Even with small earthquakes, some areas seem to experience more shaking than others, even nearby, areas. What causes this?
The type of material upon which a building is constructed greatly influences the amount of shaking that will occur from an earthquake. Thick, unconsolidated sediments, such as sand and gravel deposits or alluvium along river valleys tend to cause earthquake waves to slow down, but increase in amplitude. This, in turn, causes greater shaking and potential damage. This is known as site amplification. Buildings constructed on bedrock tend to experience much less ground motion and, therefore, less damage.
This phenomenon was clearly noted by Daniel Drake in Cincinnati during the great New Madrid, Missouri earthquakes of 1811 and 1812. Drake indicated that the shocks frightened everyone and knocked down chimneys in Cincinnati, which at that time was situated in the valley of the Ohio River. However, Drake pointed out that the shocks did not awaken residents with homes built on bedrock hills across the river in Kentucky.
It has been suggested that the Shelby County community of Anna, which suffered much damage from the earthquakes on March 2 and March 9, 1937, owes this severe shaking to the fact that the town lies above a 400-foot deep preglacial valley (Teays River) that was filled with sediment by glaciers of the Pleistocene Ice Age.
About two-thirds of Ohio is covered by unconsolidated sediments deposited by the Pleistocene glaciers. Some of these sediments consist of sand and gravel along river valleys and lake clays from post-glacial lakes and in the Lake Erie area, both of which are prone to amplification of seismic waves and, therefore, greater shaking. The Division of Geological Survey is mapping surficial sediments in Ohio and plans to use these data to produce maps that will depict areas that are more prone to shaking, if an earthquake of sufficient size would occur.
Last update July 20, 2005