Thursday, February 09, 2012

Bicentennial of New Madrid earthquakes raises question of hazard today

Bicentennial of New Madrid earthquakes raises question of hazard today

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Two hundred years ago this week, a powerful earthquake shook the Mississippi Valley near the southern tip of Illinois. It was the fourth in a series of quakes that punctuated months of heightened seismic activity stretching back to the preceding December.

In New Madrid, the Missouri settlement that gave the quakes a name, homes that had survived the previous shocks were thrown down, and sand and water spewed from fissures in the earth, covering hundreds of acres.

Nearby, the Mississippi River receded from its banks and then swelled with the force of a tsunami. Waterfalls sprang up where the fault crossed the stream, and the current of the great river seemed to run backward for a time.

[Images: Nineteenth century woodcut depicting effects of the earthquake (source to be determined); most recent USGS earthquake hazard map for 48 coterminous states from]

Despite the enormous power of the New Madrid earthquakes, there is no certain evidence that anyone died in them. At the time, the region was lightly populated, and it had few of the human structures that suffer damage when the ground shakes.

According Steve Marshak, a professor of geology who directs the School of Earth, Society, and Environment at the U of I, the New Madrid quakes were unusual in that they occurred along a line of faults in the interior of a single tectonic plate, rather than along a boundary where plates come together, which is where the great majority of quakes occur.

From a distance of 200 years, and secure in the knowledge that no lives were lost, reading the widely-available firsthand accounts of the New Madrid quakes can provide the same pleasure as watching a disaster movie. But were earthquakes of similar magnitude to occur today, the human consequences could be devastating.

The cities of St. Louis and Memphis both lie close enough to the New Madrid seismic zone to experience significant effects from a major earthquake centered there. Because of that, a great deal of scientific effort continues to go into estimating the probability of such an event.

Some scientists, especially at the U.S. Geological Survey, estimate the annual probability of a major New Madrid quake to be quite high. In fact, the 1996 update of the USGS earthquake hazard map puts earthquake hazard in the New Madrid area higher than the hazard in California.

Others, most notably Seth Stein, a professor of geology at Northwestern University, argue that the earthquake hazard in the New Madrid area is probably much lower. Stein makes this case in a lively 2010 book addressed to nonscientists called “Disaster Deferred: How New Science Is Changing Our View of Earthquake Hazards in the Midwest.”

Stein bases his argument in part on the fact that extremely precise measurements taken over the last twenty years show that the ground across the faults in the New Madrid zone is moving less than 2 millimeters per year. In contrast, he notes, the ground across the San Andreas Fault in California—where big earthquakes occur about every hundred years—moves about 36 millimeters per year.

Stein also provides extensive explanations of how seismologists estimate earthquake hazards, and a thorough critique of the methods and assumptions that account for the high hazard levels USGS assigns the New Madrid area.

All of this matters, of course, because constructing new buildings and retrofitting old ones to California standards costs a lot of money, money that would be better spent elsewhere if large New Madrid quakes are unlikely in the foreseeable future.