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Earth Sense: Quakes are rare, mild here, but not out West

POSTED: February 23, 2014 1:00 a.m.

An earthquake in South Dakota two weeks ago. Another in South Carolina last week. It would be tempting to come up with scary headlines now.

But these quakes were just two of the 1 million per year worldwide, according to the United States Geological Survey. The Edgefield, S.C., quake, 22 miles northeast of Augusta, was magnitude 4.1. That’s a relatively minor event, which is felt many miles from the epicenter but usually produces no damage.

Earthquakes tend to occur along faults, or breaks in the bedrock, and the Piedmont area of South Carolina and Georgia has lots of those. A “seismic line,” where minor quakes are common but not usually dangerous, runs along the Georgia-South Carolina state line. If you want detailed information, the South Carolina Department of Natural Resources offers an excellent free earthquake guide online, with historical data and tips on preparedness.

The last truly destructive quake in our neighboring state occurred in 1886, when 60 to 110 lives were lost and 2,000 buildings damaged in Charleston. But this doesn’t mean that the South is poised for another major shaking, because the quakes here occur when bedrock along a fault line shifts position and stabilizes again. It’s nothing like the San Andreas Fault in Southern California, where two gigantic building blocks of the globe, the Pacific Plate and the American Plate, are constantly rubbing and sliding alongside each other.

The real object of worry is San Francisco, where San Andreas isn’t moving as smoothly as it is in the Los Angeles area. Every so often, it breaks loose and produces a jolt that translates into a major earthquake on the surface. Where structures are built on soft sediment, the results can be deadly. In 1989, raised portions of the Interstate 880 freeway collapsed, crushing people in their cars below.

The USGS considers a quake “major” when its magnitude exceeds 6.7. The most recent 2012 report by that agency rates the overall probability of a magnitude 6.7 or greater earthquake in the Greater Bay Area at 63 percent, about 2 out of 3, by the year 2036 (earthquake.usgs.gov).

In plain terms, it means that if we were to compare three of these time spans, two of them would have a major quake in them, with great loss of lives and property. Federal, state and regional governments are well advised to place heavy emphasis on quake preparedness in the San Francisco Bay area.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.


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