EARTHQUAKE scientists continue to warn North Americans living east of the Rocky Mountains not to take earthquake safety for granted. Arch Johnston of Memphis State University says large parts of the eastern United States are at risk. Yet communities there have barely started to adopt seismically sound building codes. Public awareness of the hazard and emergency response preparations also are seriously lagging.
Dr. Johnston repeated that familiar - if not yet heeded - warning at a symposium where experts reviewed some of the latest eastern earthquake research. Held during the annual meeting of the American Association for the Advancement of Science, the session reflected seismologists' growing understanding of this quake threat.
The most threatened region, according to present knowledge, is centered on the New Madrid Seismic Zone. This includes large areas of Arkansas, Kentucky, Illinois, Missouri, and Tennessee, plus a bit of Indiana. It was the center of a major seismic event in 1811 and 1812 involving three quakes.
University of Nevada geologist Steven Wesnousky says the New Madrid earthquake is, in terms of area, the largest United States quake known either historically or from the geological record. It involved large ground movements over several thousands of square miles.
Johnston, working with geological evidence, estimates the largest New Madrid quake magnitude was about 8.1 to 8.3 on the Richter scale. There have been larger magnitude quakes in North America along the western zone. But their main effects were felt over smaller areas.
Western quakes generally occur where two plates in Earth's crust rub together or where one plate underrides another. Quake-generating faults are usually related to the plate boundaries. Where a fault slips to cause a quake, the bedrock typically has little overlying sediment.
The New Madrid zone is, by contrast, in the middle of a plate. Quake-generating faults (cracks in the plate) lie under a mile or more of loose river-deposited sediment. This liquefies readily during a quake, thereby greatly extending the area of the quake's influence.
Dr. Wesnousky and his research colleagues are studying the extent of that influence as it is reflected in the geological traces left by the 1811-12 events, as well as by lesser quakes. He and other researchers find ample evidence of seismic activity over a wide area. But there is no sign of any other quake in the magnitude 8 range in geological evidence extending back 10,000 years.
Johnston explained that this now leaves an uncomfortable uncertainty. Geological evidence and seismic monitoring data gathered since 1974 show relatively small quakes recur regularly. Magnitude 6 quakes - the strongest for which such statistics are available - have an average frequency of 70 to 90 years. But there is nothing to indicate whether a really big shock, such as the New Madrid event, is likely to recur.
Under these circumstances, the geologists here say, their best advice is that communities in the eastern United States should be prepared for moderate to very severe quakes with the understanding that there is no way yet to foresee how likely a really big shock may be. This ignorance also means that, while the New Madrid zone appears to have the greatest risk, no region east of the Rockies can be considered risk-free.