Earthquake Hazard & Mitigation
Marion County sits at the intersection of two of the most active intraplate seismic zones in North America: the New Madrid Seismic Zone (NMSZ) ~135–200 miles south, and the Wabash Valley Seismic Zone (WVSZ) ~50–100 miles east. Add a layered exposure to abandoned coal mine subsidence across the Centralia / Wamac / Sandoval corridor, and the County's seismic risk profile is real but manageable. This page covers what the hazard looks like, how it's monitored, and what residents, businesses, and public-safety partners can do to reduce loss before, during, and after a damaging earthquake.
1. Hazard overview
Marion County's seismic hazard is dominated by two distinct sources, plus the legacy of underground coal mining. Probabilistic seismic-hazard mapping by the USGS places Marion Co fully inside the 0.15–0.20g peak-ground-acceleration band at the 2% / 50-year exceedance level (~2,475-year return period). That is meaningful intraplate hazard — comparable to the seismic loading that drives mid-grade Seismic Design Categories in California — though spread over a much longer return period.
2. Primary seismic sources
Closer of the two zones — east edge sits ~50 mi from the county. Centered on the Wabash River near Mt. Carmel, IL / Vincennes, IN. Active modern microseismicity. Historic events: 1968 Dale, IL M5.4 (~50 mi east of Salem), 1987 Lawrenceville M5.0, 2008 Mt. Carmel M5.2, 2002 Caborn IN M4.6.
Paleoseismic evidence from sand-blow trenching supports M7-7.5 events at ~5,000-15,000 year recurrence. The zone is the most likely source of a moderate (M5-6) event affecting Marion Co in the next decade.
The 1811-1812 NMSZ sequence produced three M7+ events in three months — felt as far as Boston. Three principal fault segments are mapped:
- Cottonwood Grove (Southern): M~7.5 — 1811-12-16 mainshock. ~180 mi from Marion Co.
- Reelfoot (Central): M~7.5 — 1812-02-07. Created Reelfoot Lake. ~155 mi.
- New Madrid North (Northern): M~7.3 — 1812-01-23. ~135 mi.
USGS estimates a ~7-10% probability of an M7+ NMSZ event in the next 50 years, and a ~25-40% probability of an M6+ in the same window.
Smaller (M2.5-4) events occur regularly within both zones and across southern IL. The Operations Map's USGS Earthquakes live layer shows all events of M2.5+ within 500 km in the last 7 days, fed directly from the USGS feed and refreshed every 5 minutes.
3. Scenario damage estimates (M7.5 NMSZ)
The IEMA / FEMA Hazus-MH catastrophic NMSZ scenario models statewide consequences for a repeat 1811-12 sequence. Consequences for the Marion Co region are summarized below — these are statewide-southern-IL averages, not site-specific predictions. Local outcomes will vary with construction quality, soil conditions, and proximity to the activated segment.
| Impact area | Marion Co region — modeled outcome |
|---|---|
| Modified Mercalli Intensity | VI–VII — Strong to Very Strong shaking. Frightening; cracked plaster, fallen masonry chimneys, displaced furniture, broken windows. Most modern wood-frame homes survive structurally. |
| Building damage (residential) | ~5-15% of unreinforced masonry (URM) buildings sustain Moderate or worse damage. Wood-frame ~1-3%. Highest losses on alluvial soils and over coal mine workings. |
| Critical facilities at risk | Pre-1990 fire stations, schools, and the county courthouse — any URM structure without modern seismic detailing. Salem Township Hospital + SSM Health St. Mary's Centralia: assess seismic readiness of ICU + pharmacy. |
| Bridges | ~3-8% of older steel/concrete bridges sustain damage requiring inspection before reopening. Bridges over Crooked Creek, Skillet Fork, and Salem Creek alluvium most exposed. |
| Pipelines (Patoka cluster) | 4-operator hub at parcel 0115100002 (Phillips 66 / Energy Transfer / Marathon / Enterprise) — pipelines are typically resilient to M7-class shaking but valves, pumps, and tanks are not. A release here is a credible secondary hazard. |
| Lifelines — power | Rolling outages 4-72 hours likely. Ameren regional substations need walk-down inspection. Plan for self-supplied generation at the EOC, hospitals, and PSAPs. |
| Lifelines — water / wastewater | Cast-iron mains in older Salem and Centralia neighborhoods are vulnerable. Assume boil-water orders 24-72 hours. |
| Telecom / 911 | Cell tower antennas and microwave dishes sometimes shift. STARCOM21 + amateur radio (ARES/RACES) are the planned degraded-comms backbone. See EOP § Communications. |
| Mass care / shelters | Ten-shelter network on the map activated by Red Cross + MCEMA. Surge capacity ~1,200 cots short-term; sheltering longer than 72 hr requires Region 9 / IEMA support. |
| Mutual aid posture | MABAS Region 9 (fire) + ILEAS (LE) + IPWMAN (public works) + ESF-8 coalition (medical surge). USAR via IL Task Force 1. Federal escalation through IEMA → FEMA Region V. |
4. Coal mine subsidence
Marion County is in the Illinois Coal Basin. From the late 1800s through the mid-1900s, multiple underground mines worked the Herrin No. 6 and Springfield No. 5 seams, primarily in and around Centralia, Wamac, and Sandoval. The most famous is Centralia Coal Co. Mine No. 5, where on March 25, 1947 a coal-dust explosion killed 111 miners — one of the deadliest U.S. mine disasters and the catalyst for the federal Coal Mine Safety Act and Illinois surface-protection statutes.
All of Marion County's underground mines are now closed, but their workings remain. Surface-level effects can include slow trough subsidence (gradual settling that cracks foundations and walls), sudden sinkhole formation (rarer but possible over shaft collars and shallow workings), well/septic damage, and accelerated damage during earthquake shaking. The Centralia / Wamac corridor carries the highest exposure; smaller residential exposure exists at Sandoval and Odin.
Risk: Moderate–High. Densely worked room-and-pillar mines under residential, commercial, schools, hospitals, and infrastructure. Active claims area for the Illinois Mine Subsidence Insurance (MSI) Fund — multiple structural settlement claims annually.
Risk: Low–Moderate. Surface zone over abandoned shaft mine workings, primarily under the village of Sandoval and the IL-161 corridor.
Risk: Low. Small abandoned drift mine workings, mostly under cropland with limited residential exposure on the village edge.
5. Liquefaction susceptibility
Liquefaction occurs when saturated, loose, fine-grained soils temporarily lose their shear strength under earthquake shaking and behave like a fluid. The hallmark is sand-blow ejecta at the surface. The 1811-12 NMSZ events produced sand blows across hundreds of square miles of southern Illinois — including documented occurrences in the Wabash Valley and Skillet Fork drainages.
Marion County's liquefaction exposure follows its major drainage corridors. The map layer shows three identified zones:
- Crooked Creek alluvial corridor — Moderate-High susceptibility. Crosses the county E-W in the south-central portion. Bridges, levees, and buried utilities along this corridor are at elevated risk during M5+ shaking.
- Skillet Fork alluvial corridor — Moderate. Mostly rural / cropland, with bridge crossings and farmsteads.
- Salem Creek alluvial corridor — Moderate. Crosses the city of Salem; affects N/S Broadway, IL-37 bridges, and select utility crossings.
Properties on these corridors should consider site-specific geotechnical assessment for new construction or major renovation. Existing critical facilities (PSAPs, hospitals, fire/EMS stations) on alluvium should request a structural seismic walk-down by a qualified engineer.
6. Monitoring & alerts
Operations Map shows all M2.5+ earthquakes within 500 km in the past 7 days, fed from the USGS feed via the EMA worker. Refreshes every 5 minutes. New M3.5+ events in the past 30 minutes are auto-posted as Operations and broadcast on the live feed.
Free email/SMS alerts customizable by location and minimum magnitude. EMA staff and partner agencies should subscribe with a 500 km radius around Marion Co (38.627°N, 88.946°W) and a M3.5 minimum.
Crowdsourced shaking-intensity reports. After any felt event in the area, EMA encourages residents to file a DYFI report. Aggregated reports help generate ShakeMaps for response planning.
Phone-sensor early-warning network. Coverage is strongest on the West Coast but the app provides global event notifications and educational content useful for staff and residents.
Multi-state EMA coordination body for NMSZ + WVSZ states. Hosts the annual Great Central US ShakeOut (3rd Thursday in October). Marion Co participates as a registered jurisdiction.
State-level coordination and training. IEMA Region 9 includes Marion Co and conducts NMSZ tabletop exercises on a multi-year cycle.
Within minutes of any felt event, USGS publishes a ShakeMap showing modeled shaking intensity by location. Use during response to prioritize damage-assessment patrols.
Research-grade broadband seismometer network covering the central US — including stations in IL, IN, MO. Useful for after-action analysis.
7. During the shaking — Drop, Cover, Hold On
The single most important action during earthquake shaking, everywhere in the world, at every age:
- DROP to your hands and knees before the shaking knocks you down.
- COVER your head and neck with one arm. If a sturdy desk or table is nearby, get under it. If not, crawl next to an interior wall away from windows.
- HOLD ON to your shelter (or your head and neck) until the shaking stops.
Do NOT run outside during shaking — most injuries happen from falling debris while people try to flee. Do NOT stand in a doorway — modern doorways are not stronger than the rest of the house. Do NOT use the "triangle of life" — that is contradicted by every major earthquake-safety body (USGS, FEMA, ARC).
If you are…
- In bed: stay there. Cover your head and neck with a pillow.
- Driving: pull over (away from bridges, overpasses, power lines). Stay in the vehicle until shaking stops.
- Outdoors: move to an open area away from buildings, trees, and power lines. Drop to the ground.
- In a wheelchair: lock the wheels, cover your head and neck with arms or pillow, hold on.
- In a high-rise: drop / cover / hold on. Do NOT use elevators. Expect fire alarms and sprinklers to activate.
- Near the coast (lakes here, not ocean): after shaking stops, move to higher ground if a seiche is possible — Carlyle Lake and similar reservoirs can produce small standing waves during long-duration shaking.
8. Structural mitigation (homes & buildings)
The five most cost-effective structural retrofits for residential buildings in Marion County, in priority order:
- Anchor the sill plate to the foundation. Pre-1980 wood-frame homes often were not bolted — the house can slide off the foundation during shaking. A licensed contractor can install foundation bolts at ~$3,000–$8,000 for a typical home. This is the highest-impact retrofit.
- Brace cripple walls (short walls between foundation and first floor). Plywood sheathing on the inside of cripple walls prevents collapse. Often combined with sill bolting.
- Strap or replace the chimney. Unreinforced masonry chimneys are the leading source of structural earthquake damage in single-family homes in this hazard band. Bracing or replacement with a metal flue is inexpensive.
- Brace the water heater. Two heavy-gauge straps (one upper third, one lower third) anchored to wall studs. Prevents the heater from toppling, which is the leading source of post-earthquake fires and water damage. ~$50 in materials.
- Identify and reinforce soft-story conditions. Older homes with garages or carports under living space ("soft story") collapse disproportionately. A structural engineer can specify shear-wall additions.
For commercial / public buildings
- URM retrofit. Unreinforced masonry buildings (older brick storefronts in downtown Salem and Centralia) benefit dramatically from parapet bracing and tie-downs. FEMA P-774 covers the engineering basis.
- Tilt-up concrete: verify wall-roof connections meet current code. Pre-1997 tilt-up structures are a recognized risk class.
- Critical facility seismic walk-down. Hospitals, fire/EMS stations, PSAPs, and the EOC should have a structural engineer perform an ASCE 41 Tier 1 screening every 5–10 years.
- Non-structural restraints in healthcare: HVAC equipment, medical gas piping, pharmacy shelving, and ICU equipment all benefit from seismic anchorage. FEMA E-74 is the canonical guide.
9. Non-structural mitigation (interior)
Non-structural items — furniture, appliances, contents — cause the majority of earthquake injuries in modern buildings. Most fixes are inexpensive and can be done in a weekend.
- Strap your water heater (top + bottom) to wall studs.
- Anchor tall bookcases, china cabinets, and dressers to wall studs (L-brackets at top).
- Secure flat-screen TVs with TV-anchor straps to the wall (not just the stand).
- Install latches on kitchen cabinets that hold glassware or heavy items.
- Move heavy or breakable items off high shelves to lower shelves.
- Anchor desktop computers, copiers, and printers with double-sided "earthquake putty" or velcro.
- Hang mirrors and pictures with closed hooks (not open J-hooks) — and not over beds, sofas, or desks.
- Locate and label gas, water, and electrical shutoffs. Keep a wrench tied near the gas meter.
- Install flexible gas connectors at the water heater, dryer, and stove.
- Replace overhead light fixtures with safety-cabled or recessed designs over beds and desks.
- Fasten the refrigerator and chest freezer to wall studs with appliance straps.
- Relocate hazardous chemicals (cleaners, pesticides, paints) to low, latched cabinets.
Cost benchmark: A typical Marion County home can complete the entire non-structural checklist for under $200 in materials. The highest-leverage items are the water heater straps, TV anchors, and tall-furniture brackets.
10. Family preparedness
The standard FEMA / ARC recommendation is at least 3 days of supplies for each person — water (1 gal/person/day), shelf-stable food, first aid, medications, flashlight, hand-crank radio, batteries, cash, copies of ID/insurance/medical info, sturdy shoes near the bed, and a whistle.
Marion Co adds: a basic respirator (N95 or KN95) for post-event dust, leather gloves for debris, and a Mylar emergency blanket per person (winter outages).
Local phone networks may be saturated for hours. Designate an out-of-state contact as the family check-in point — long-distance calls often complete when local ones do not. Texts succeed when calls fail. Every family member should memorize the out-of-state number.
Establish a meeting place outside the home (mailbox / driveway) and a secondary location away from the neighborhood (church, library, park) in case the area is evacuated.
Marion Co participates in the annual Great Central US ShakeOut (3rd Thursday of October). Schools, businesses, and households simultaneously practice Drop / Cover / Hold On. Free to register; the practice itself takes one minute.
Households with elderly residents, mobility-impaired members, oxygen-dependent members, or service animals: register with MCEMA's Functional Needs Registry so responders can prioritize wellness checks. Contact crose@marionco.illinois.gov.
Plan for at least 5 days of pet food + water and a kennel/leash kit. Livestock owners: pre-stage gates and identify alternate watering sources. Most public shelters do not accept pets — pre-arrange with a friend, kennel, or designated pet shelter.
Expect aftershocks — often felt within minutes and continuing for days/weeks. After major shaking: check yourself, then others; check for fire and gas; then exit if the structure shows damage. Use texts, not calls. Listen to NWS Weather Radio / local AM/FM for official updates.
11. Earthquake & mine subsidence insurance
Standard homeowners insurance in Illinois does NOT cover earthquake damage. It must be purchased as a separate policy or endorsement. Mine subsidence damage is auto-included for residential properties in Marion County via the Illinois Mine Subsidence Insurance (MSI) Fund, unless explicitly declined in writing.
Available as a rider on most homeowners policies. Typical deductibles are 5–15% of dwelling coverage (much higher than standard policies). Premiums in Marion Co's hazard band are moderate compared to West Coast rates. Worth pricing if you carry a mortgage on a brick home or live over a mine subsidence zone.
State-mandated coverage in 34 designated counties (including Marion). All residential policies under $750k automatically include up to $750k in mine subsidence coverage. Property owners must actively decline in writing to opt out. Commercial owners must affirmatively request the endorsement.
Illinois requires sellers to disclose known mine subsidence damage. Buyers in Marion Co — especially Centralia, Wamac, Sandoval — should request the seller's MSI claim history and pull the ISGS county mine map for the property.
12. Resources & further reading
Real-time feeds, hazard maps, ShakeMap, DYFI, ENS, scenario modeling.
The basis for IBC seismic-design loading nationwide. 2023 update.
Multi-state coordination, ShakeOut, scenario products.
Authoritative IL geology + coal mine maps + by-county PDFs.
Practical homeowner mitigation guide with photos and product references.
Engineering-grade nonstructural mitigation reference for hospitals, schools, EOCs.
Engineering basis for retrofit of unreinforced masonry buildings.
Annual drill — 3rd Thursday of October. Free to register.
Auto-included coverage for Marion Co residential policies.
Frequently asked questions
How likely is "the big one" in my lifetime?
USGS estimates a 7–10% probability of an M7+ NMSZ event in the next 50 years and a 25–40% probability of an M6+. Smaller (M5–6) events in the closer Wabash Valley Zone are individually less catastrophic but more likely — one or two per decade is realistic.
I rent. What can I do?
Plenty. Most non-structural mitigation does not require landlord approval — strap your water heater (with permission), anchor tall furniture, secure your TV, latch cabinets. Get renters insurance with an earthquake endorsement. Build a 72-hour kit. Identify shelters and exit routes in your building. Practice ShakeOut.
Should I buy earthquake insurance?
Worth pricing. The decision turns on dwelling type (URM is highest exposure, modern wood-frame is lowest), proximity to a mine subsidence zone, and household financial reserves. A licensed Illinois agent can quote both an earthquake rider and the (typically already-included) MSI coverage in one conversation.
My house is over an old coal mine. What now?
First — you almost certainly already have MSI coverage. Confirm with your insurance declarations page. Second, watch for the warning signs: doors/windows that suddenly stick, new diagonal cracks in masonry or drywall, separating joints, cracked foundation slabs, well-yield changes. If you see any, document with photos and contact your insurer to open an MSI claim. Third, pull the ISGS county mine map for your address to know exactly what is under you.
Where do I report a felt earthquake?
USGS "Did You Feel It?" — file a report at earthquake.usgs.gov/data/dyfi. Aggregated reports help generate ShakeMaps that EMA, MABAS, and ILEAS use to prioritize damage assessment.
Is there an earthquake "annex" in the Marion Co EOP?
Earthquake response is folded into the all-hazards EOP rather than treated as a standalone annex. ESF-9 (Search & Rescue) and ESF-3 (Public Works) are the most directly activated ESFs after a damaging event. See the EOP page for the full task organization.