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

Nowhere are the economic risks higher, however, than areas such as Chesterfield, where river bottoms are undergoing rampant development. Soils in the floodplain are typically unstable and subject to liquefaction in the event of an earthquake. The sand boils in southeast Missouri, which remain evident from the New Madrid earthquakes of 1811-12, were caused by liquefaction. The phenomenon often occurs when seismic activity transforms loosely packed sand and silt into a fluid mass as a result of extreme pressure in the groundwater. The water table in the Missouri River bottom in St. Louis County is sometimes only a few feet from the surface. Whereas levees can redirect floodwaters elsewhere, nothing can stop a major earthquake from shaking the alluvial plain and turning parts of it into a giant cache of quicksand.

"Good firm soil will not amplify the ground motion as much as a weak soil," says Theiss, who is a member of the Building Seismic Safety Council, a professional group that oversees building standards nationwide. "If you're near a river and you build on soil that was previously on a floodplain, that's the worst type of soil, because it magnifies the ground motion." Design techniques exist that can compensate for building on unstable soil, but the latest upgrade in the USGS recommended standards are not currently being applied, Theiss says.

"The conclusion that they (USGS) have reached, as far as the St. Louis area is concerned, is that the ground motion that we should be designing for is greater than the current ground motion that's specified in the building code," Theiss says. Developers, however, are still only meeting the minimum code specifications, Theiss says.

State geologist David Hoffman in the Benton Hills, where he has investigated seismic activity.
Jennifer Silverberg
State geologist David Hoffman in the Benton Hills, where he has investigated seismic activity.

"There's nothing that prohibits you from going beyond the code," says Michael Werner, St. Louis County deputy director of code enforcement. "It makes sense to construct just about anywhere, so long as you take the proper precautions," he adds. "That's why you have engineers who not only comply with the law but also use what's called sound engineering judgment in the design."

But sound judgment often bows to the bottom line. "We tell our clients that there are new maps out there, and we recommend designing to them," says Theiss. "(But) they say: "We're just going to go by the code, period.' That's the prevailing attitude." Those code standards are subject to change again next year when a new nationwide building-code standard code takes effect, Theiss says.

The situation outside the St. Louis area is much worse because of a lack of a statewide building code. State law actually prohibits smaller counties from establishing an office to enforce building codes. At the same time, another Missouri law, enacted in 1991, mandates that towns and counties where earthquake damage is anticipated require new buildings to meet national seismic safety standards. The statute applies to more than three dozen counties in the eastern part of the state. It's a classic Catch-22, because there is no means of enforcing the rule.

The laws of nature are not bound by human contradictions, though, and in the Benton Hills the earth itself is deliberating its next move.

Hoffman, the state geologist, is standing in a 15-foot-deep trench that was excavated on the farm of Leonard Weber in Scott County. The excavation is one of a pair that have not been refilled since he completed his research a few years ago. He scrapes the walls of the trench with a hoe. Turning the implement around, he uses its handle as a lecture tool, pointing out a distinct soil layer that juts up toward the top of the ditch.

"The Peoria loess started to be deposited about 25,000 years ago and ended deposition about 10,000 or 12,000 years ago -- and it's offset," says Hoffman. "So we know the fault had to happen after that was deposited. That tells us that it happened within the last 10,000 years. 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."

For now, the secrets of the Benton Hills defy further explanation. "There are many lifetimes' worth of study here," says Hoffman. "It's had a long, complicated history. We've just sort of scratched the surface on it."

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