BOSTON--The bluffs of the Mississippi River -- though they shielded Confederate inhabitants from the bombardments of Yankee gunboats during the Civil War-- will not protect the current residents from the strong ground shaking produced by potential large earthquakes.
Recent U.S. Geological Survey research reveals that there is little difference between the ground motions recorded on the Mississippi River bluffs relative to the ground motions recorded on the nearby floodplain. This evidence will be presented in the poster, Earthquake Ground Motions in the Central U.S., by Mark Meremonte and other USGS scientists from Golden, Colo., as part of the session, USGS Earthquake Hazards Program, Central and Eastern USA, on Thursday, May 28 from 8:30 a.m. to 12 p.m. in Hall C of the Hynes Veteran's Memorial Convention Center.
Most topographic highs on Earth, such as bluffs, are underlain by rock that is more resistant to erosion than the alluvium -- clay, silt, sand, gravel or similar material deposited by running water -- that generally underlies the adjacent valleys or floodplains. As a rule, seismic waves travel faster through rock than through alluvium. As they travel from high-velocity rock to low-velocity alluvium, seismic waves "pile up" -- like slow-moving traffic on a freeway -- and their amplitudes increase.
The topographic relief of the bluffs along the Mississippi River suggest that the area is also underlain by rock with a high seismic velocity and a corresponding tendency to de-amplify the ground motions of the floodplain.
"Alluvium tends to amplify ground motions relative to those of nearby rock," said Edward Cranswick, geophysicist. "On the other hand, sites underlain by rock experience less shaking than sites underlain by alluvium. Ridges are usually underlain by resistant rock, and therefore they usually shake less than the adjacent valleys underlain by alluvium. According to aftershock studies, sites in the Santa Monica Mountains experienced less than a one quarter of the strong ground shaking experienced by some sites in the adjacent San Fernando Valley that were severely damaged by the 1994 Northridge, California, Earthquake."
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