To our surprise, almost 20 years of very precise GPS data across the New Madrid seismic zone don't show the motion we'd expect if a large earthquake was on the way. Hence it looks like the major earthquakes that have happened in the past thousand years are ending.

Because of these interactions, faults within continents form what's called a "complex system." Complex systems are ones in which the whole system behaves in a way more complicated than can be understood by studying its component parts. For example, the human body is more complicated than we can understand by studying individual cells, the economy is more complicated than can be explained by studying individual business transactions, and studying one ant doesn't tell us how an ant colony behaves. I've been exploring this concept with Mian Liu of the University of Missouri, Eric Calais of Purdue, and Qingsong Li from the Lunar and Planetary Institute.

A nice analogy, suggested by Bob Smalley of the University of Memphis, is the game "Booby-Trap". It's a box with a spring bar pushing on lots of small pieces. When you remove one piece some of the others slide around in a complicated way, depending on the pattern of the pieces and how much each is being pushed. Some can be removed without making anything happen, but removing others makes many pieces slide. Removing a small piece can have a big effect, and removing a large piece can do nothing. All this makes it hard to tell what will happen before you remove a piece. The game is somewhat like earthquakes within continents, except that we don't have to watch for thousands of years to see what's going on.

This idea lets us think about earthquake hazards in a more sophisticated way than just saying earthquakes will happen in the future where they happened in the past. In particular, there's no sign that a major earthquake is on the way in the central U.S. any time soon, especially because many of the small earthquakes that occur today look like aftershocks of past large earthquakes rather than indicators of future ones. That means that predicting where big earthquakes will happen based on small quakes is like the 'Whack-a-mole' game -- you wait for the mole to come up where it went down, but it's likely to pop up somewhere else. So instead of just focusing on where small earthquakes happen, we need to use methods like GPS and computer modeling to look for places where the earth is storing up energy for a large future earthquake. We don't see that in the Midwest today, but we want to keep looking.

A movie showing how "boobytrap" works

A movie from YouTube showing "Whack-a-mole"

An interview with the BBC describing these concepts (mp3)

For news stories about these concepts click here, or here

Applying these ideas to the New Madrid seismic zone

Paper discussing the complex fault concept: Stein, S., M. Liu, E. Calais, and Q. Li, Midcontinent earthquakes as a complex system, Seis. Res. Lett., 80, 551-553, 2009. For html click here, For pdf click here

Paper discussing the long aftershock sequence concept: Stein, S. and M. Liu, Long aftershock sequences within continents and implications for earthquake hazard assessment, Nature, 462, 87-89, 2009. For pdf click here