Strong quake expected east of Rocky Mountains.

Strong quake expected east of Rocky Mountains Strong quake expected east of Rocky Mountains

Easterners who fantasize about California sliding into the sea may be in for a jolt. Chances appear high that a damaging quake will strike east of the Rocky Mountains in the next 30 years, according to new calculations.

"We wanted to tell people in the East that there is a level of eartquake hazard here. While it's nowhere near California's and you shouldn't be frightened or alarmed, you should be aware that the hazard is probably greater than you think," says G.A. Bollinger of the Virginia Polytechnic Institute and State University in Blacksburg.

He and Stuart P. Nishenko of the National Eartquake Information Center in Golden, Colo., have formulated the first estimate of earthquake risk for the eastern half of North America by analyzing historical records going back to 1727 and seismographic data accumulated over the last 15 years. Using the frequency of past quakes, they calculated the chances of a shock occurring within coming decades.

The region east of the Rockies stands a 40 to 60 percent chance of experiencing an earthquake of magnitude 6 or greater before the year 2020, the seismologists conclude in the Sept. 21 SCIENCE. Noting that seismic waves travel particularly well through the less-fractured rock in the eastern half of the continent, Nishenko and Bollinger say this region faces about two-thirds California's risk of sufferin a damaging earthquake within 30 years.

Their forecasts do not predict where earthquakes might occur, but seismologists are keeping a close watch on several seismically active areas, including New Madrid, Mo. (which spawned the country's most violent known earthquakes in the winter of 1811-1812), Quebec's St. Lawrence River (site of five strong quakes in the last three centuries) and Charleston, S.C. (severely damaged in 1886).

Archibald C. Johnston, a seismologist at Memphis State (Tenn.) University, says Nishenko and Bollinger's forecasts probably underestimate the East's quake hazard. Johnston has analyzed seismic activity around New Madrid and calculated a 40 to 60 percent probability that this fault will produce a magnitude 6 earthquake within 15 years.

Bollinger says the discrepancy underscores the primitive state of earthquake risk estimates for eastern and central North America. Geologists cannot directly study faults in this region because they lie buried deep below ground, whereas many California faults break the surface. Eastern faults also accumulate stress more slowly and can remain quiet for several hundred years, further hampering study.

Despite the difficulties, seismologists have succeeded in convincing some state and municipal officials of the earthquake hazards in their areas. In the last several years, many states and cities east of the Rockies have incorporated seismic regulations into their building codes for the first time. New York City is currently considering its own set of seismic standards.