Central U.S. risk higher than thought.

University of Illinois geologist Stephen Marshak is not saying that disaster is coming, but he warns that earthquake risk in some of the nation's midsection may have gone unnoticed. More than 1,000,000,000 years ago, extensive regions of the midcontinent stretched until they broke along two sets of faults. One set trends northeast; the other, northwest. In some places, northwest-trending faults link end to end to create long zones of broken, weakened crust. One runs from South Carolina, across southernmost Illinois, to northern Idaho.

The distribution of contemporary earthquakes implies that the most active seismic regions occur where this linked fault zone crosses northeast-trending faults, Marshak explains. The intersections between two crossing fault sets appear to be weaker than surrounding areas and may be more likely to rupture and cause earthquakes. The New Madrid seismic zone of southeast Missouri, for example, occurs at such an intersection. There are numerous other fault-intersection zones, he points out.

"The historical record provides just a snap-shot of seismic activity that spans the last two centuries or so. Thus, this record may not point to all the intersection zones that should be studied for their potential risks. Nobody can say that there is going to be a big earthquake on any particular fault at any particular time. But our research hints that any fault-intersection zones should be considered as a...