ON SHAKY GROUND

The earthquake hazard here may be greater than you think. And planning for it is less than you'd expect.

The Missouri State Emergency Management Agency (SEMA) anticipates that a 6.7 shock would significantly damage more than 32,000 buildings in eastern Missouri. Many buildings would totally or partially collapse. Some houses could shift on their foundations. Towers and chimneys would fall. The estimated damage represents 6 percent of the total number of structures in the affected area and would be comparable to the 1994 quake in Northridge, Calif., which killed 57 people, injured 1,500 and caused $15 billion in property damage.

In contrast to California, however, the consequences here could be more far-reaching because faults in the Mississippi Valley are buried under sedimentary deposits up to a mile deep. These conditions allow seismic waves to travel as much as 20 times farther than they do in California. As a result, a moderate New Madrid quake would shake a seven-state region -- Missouri, Illinois, Arkansas, Tennessee, Kentucky, Mississippi and Indiana -- like a bowl of jelly.

"If you have an earthquake in New Madrid, I can't guarantee you that any of those bridges from Vicksburg, Miss., to Louisiana, Mo., are still going to be functioning," says Ed Gray of SEMA. Under such circumstances, all rail shipments would, out of necessity, be routed through Chicago, resulting in massive delays in the delivery of goods. It could also stop highway traffic along Interstate 55, impede barge traffic on the Mississippi and Ohio rivers, and disrupt air service to the hub cities of St. Louis and Memphis. Electrical-power outages would almost inevitably occur.

According to state geologist David Hoffman, "There's been two or probably three periods of movement in the last 10,000 years. That suggests a recurrence interval somewhere on the order of 3,000-5,000 years. The fact that these faults go up to the surface suggests moderate to large-scale earthquakes."
Jennifer Silverberg
According to state geologist David Hoffman, "There's been two or probably three periods of movement in the last 10,000 years. That suggests a recurrence interval somewhere on the order of 3,000-5,000 years. The fact that these faults go up to the surface suggests moderate to large-scale earthquakes."

But what might be the most serious danger from the next New Madrid earthquake pertains to another valuable resource that courses through the region. "You have four of the five major natural-gas pipelines come right through the soup in New Madrid, the soft alluvial soil," says Gray. "They carry gas all the way to Detroit, Chicago, Indianapolis and Pittsburgh. If (the earthquake) happened during the winter, you're going to have major-league problems on your hands. Try to explain to somebody why you cannot heat a nursing home or keep a hospital warm."

Unfortunately, the prospect of future New Madrid rumblings is not the state's only seismic worry. Recent jolts to the region have stemmed from disturbances outside what is normally defined as New Madrid seismic zone. The fault that is not even known could wreak the greatest havoc. "If any one of these goes off periodically, and any of them happen to be near a metropolitan area, (it) can do tremendous damage," says Buddy Schweig of the USGS in Memphis. Seismologists weren't even aware of the fault that caused the 1994 Northridge earthquake until after the ground started moving.

Nothing precludes a similar freak occurrence from happening here. Indeed, there is ample precedent for such an anomaly. On Nov. 9, 1968, for example, the seismographs in the St. Louis University geophysics department recorded an earthquake that registered 5.5 on the Richter scale. The epicenter was located 120 miles southeast of St. Louis, near the Illinois-Indiana state line. According to various reports, the quake made the Gateway Arch bounce, caused cars parked near the St. Louis County Courthouse to skip a few inches and cracked the rear wall of the synagogue on Skinker Boulevard that now houses the Missouri Historical Society Library. An 11-year-old boy sustained a head injury when he was hit by falling debris outside his home on South 11th Street. Brick chimneys and parapets rained down into the streets across the city. The tremor was felt as far away as Milwaukee.

Martin J. Walsh, then deputy building commissioner for the city of St. Louis, recalls helping coordinate the emergency response. Building inspectors, with the aid of the fire department, were dispatched and initially determined that 16 buildings had sustained structural damage. Fortunately for one city official, the earthquake took place on a Saturday. "After we thought we had cleaned it all up, we went to work Monday morning and found the plaster had fallen in the safety director's office in City Hall," says Walsh. "It would have killed him if that would have happened when he was there."

Ten years later, on Sept. 20, 1978, another twitch of the earth -- this one registering 3.5 on the Richter scale -- rattled the St. Louis area. As earthquakes go, it amounted to barely a squiggle on the seismograph. No damage or injuries were reported, but the tiny tic nonetheless remains noteworthy because it was centered virtually under somebody's backyard in South St. Louis. The source of the spasm was the St. Louis Fault, which was first discovered running through a quarry on South Broadway in 1948. The geologist who discovered the fault subsequently traced its path from Valmeyer to Alton, Ill., a distance of 45 miles.

In the past decade, the strongest tremor in the region has come out of the Benton Hills south of Cape Girardeau. It registered 4.6 on the Richter scale and was felt over a seven-state region on Sept. 26, 1990.

Seismographic data indicated that the epicenter was located near New Hamburg in Scott County, Mo., which is outside the New Madrid seismic zone. After the shaking stopped, the DNR and the USGS decided to take a closer look at the Benton Hills, and this is when Hoffman and his team began their trenching project on a farm a few miles south of New Hamburg. The excavations showed folds in the soil layers suggesting that seismic activity had occurred within the last 10,000 years.

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