Scientists explain swarm of small quakes

In less than 24 hours, a swarm of 28 small earthquakes was recorded near Ridgely, Tenn., a city 109 miles southeast of Cape Girardeau.

Ridgely is at "a junction of two major faults in the New Madrid seismic zone," said Gary Patterson, geologist and spokesman for the Center for Earthquake Research and Information at the University of Memphis.

Multiple earthquakes are common near Ridgely, he said, but are limited to four or five events. Every 20 years or so, he said, a big swarm, such as the one that started Saturday, occurs.

He said it is not clear if the weight of water from recent rains caused the quakes, but scientists are studying the possibilities.

"Certainly the occurrence of earthquakes is controlled by the amount of overburden -- how much weight is above it," he said. But swarms have occurred in both wet and dry weather.

The Ridgely quakes occurred about four miles under the ground's surface.

According to the U.S. Geological Survey, the central Mississippi Valley is the most earthquake-prone region of the United States east of the Rocky Mountains. Patterson said planning is better than panicking.

"We don't believe it's any cause for increased levels of concern. It's always prudent to prepare when you're at risk for a magnitude 5 earthquake," he said.

Scientists who follow the small Midwestern quakes linked to the New Madrid fault think in terms of a great quake as a magnitude 8 or greater, which would devastate multiple states.

"We have to plan for the worst-case scenarios, but it doesn't do a whole lot of good to scare the public with 8.4 when we're not ready for a magnitude 6," he said.

The 28 quakes over the weekend weren't felt by most people. Patterson said it takes a magnitude 2.5 quake draw the attention of people on the ground.

A magnitude 4 quake would feel alarming, he said, because it would rattle loose items, cause some damage to brick joints and cause hanging items, such as chandeliers, to sway.

Significant damage occurs at magnitudes 5 and up. A magnitude 6 quake would be felt over an eight-state area, he said.

But damage is impossible to predict, because it depends on how soils react to the underground vibrations and whether the surface is farmland or a city street. Older, taller structures fare the worst in earthquakes.

Patterson said any disaster plan a business, individual or family has for a tornado or flood will also work for an earthquake.

"Quakes are different in that there's no evacuation and no warning. You have to have a plan and have to be able to implement it immediately," he said.

People should react to the vibrations by getting away from bookcases and chimneys.

"We need to talk to one another about this. There won't be time to talk if we have one of these infrequent but damaging earthquakes," he said.

pmcnichol@semissourian.com

335-6611, extension 127

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