Wednesday, February 23, 2011

The New Madrid Seismic Zone: Much Ado About Something ... Unexpected

In 1811 and 1812, four major earthquakes (M7.4 to M8 on the Richter scale) centered in New Madrid, Missouri changed the course of the Mississippi River near Memphis, and may have caused the river to flow temporarily upstream. The entire fault system has been named after those earthquakes' epicenter. The biggest part of the New Madrid system sits in Missouri; it then extends along the Mississippi River Valley from northeast Arkansas through southeast Missouri and into western Tennessee and western Kentucky. The more than 4,000 earthquakes recorded there since 1974 make it the single most active fault system in the United States.

Is a "big one" centered in New Madrid on the near horizon? Anxiety runs high, and the local data are fearsome.

Yet the New Madrid (pronounced New MAD-rid) seismic zone is a mid-continent, or intraplate fault system, a rift occurring within the interior of a tectonic plate, and that may be key in forecasting the fate of the New Madrid area. Publishing in the journal Lithosphere, Mian Liu, professor of geological sciences at the University of Missouri-Columbia, Seth Stein, professor of earth and planetary sciences at Northwestern University, and Hui Wang, a Chinese Earthquake Administration researcher, suggest that widespread concern about the New Madrid area may be misplaced—literally. Examining about 2,000 years of records of mid-continent earthquakes from China, they found that big earthquakes never struck twice in the same place. Rather, after an initial earthquake, a series of aftershocks quaked the immediate area, sometimes with intensity approximating that of the initial shock. Additional trembling sometimes occurred with astonishing frequency over hundreds of years. The next major earthquake, however, was always further along the fault system. The investigators' GPS data show that an earthquake and prolonged resettling on one spot of a fault system inevitably increases tectonic stress on other sections of the system.

All of which suggests that New Madrid itself will probably not host the next mid-continent big one. On the other hand, St. Louis and Memphis—both of which are heavily populated, and situated near the furthest reaches of the New Madrid fault system—might. It could be what FEMA calls a "maximum of maximums" event. In anticipation of such a catastrophe, FEMA will lead a national-level exercise in May of 2011. Federal, state, and local governments will be involved, as well as businesses, private groups, and even faith-based groups. As May approaches, FEMA will make more exercise information available.

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