This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Thursday, November 10, 2011

New Madrid Fault: To Break or Not to Break?

Geophysicist Seth Stein, described in the Nature
article as "a hyper-intelligent version of George Costanza,
the ever-complaining character from the television show

With a title reminiscent of a best selling science book "How I killed Pluto, and Why it Deserved it," Nature today has featured "Seth Stein: The quake killer." In spite of the impression you might get from the article and from this picture, Seth does have a great sense of humor! 

The quake that he's killed is the feared future quake on the Reelfoot fault in the New Madrid region of the 1811-1812 earthquakes.  Seth adamantly opposes the established position of the U.S. Geological Survey (USGS) which puts the probability of a large earthquake there as high as the regions along the West Coast and Alaska. The article, by Richard Monastersky, in this week's Nature, is long and well done (thank the world for great science writers!). Stein makes several arguments, the strongest being that surveys across this region with Global Positioning System (GPS) instruments show no evidence that strain is accumulating in advance of future quakes. Stein has been monitoring the region for the past 20 years. Another argument that he makes is that there is no topography to speak of in the region, topography that would be expected if there was a long history of faulting. 

USGS earthquake hazard map
For a long time the earthquakes that occurred here in 1811-1812 have been assigned a magnitude between 7.8 and 8.4, which would make them bigger than any earthquakes in the history of the mainland U.S. (Alaska has had a bigger quake). However recent work not only by Stein, but by USGS and other geologists as well, now have downgraded these to a magnitude between 6.8 and 7.0, a factor of twenty five times less energetic.

The faults in this area are hundreds of millions of years old. But what reactivated those faults and triggered those quakes. Stein's idea, with Roy van Arsdale, is that they are responses to the unloading of about 12 meters of sediment by erosion from the Mississippi River during the 16,000-18,000 years since the end of the last ice age. Reduction of pressure by removal of that weight altered the stresses along the old faults enough to reactivate those faults in the region that are ready to go. However, once a fault breaks, there is not enough stress in the region to trigger another big quake along it. The seismic activity would end on this fault, but other faults in the area could still be under stress. A bottom line is that the big picture should focus on a broad area of faults, not simply on the New Madrid area itself.

The implications are not trivial.  Reinforcement of old buildings in this part of the country is an expensive proposition, and whether you have to reinforce for a magnitude 6.8 or magnitude 8.4 earthquake makes a huge difference in the cost.

Stein has written a trade-science book "Disaster Deferred: How New Science is Changing our view of Earthquake Hazards in the Midwest." It's a great read!

No comments: