Our studies, led by recent graduate student Andy Newman as part of his thesis work and first reported in 1999, used Global Positioning System satellite surveying and found that the deformation rate in the New Madrid zone was much less than previously thought. A similar result emerged from reanalysis of the earthquake history. Hence it seems that the risk posed by large earthquakes in the the Central U.S.'s New Madrid Seismic Zone to cities such as Memphis and St. Louis is lower than previously thought, whether or not the NMSZ is presently shutting down.

The results illustrate the need to reassess ideas about the earthquake risk in the area, such as the National Seismic Hazard maps prepared by the U.S. Geological Survey, which are used to determine the appropriate level of earthquake-resistant construction. It had been assumed that large earthquakes like those that struck the area in 1811 and 1812 were magnitude 8 events, which should recur in the New Madrid seismic zone (NMSZ) every 500-1,000 years. Using this model, the assumed seismic hazard in the midcontinent is quite high, in some ways exceeding that in California. In particular, the predicted peak ground motion for the NMSZ exceeds that in Los Angeles, and the predicted highest ground motion area for the NMSZ is larger than for Los Angeles or San Francisco.


GPS antenna set up over geodetic marker at Village Creek State Park, Arkansas

A different picture emerges from the study that we conducted with colleagues from the University of Missouri, the University of Miami Geodesy Lab and Grand Valley State University, with funding from NASA and technical assistance from the University NAVSTAR Consortium (UNAVCO), national consortium of universities supporting GPS research. Using GPS, the positions of a network of geodetic markers in Missouri, Tennessee, Illinois, Arkansas, and Kentucky have been measured to accuracies of better than an inch since 1991. After the data were analyzed at University of Miami, they show little or no motion across the seismic zone. Moreover, GPS sites all over eastern North America also show little if any motion across the seismic zone. These results have since been confirmed by more detailed studies as discussed in popular terms here or in more technical terms here , here (pdf) , and here (pdf) .

The small -or zero - motion observed implies that it would take a very long time to accumulate the motion needed for large earthquakes. At least 500-1,000 years would be needed before a future magnitude 7 earthquake. Because magnitude 7 earthquakes are ten times smaller than magnitude 8s, these results mean that the largest earthquakes in the New Madrid Zone are 10 times smaller than assumed in the hazard maps.

The GPS results agree with our analysis of the earthquake history of the area. In the 1950s, seismologists B. Gutenberg and C. Richter noted that in a given area, the time between earthquakes of a certain size is approximately ten times longer than that for earthquakes one magnitude unit smaller. Since 1816, the New Madrid zone has had earthquakes with magnitude greater than 5 about every 10 years, and earthquakes with magnitude greater than 6 about every 100 years. Thus magnitude 7 earthquakes should occur about every 1,000 years, whereas magnitude 8 earthquakes should be about 10,000 years apart.

Additional information comes from earlier geological studies showing that earthquakes similar to those in 1811 and 1812 occurred in about 1300 AD and 900 AD. Although these earthquakes have been thought to be magnitude 8 earthquakes, the GPS data and the earthquake history, together with fact that these were about 500-1000 years apart, make it likely that the earlier earthquakes were much smaller, probably magnitude 7.

The GPS measurements also raise the possibility that the New Madrid fault system is shutting down, and that future large earthquakes will occur on other faults. Until recently about all we could say was that future earthquakes might occur in places where past ones had. Now we can actually test that idea by looking at the motion accumulating for possible future earthquakes. Although we can't be sure yet, the longer the GPS data continue to show essentially no motion, the more likely it seems that the fault is shutting down and won't cause large earthquakes for a very long time.

The new results illustrate that estimates of seismic hazard in the area, such as the hazard map, should be reduced. The current map reflects crucial parameter assumptions, many of which have considerable uncertainty due to little seismological data from any but small earthquakes. For example, although it has been traditionally assumed that the largest earthquakes were magnitude 8, magnitude 7 seems more likely. Similarly, the current map uses as relation that predicts significantly larger ground motion than the alternatives. As a result, alternative plausible parameter assumptions significantly lower the estimated hazard, as shown in the alternative maps developed with John Schneider and A. Mendez. These uncertainties should be recognized when using hazard maps.

An interesting implication is that although proposed building codes for the area call for expensive California-level antiseismic construction, work with structural engineer Joseph Tomasello indicates that less expensive measures are likely to make more economic sense. This research has spawned considerable discussion about the appropriate policy both for the Memphis area and for neighboring states Kentucky and Arkansas , including whether upgrades of existing structures, which can be very expensive, makes sense.

The GPS results give important new insight not only into earthquake hazards in the midwest, but into the fundamental mystery of why earthquakes occur here at all. The New Madrid Seismic Zone is in the center of the North American plate. Hence although it is easy to see how earthquakes occur along the San Andreas fault - the boundary between the moving Pacific and North American plates - it is hard to see why they occur within the plate. The GPS data show that the North American plate is amazing rigid - on average parts of eastern north America move relative to each other by less than 1/25 of an inch per year! Somehow, this tiny motion adds up over geologic time to cause large earthquakes.

Similar issues arise in other areas where seismicity occurs within the interiors of continents. Hence we are looking into a broad class of questions about earthquake recurrence and hazards.

Much future research by many investigators will be needed to answer these questions.

For the Science paper, click here

For the velocity vectors click here

For the New York Times story about this project click here

For the Science Daily story about this project click here

For the SRL paper illustrating the uncertainties in hazard maps (pdf) click here

For the EOS paper discussing the implications for building codes (pdf file) click here

For further discussion of the EOS paper (pdf file) click here

For the press release describing the EOS paper click here

For a NY Times Op-ed column on these issues click here

For an overview talk focusing on New Madrid seismology, hazard, and policy issues in context of other continental intraplate earthquakes (pdf) click here

For an opinion piece from Seismological Research Letters discussing general policy issues related to cost-effective natural hazard mitigation (pdf) click here

For a Power Point presentation, based on the SRL piece, discussing general policy issues related to cost-effective natural hazard mitigation click here

For a column from Geotimes discussing the general issues of cost-effective natural hazard mitigation (pdf) click here

For a presentation to the Arkansas legislature's committee on economic development on these issues click here

For a discussion of earthquake probabilities in the New Madrid Zone Click here

References

Newman, A., S. Stein, J. Weber, J. Engeln, A. Mao, and T. Dixon, Slow deformation and low seismic hazard at the New Madrid seismic zone, Science, 284, 619-621, 1999. For pdf click here

A. Newman, J. Schneider, S. Stein, and A. Mendez, Uncertainties in seismic hazard maps for the New Madrid Seismic Zone, Seis. Res. Lett., 72, 653-667, 2001. For pdf (B&W only) click here

Stein, S., J. Tomasello, and A. Newman, Should Memphis Build for California's Earthquakes? EOS, 84, 177,184-185, 2003. For pdf click here

Stein, S. and A. Newman, Characteristic and uncharacteristic earthquakes as possible artifacts: applications to the New Madrid and Wabash seismic zones, Seis. Res. Lett., 75, 173-187, 2004. For pdf click here

Stein, S., No free lunch, Seis. Res. Lett., 75, 555-556, 2004. For pdf click here

Calais, E.,G. Mattioli, C. DeMets, J.-M.Nocquet, S. Stein, A. Newman, and P. Rydelek, Tectonic strain in plate interiors? Nature 438, doi: 10.1038/nature04428 (2005). For pdf click here

Stein, S. New Madrid GPS: much ado about nothing?, EOS, 88, 58-60, 2007. For pdf click here

Stein, S. Approaches to continental intraplate earthquake issues, in Continental Intraplate Earthquakes, Special Paper 425, 1-16, S. Stein and S. Mazzotti, eds., GSA, Boulder, CO, 2007. For pdf click here

McKenna, J., S. Stein, and C. Stein, Is the New Madrid seismic Zone hotter and weaker than its surroundings? in Continental Intraplate Earthquakes, Special Paper 425, 49-58, S. Stein and S. Mazzotti, eds., GSA, Boulder, CO, 2007. For pdf click here

Stein, S. and J. Hebden, Time-dependent seismic hazard maps for the New Madrid seismic zone and Charleston, South Carolina areas Seis. Res. Lett., submitted 2008. For pdf click here

Investigators:

Seth Stein
Department of Earth and Planetary Sciences, Northwestern University
Evanston, Illinois 60208
(847) 491-5265 FAX: (847) 491-8060

Andrew Newman
School of Earth and Atmospheric Sciences, Georgia Institute of Technology
Atlanta, GA, 30332-0340
Phone: 404-894-3976 Fax: 404-894-5638 Email: anewmangatech.edu

Timothy Dixon and Ailin Mao
Rosenstiel School of Marine and Atmospheric Sciences
University of Miami 4600 Rickenbacker Causeway Miami, FL 33149
Telephone: 305 361 4660 Fax: 305 361 4632

John Weber
Department of Geology, Grand Valley State University,
Allendale, MI 49401, USA
(616) 895-3191

Joseph Tomasello
The Reaves Firm, 5118 Park Ave. Memphis, TN 38117
(901) 761-2016 joet@reavesfirm.com

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