Models and data suggest that mid-continent earthquakes may well have been aftershocks from earthquakes that occurred more than 100 years earlier.
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So it was with interest that I read a recent Nature paper reporting that scientists might have been misinterpreting some aftershocks as earthquakes, leading them to overestimate the risk on some faults and underestimate the risk on others.
At present, earthquake prediction—and I use that word in its loosest possible sense—consists of looking at the frequency of small earthquakes along a fault of interest. These smaller earthquakes indicate that it is on the move and that, somewhere along the fault, the pressure is probably building up for a big one. Unfortunately, it is currently impossible to determine where the pressure point is—if, indeed, it does exist—and when it will give.
To make matters worse, events after an earthquake look remarkably similar to events before. After a major quake, the pressure point has relaxed, and there are a whole lot of small earthquakes as all the smaller pressure points get released. On short time scales, the aftershocks are pretty easy to distinguish from the small shocks that may presage an earthquake. The complicating factor is that the Earth really doesn't care about our timescales.
Some fault lines are very inactive—the boundary has basically fused and there is very little movement. These are located in places like New Madrid in Missouri, central China (that one should ring a bell), and Quebec. These places are considered pretty geologically inactive and yet, occasionally, they get hit by an earthquake—usually a big one. Typically, these earthquakes are not presaged by a series of small quakes, so they are even more unexpected.
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http://arstechnica.com/science/news/2009/11/earthquakes-from-slow-moving-faults-last-much-longer.arsOh Yay! Attacks from the past.
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