault Reactivates
He discovered that an ancient fault line from the Precambrian era--4.6 billion to 570 million years ago--has reactivated and probably caused the 2002 earthquake. According to Dr. Won-Young Kim, This area was once as seismically active as the Gulf of California is today. The reactivation of this fault may be due to the forces that are moving the North American Plate over the Earth's mantle. The depth of this earthquake suggests that these forces are quite large, even though they are far away from present plate boundaries.
Chicago and St. Louis Feel Earthquake
On April 18, 2008, the 102nd anniversary of the 1906 San Francisco earthquake, a magnitude 5.2 earthquake struck near the community of West Salem, Illinois. It jolted communities across southern Illinois, southern Indiana, western and central Kentucky and eastern Missouri. People in Chicago and St. Louis, 123 miles away, felt its vibrations.
Another Earthquake before 2040
Seismologists and geologists at St. Louis University predict that a future earthquake in the region is likely, forecasting a 90 percent chance of a 6.0 magnitude earthquake, or greater, before 2040. They say it will very likely originate in the Wabash Valley Seismic Zone or in the New Madrid Fault Zone.
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