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IGWS Geologic Hazards

Earthquakes

Geological Survey Bureau geologists have information on historic Iowa earthquakes and significant earthquakes in other areas of the midcontinent. Only 12 earthquakes with epicenters in Iowa are known in historic times. The first known occurred in 1867 near Sidney in southwest Iowa, the most recent in 1948 near Oxford in the east-central part of the state. The largest (Mercalli magnitude VI) occurred near Davenport in southeast Iowa in 1934. None of these events was instrumentally recorded.

A map of the uniform building code (UBC) seismic zones is available online, as well as an article titled Iowa Perspectives on Midwestern Earthquakes.

For more information about earthquakes, contact Ray Anderson.
 

Mine Subsidence

Most of Iowa's coal mines were underground mines, possibly as many as 6,000 in 38 counties, potentially affecting 80,000 acres. Iowa contains eight underground limestone mines. Long-lasting detrimental effects of underground mining include subsidence, the process by which the land surface sinks from collapse of the mine roof or failure of the support pillars.

In addition to several publications on subsidence-prone areas of Iowa, the Survey has an extensive collection of restored coal mine maps, primarily blueprints, and maps of underground limestone mines. These maps are useful in the evaluation of suspected subsidence events; the maps were utilized in preparing reports on What Cheer, Centerville, and Des Moines (these reports contain detailed maps of compiled mine locations). Copies of the individual mine maps are available through photographic reproduction on a case-by-case, as-needed basis. Currently several staff are familiar with these maps and mine subsidence issues in general.

Visit Popular Browsing Subjects for several articles about mining in Iowa.

For more information about mine subsidence, contact Mary Howes, Bob McKay, or Paul Van Dorpe.
 

Karst Subsidence

Karst refers to geologic, hydrologic, and landscape features associated with the dissolution of soluble rocks, such as carbonates and evaporites. A common feature of karst landscapes are sinkholes, which form when the land surface collapses into subsurface voids formed in the slowly dissolving rock. In Iowa, carbonate rocks form the uppermost bedrock over roughly the eastern half of the state and are mantled with a variable thickness of glacial and other unconsolidated materials. Where these unconsolidated materials are less than 50 feet, and particularly less than 25 feet thick, sinkholes may occur. There are three areas in Iowa where large numbers of sinkholes exist: (1) within the outcrop belt of the Ordovician Galena Group carbonates in Allamakee, Clayton, and Winneshiek counties; (2) in Devonian carbonates in Bremer, Butler, Chickasaw, and particularly Floyd and Mitchell counties; and (3) along the erosional edge of Silurian carbonates in Dubuque and Clayton counties.

Most of Iowa's sinkholes occur in rural areas where their main impact is rendering some land unsuitable for row-crop agriculture. Sinkholes have also resulted in the failure of farm and other types of ponds, roads, and one sewage-treatment lagoon. As sinkholes sometimes allow surface runoff to directly enter bedrock aquifers, their presence has implications for groundwater quality. Locations of known sinkholesfrom U.S. Department of Agriculture - Natural Resources Conservation Service (formerly the Soil Conservation Service) county soils maps, U.S. Geological Survey topographic maps, and staff field observationsare shown on the map "Groundwater Vulnerability Regions of Iowa," and are available as a GIS coverage. Numerous publications on groundwater quality in areas of sinkholes are available online.

For more information about karst subsidence, contact Bob Libra.

Floodplains

Iowa's diverse rivers and streams share a common set of hazards associated with flooding and high river stages. These include inundation, sedimentation, and channel erosion. The Survey maintains records on the composition of floodplain deposits and has personnel who are knowledgeable about the behavior of Iowa's rivers and their impacts on society.

Articles regarding the flood of 1993 and gully erosion are available online.

For more information about floodplains, contact Deb Quade.

Slope Stability (landslides)

Layering of diverse geologic materials may lead to slumping, creep, and other slope instability. In Iowa, these hazards are frequent along steep valley walls and deep road cuts. The Survey maintains an inventory of landslides hazards in Iowa and has conducted investigations on several areas of recurring slope stability problems.

For more information about slope stability, contact Deb Quade.


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