Illinois, perceived as geologically stable, is situated within a region susceptible to seismic activity. While large earthquakes are not a frequent occurrence, the state lies within the influence of significant seismic zones. Understanding these realities provides a clearer picture of Illinois’ natural landscape and helps residents comprehend the potential for ground movement.
The New Madrid Seismic Zone’s Influence on Illinois
The New Madrid Seismic Zone (NMSZ) represents a substantial source of seismic activity in the central United States, extending approximately 150 miles. This zone primarily spans northeastern Arkansas, southeastern Missouri, western Tennessee, and western Kentucky, with its northern reach extending into southern Illinois. It is considered the most active seismic area east of the Rocky Mountains.
This seismic zone is an example of an intraplate earthquake region, meaning the seismic activity occurs within the North American tectonic plate rather than at its boundaries. The faults within the NMSZ are not visible at the surface; they are deeply buried beneath thick layers of river sediments. These hidden faults are associated with the Reelfoot Rift, an ancient subterranean structure. The NMSZ remains active, experiencing an average of about 200 earthquakes each year, though most of these tremors are too small to be felt by people.
Geographic Scope and Historical Tremors
The effects of the New Madrid Seismic Zone extend significantly into southern Illinois. Although the primary fault lines of the NMSZ are not directly beneath Illinois, their activity profoundly impacts the state. This influence was demonstrated during the large New Madrid earthquakes of 1811-1812.
This series of events included three powerful earthquakes, with estimated magnitudes ranging from 7.0 to 8.0. These historic quakes were felt across a vast area, reaching as far as Canada, the Gulf Coast, Boston, and Washington D.C. They caused considerable landscape changes, including the formation of fissures, landslides, and areas of soil liquefaction, where saturated soil temporarily loses its strength and behaves like a liquid. Southern Illinois experienced these ground deformations, with ground changes and sand blows. Another event, the 1895 Charleston, Missouri, earthquake, with an estimated magnitude of 6.6, also caused damage and was felt across Illinois.
Beyond New Madrid: Other Illinois Faults
Beyond the influence of the New Madrid Seismic Zone, Illinois is also affected by other fault systems. The Wabash Valley Seismic Zone (WVSZ) is another seismic area impacting the state, particularly its southeastern region. This zone extends from southeastern Illinois into southwestern Indiana.
The Wabash Valley Seismic Zone has a history of seismic activity and is considered capable of producing earthquakes with magnitudes of 7.0. On April 18, 2008, a magnitude 5.4 earthquake struck near Mount Carmel, Illinois, within this zone, and was felt across 16 states. Geological studies reveal evidence of prehistoric earthquakes in the WVSZ, with some estimated to have reached magnitudes of 7.1. Additionally, other minor faults, such as the Cottage Grove Fault in the Southern Illinois Basin, were identified following events like the 1968 Illinois earthquake, which had a magnitude of 5.3.
Living with Seismic Activity: Preparedness
Given these seismic zones, Illinois faces a medium earthquake hazard, with an estimated 10% chance of experiencing damaging shaking within the next 50 years. While major events are rare, the geological conditions in the central United States mean that ground shaking from an earthquake can affect a much larger area compared to similar-sized earthquakes in more tectonically active regions like California.
Since earthquakes cannot be predicted, preparedness is a key approach for residents. The Illinois Emergency Management Agency encourages residents to prepare for a minimum of two weeks of self-sufficiency following a major event. A key safety action during an earthquake is “Drop, Cover, and Hold On”: drop to the floor, take cover under a sturdy piece of furniture, and hold on until the shaking stops. Securing household items, such as strapping water heaters and large appliances to wall studs, anchoring light fixtures, and fastening shelves, can help prevent injuries and property damage. Developing a family emergency plan and assembling an emergency kit are also important steps.
A basic kit should include:
- Water (one gallon per person per day for at least three days).
- Non-perishable food.
- A first-aid kit.
- Flashlights.
- A battery-powered radio.
- Extra batteries.
- Any necessary medications.