The Wabash Valley Seismic Zone ranks as one of the most significant earthquake hazard areas in the central United States east of the Rocky Mountains. While less famous than the New Madrid Seismic Zone to the south, the Wabash Valley has produced damaging earthquakes in recorded history and is capable of generating major earthquakes that would affect millions of people across the Midwest.
For the science behind how faults produce earthquakes, see what causes earthquakes. For information on the nearby New Madrid zone, see New Madrid Seismic Zone.
Geography: Location and Extent
Zone Boundaries
The Wabash Valley Seismic Zone encompasses an area of approximately 10,000 square kilometers in southeastern Illinois and southwestern Indiana, centered on:
- Wabash River valley — from roughly Mount Carmel, Illinois, to Vincennes, Indiana
- Southeastern Illinois — including White, Wabash, Edwards, and Wayne counties
- Southwestern Indiana — including Posey, Gibson, Knox, and Sullivan counties
The zone is bounded by:
- North: Approximately 39.5°N latitude (near Terre Haute, Indiana)
- South: The transition to the New Madrid Seismic Zone (around 38°N)
- East: Western Indiana beyond the Wabash River influence
- West: Central Illinois beyond the zone of concentrated seismicity
Nearby Population Centers
| City | State | Population | Distance to Zone |
|---|---|---|---|
| Evansville | Indiana | ~118,000 | Within zone |
| Terre Haute | Indiana | ~58,000 | ~60 km |
| Vincennes | Indiana | ~16,000 | Within zone |
| Mount Vernon | Indiana | ~6,000 | Within zone |
| Mount Carmel | Illinois | ~6,500 | Within zone |
| Olney | Illinois | ~8,000 | ~30 km |
| Champaign | Illinois | ~88,000 | ~140 km |
| Indianapolis | Indiana | ~887,000 | ~180 km |
Geology: Tectonic Setting
Ancient Rift System
The Wabash Valley Seismic Zone lies within the Reelfoot Rift, a failed continental rift that formed approximately 750–530 million years ago during the breakup of the supercontinent Rodinia. The rift is a zone of weakened, faulted crust that extends from northeastern Arkansas through the New Madrid region and into southern Illinois and Indiana.
Although the rift stopped spreading hundreds of millions of years ago, the weakened and faulted crust remains seismically active. Compressional stresses transmitted through the North American Plate from distant plate boundaries reactivate these ancient faults, producing earthquakes.
Fault Systems
The Wabash Valley Seismic Zone contains several fault systems:
| Fault System | Length | Orientation | Activity |
|---|---|---|---|
| Wabash Valley Fault System | ~150 km | NE-SW | Active, generates most recorded earthquakes |
| New Harmony Fault | ~50 km | N-S | Active, associated with 2008 earthquake |
| Cottage Grove Fault System | ~120 km | E-W | Lower activity, may contribute to regional hazard |
| Rough Creek Graben | ~200 km | E-W | Ancient rift structure, uncertain activity |
Most of the seismicity concentrates along northeast-southwest trending faults that follow the structural grain of the underlying rift system.
Earthquake History
Paleoseismic Evidence
The most significant evidence for large earthquakes in the Wabash Valley comes from paleoliquefaction studies — investigations of ancient sand blows and soil deformation caused by liquefaction during past earthquakes. These studies have revealed:
- ~6,100 BCE event: Evidence for a M6.5–7.5 earthquake based on widespread liquefaction features
- ~2,000 BCE event: Evidence for another M6.5–7.0 earthquake
- ~480 BCE event: Possible additional large earthquake
- Total: At least 3–5 significant earthquakes (M6+) in the past 10,000 years
This paleoseismic record indicates an approximate recurrence interval of 2,000–4,000 years for major earthquakes in the Wabash Valley.
Historical Earthquakes
The Wabash Valley has produced numerous felt earthquakes since European settlement of the region:
| Date | Location | Magnitude | Effects |
|---|---|---|---|
| September 27, 1909 | Near Olney, IL | M5.1 | Felt in 12 states; chimneys damaged |
| November 9, 1968 | Southern Illinois | M5.5 | Felt in 23 states; minor damage |
| April 18, 2008 | Mt. Carmel, IL | M5.4 | Felt in 28 states; minor damage |
| June 18, 2002 | Evansville, IN | M5.0 | Felt regionally; no significant damage |
| April 18, 2008 (aftershock) | Mt. Carmel, IL | M4.6 | Followed main shock by ~5 hours |
The 2008 Mt. Carmel Earthquake
The April 18, 2008, M5.4 earthquake near Mt. Carmel, Illinois, was the most significant earthquake in the Wabash Valley in over 40 years. Key facts:
- Time: 4:36 a.m. local time (CDT)
- Depth: Approximately 12 km (7.5 miles)
- Felt area: 28 states, from Georgia to Wisconsin
- Intensity: MMI VI (strong shaking) in the epicentral area
- Damage: Minor — cracked masonry, fallen objects, damaged chimneys
- Aftershocks: Numerous, including M4.6 on the same day
The earthquake caused no fatalities or serious injuries but reminded residents of the region's seismic hazard. Scientists at the University of Illinois and Indiana University used the earthquake to improve understanding of regional fault systems.
Seismic Hazard
Earthquake Probability
The USGS National Seismic Hazard Maps include the Wabash Valley as a significant seismic source zone. Key hazard parameters:
- Background seismicity: Approximately 50–100 small earthquakes (M<3) per year
- Moderate earthquakes (M5–6): Approximately once every 30–50 years on average
- Large earthquakes (M6.5+): Recurrence interval of 2,000–4,000 years based on paleoseismic evidence
- Time since last large event: Approximately 4,000 years (conservative) to 2,500 years (possible 480 BCE event)
Shaking Hazard
The central United States presents special challenges for earthquake hazard:
- Low attenuation: Seismic waves travel farther through the stable continental crust before losing energy, affecting larger areas than similar earthquakes in California
- Unfamiliar hazard: Many residents and builders are not accustomed to earthquake considerations
- Older buildings: Much of the building stock predates modern seismic codes
A M6.5 earthquake in the Wabash Valley could produce:
- Severe shaking (MMI VII–VIII): Within 50 km of the epicenter
- Strong shaking (MMI VI): Throughout southern Illinois and Indiana
- Moderate shaking (MMI IV–V): Felt from St. Louis to Indianapolis to Louisville
Estimated Impacts
A hypothetical M7.0 earthquake in the Wabash Valley could cause:
| Impact | Estimate |
|---|---|
| Affected population | 3+ million (within 100 km) |
| Buildings damaged | Thousands of structures |
| Bridge closures | Numerous Ohio and Wabash River crossings |
| Economic losses | Billions of dollars |
| Liquefaction | Widespread in river valley alluvium |
| Power outages | Extended, affecting multiple states |
Monitoring
Seismic Networks
The Wabash Valley is monitored by multiple seismic networks:
- Central and Eastern U.S. Seismic Network (CEUSN): Part of the USGS Advanced National Seismic System
- University of Illinois Seismic Network: Stations throughout Illinois
- Indiana University Seismic Network: Stations in southwestern Indiana
- St. Louis University Earth and Atmospheric Sciences: Regional network coverage
Research
Ongoing research programs study the Wabash Valley Seismic Zone:
- Paleoseismic investigations to refine earthquake history and recurrence intervals
- GPS measurements to detect any ongoing crustal deformation
- Seismic tomography to image subsurface fault structures
- Probabilistic hazard assessments for regional planning
Preparedness
Residents and businesses in the Wabash Valley region should:
- Learn about earthquake hazards: Understand that the region is seismically active
- Prepare an emergency kit: Food, water, medications for at least 72 hours
- Secure heavy items: Strap water heaters, secure bookshelves
- Know building vulnerabilities: Older unreinforced masonry buildings are most at risk
- Have a family communication plan: Know how to contact family after an earthquake
For detailed preparedness guidance, see what to do during an earthquake and earthquake emergency plan.