Geologic Hazards

Ohio has recorded at least 200 earthquakes above 2.0-magnitude since the earliest record in 1776. Most of these events have been small, in the magnitude range, but at least 15 earthquakes centered in the state have caused damage. The largest earthquake in Ohio occurred on

Ohio’s earthquakes are concentrated in two primary zones, but also occur with less frequency and intensity in other areas of the state. The Western Ohio Seismic Zone includes Allen, Auglaize, Mercer, and Shelby Counties and was particularly active in the 1930’s but still has periodic earthquakes. These earthquakes occur at depths of about three miles along faults associated with a Precambrian failed rift zone. The rift zone trends northwest from Champaign County and into Indiana.

The Northeastern Ohio Seismic Zone includes a broad area from eastern Cuyahoga County east to the Pennsylvania border and south to the vicinity of Akron. Most activity in this area has been concentrated in Lake County and beneath Lake Erie offshore from Lake and Ashtabula Counties. A sequence of earthquakes centered at Ashtabula, beginning in 1987, is interpreted to have been induced by fluids from a deep injection well. The largest earthquake in this zone occurred on

Small earthquakes have occurred in a broad area of southern Ohio. A few of them have caused minor damage. A small number of minor earthquakes have been scattered across some other areas as well.

Downslope movements of rock and soil, collectively called landslides, cause many millions of dollar of damage yearly in Ohio. These gravity driven movements occur in many areas of the state where slopes and failure-prone materials exist. Although such movements occur naturally, many landslides are triggered by human activities.

Slow, downslope movements of unconsolidated surface materials called creep, is widespread as can be noted by tilting fence posts, cemetery monuments, telephone poles, and other objects that through time have slowly moved from their original vertical position. Creep is partly a result of alternate freeze-thaw cycles pushing grains apart.

Rotational slumps involve large blocks of rock or sediment that move as a unit along a glide plane. The bottom portion, or toe, of the slump moves outward and downward whereas the top portion of the slump block moves downward but tilts backward. Trees on top of the tilted block lean upslope. Rotational slumps commonly occur in red-colored shales of Pennsylvanian age in southeastern Ohio.

Earthflows occur in unconsolidated materials that are saturated with water and lose the bearing strength of grain-to-grain contact. The sediment moves down slope in a series of lobate flows.

All of these movements are relatively slow and, although seriously damaging to property and structures, generally do not pose an immediate threat to human life. However, rockfall is a category that can be of considerable danger because it involves a large amount of rock that suddenly moves downslope with little or no warning. Generally, rockfalls occur in thick, massive sandstones that are cut by vertical joints. Undercutting of the less-resistant rock beneath the sandstone, either by natural processes or human activities, removes support for the overlying sandstone. Water that penetrates a vertical joint tends to pry the block loose from the outcrop by freezing. Ohio’s only landslide fatality occurred in 1986 along U. S. Route 52 near Ironton when a rockfall crushed a passing car on the highway below.

The most significant landslide damage in the state, in terms of financial loss, occurs in the Cincinnati area. Steep slopes in this densely populated region are prone to landslides where an Ordovician-age unit, the Kope Formation, is present. This shale and limestone unit is prone to slumps and earthflows when vegetation is removed and the slope is undercut by excavations. Lake clays of Pleistocene age are also prone to landslides in this area.

Bluffs of till deposited by the glaciers of the Ice Age along the Lake Erie shore from Huron eastward to Ashtabula are particularly prone to landslides as waves undermine the bluffs, leading to collapse. Significant retreat of the shoreline has resulted in loss of homes, roads, and other structures.

Silts and clays deposited in former glacial lakes in the valley of the Cuyahoga River between Akron and Cleveland are susceptible to landslides, particularly when these sediments become saturated with water. This has been a problem for homes and roads in the area.

In southern Ohio, the Bedford Shale of Late Devonian age is prone to slumping in the valley of the Scioto River between Portsmouth and Circleville. Lake clays deposited in association with Lake Tight, and ice-dammed lake formed in the Early Pleistocene, are prone to landslides in some areas in southern Ohio.

In southeastern Ohio, red-colored shales of Late Pennsylvanian or Early Permian age tend to lose their bearing strength when they become saturated with water, leading to slope failure in the form of slumps and earthflows. Although the area is not densely populated, these landslides have been particularly troublesome along roads vital to transportation.

Most landslides in any category usually occur when the slope is undercut by natural erosion or by human construction activities, when vegetation, the roots of which bind earth materials, is removed, or the rock or when sediment becomes oversaturated with water. Care exercised in construction can commonly prevent future landslides if these factors are taken into consideration. In addition, avoiding construction on or through landslide-prone geologic units will avert the problem.

Radon is a naturally occurring radioactive gas that is a product of the breakdown of small amounts of uranium in rock or soil. It is thought to be a carcinogenic agent if it is concentrated in buildings in sufficient quantities. The U. S. EPA has set a concentration of 4 Pico curies per liter as an “action level’ at which steps should be taken to reduce long-term exposure to this gas. Radon enters homes from the soil or bedrock through basements or foundations in contact with the ground and is generally in highest concentrations in the lowest part of the structure. Radon concentrations can be tested in buildings by purchasing kits or canisters at many hardware and home-supply stores.

Home tests by nearly 140,000 individuals across Ohio indicate that most areas of the state do not have significantly high concentrations of radon, although such statistics do not guarantee that specific sites or homes will not have a problem. However, several counties in the state have yielded results suggesting that high radon concentrations may be pervasive in some areas. Knox and Pickaway Counties, and in particular, Licking County, have yielded consistently high radon readings from home tests.

Some areas yielding elevated radon levels are underlain by the Ohio Shale of Late Devonian age. This unit is known to contain uranium. Glaciers of the Ice Age ground up the shale and redeposited it in areas not underlain directly by this unit, leading to elevated radon levels. The glaciers also incorporated weathered soils formed on extensive limestone and dolomite bedrock in western Ohio. These weathered soils tend to concentrate uranium. Therefore, glacial sediments may have higher levels of radon derived from this source. In addition, the permeability of soil, rock, and sediment contribute to radon concentrations in homes as the gas can move more easily through cracks or between grains.

For extensive information on radon in Ohio, please visit the Ohio Radon Information System Web site maintained by the University of Toledo. This Web site has statistics on radon concentrations in counties and zip codes, geologic sources of radon, mitigation, and much other information.

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