Early in the morning of May 16, while most of America was being titillated and transfixed by the appearance in court of the then-suspect Dominique Strauss-Kahn, an urgent message was suddenly received at the headquarters of the Federal Emergency Management Agency (FEMA) in Washington, D.C.

Reports were streaming in of a catastrophic earthquake, magnitude 7.7, that had struck the Midwest near the town of Marked Tree, Ark. First reports were alarming: phenomenal property damage; casualty figures were unprecedented; transportation links were severed; and cities like St. Louis, Memphis, Little Rock, and Cincinnati had been thrown into utter turmoil. Eight states were believed to have been directly affected, and it was thought the death toll would be in the thousands.

A gigantic federal relief mission swung into action. Nine thousand National Guardsmen were ordered to be deployed. Triage centers were opened in all the affected cities—a list that grew longer as a secondary magnitude 6.0 earthquake struck close to the city of Mt. Carmel, Ill. The Red Cross deployed emergency teams. Power companies were given priority to restore electricity and gas supplies. Heavy equipment was sent in to clear highways and railway tracks.

Within 72 hours some kind of order was restored. Hospitals found themselves more able to cope with the vast number of patients suffering injuries. Refugees fleeing in panic were being assembled into special camps. Temporary tent cities were set up along the main refugee routes.

It was all, of course, an exercise. Its name was NLE2011—a National Level Exercise run by planners at FEMA who were clearly very eager never to be caught wrong-footed again, as they had been after Hurricane Katrina, in 2005. But the exercise was itself somewhat disaster-prone, turning out to be spectacularly ill-timed—for at the very moment it began the country was occupied by the record floods along the Mississippi and then by the outbreak of tornadoes that climaxed with the near-total destruction of the city of Joplin, Mo. Emergency responders had their hands full with very real catastrophes, without having a Washington-created disaster of even greater magnitude added to their burden.

Yet the exercise was of vital importance—as the two somewhat smaller earthquakes yesterday have amply shown.

For the temblors that struck yesterday in the hitherto unknown hamlets of Trinidad, Colo., and, more significantly, in Mineral, Va., have served to remind the country of three abiding seismic truths. First, that earthquakes occurring east of the Rocky Mountains extend their effects over much greater distances than do the very much larger, but more contained quakes on the Pacific Coast. Second, that the population east of the Rocky Mountains has precious little idea of how to prepare for an earthquake, so seldom do large events occur in the region. Third, and most important, as FEMA would now be the first to point out, there is a very real potential for a truly disastrous earthquake to occur in the Midwest—and that when it comes it would most probably be in the very region chosen for the NLE2011 three-day quake-game—centered around the otherwise forgettable city of New Madrid, Mo.

For it has happened there before, big time. This year marks the bicentennial of the great swarm of earthquakes that afflicted New Madrid between December 1811 and February 1812—hundreds of them, day after day, but punctuated by four enormous ruptures, two occurring on Dec. 16, and one each on Jan. 23 and Feb. 7. These caused spectacular effects all across the then young, sparsely settled United States—toppling church steeples in South Carolina, ringing church bells in Boston, causing the Mississippi to reverse it course, and sinking numerous properties deep into the liquefied earths of the prairies.

The casualties from this string of over-magnitude 7.0 quakes—perhaps 7.5, which would approach the magnitude of the great San Francisco quake of 1906—were light, principally because in 1811 there were so few people in the region. But today is different. There are tens of millions close by—which is why FEMA, triggering its exercise-quake in an Arkansas town just a few miles from New Madrid, chose to send its stark warning to the cities of St. Louis, Memphis, Little Rock, and Cincinnati, all of which would most likely be severely affected should such a quake happen again.

As it most assuredly will. For there is no doubt that there will be another New Madrid event, quite probably as large as two centuries ago, and yet much more devastating.

That is certain, but when, no one can say. The U.S. Geological Survey will only suggest a probability: that there is a 10 percent change of a magnitude 7 quake in the next 50 years, and a 25 to 40 percent chance of a magnitude 6. For some, that is as dire as is necessary.

A thousand miles east of New Madrid, in New York, and despite the worry caused by the dramatic events of yesterday, the chances of real damage and danger are slim. Large spreading quakes—and the 5.8 magnitude event in Virginia that affected Manhattanites over their lunch hour, did spread like wildfire, as is so characteristic of large eastern quakes—occur very seldom, once in a century or more. And most of New York is built on ancient shield rock, strong and secure and less liable to shake and damage itself than softer and more pliable rocks.

The single exception is in Lower Manhattan: Battery Park City. This 92-acre gathering of real estate was built on landfill, mainly on sand and gravel excavated and ground up from the building of the World Trade Center. But as was demonstrated in the 1811 New Madrid quake, soft ground suddenly and precipitously turns to liquid during a quake—and there is little doubt that a major seismic event in New York would do the same for the ground underlying Battery Park City. The result, just as happened in the reclaimed land of the Marina District in San Francisco in 1989, could be widespread damage to property, and probably significant numbers of casualties.

Naturally, after an event like yesterday’s—one that triggered the evacuation of the White House and the Pentagon, and briefly unsettled even the most phlegmatic New Yorkers—the nation’s attention is turned to the likely fate of its great Eastern cities. It shouldn’t be: far greater danger attends the cities of the Midwest, which are perched on a ticking seismic time bomb. This is something the planners in Washington are just now starting to realize, and for which they are planning their own Doomsday scenario—even if their timing, this year, could have been a great deal better.