Small earthquakes in unexpected locations are often a cause for concern. The worry is that these rumbles are harbingers of bigger quakes to come. But not always - a new study suggests that many of these tremors aren't warnings, but aftershocks. In particular, those that happen in the middle of continents, far away from the major fault-lines that separate tectonic plates, probably reflect past quakes rather than future ones.
Earthquakes are a common occurrence on the boundaries between tectonic plates, and they occur at predictable spots. But they can often strike areas that are far away from such boundaries and where old fault-lines have seen little seismic activity over the past hundred years. The central United States, for example, experiences many such unexpected tremors.
But Seth Stein from Northwestern University and Mian Liu from the University of Missouri think that many of these small quakes are aftershocks of two bigger magnitude-7 tremors that shook the Midwest around 200 years ago.
The first hit a town called New Madrid in 1811 and triggered three shocks of similar magnitude that, together, reactivated an ancient set of faults in the continent's interior. The second big one hit Charleston, South Carolina in 1886. Low-level seismic activity in both areas, New Madrid and Charleston, is often interpreted as a sign that they will once again be hit by large earthquakes in the future, painting two imaginary bull's-eyes of risk in middle America.
New Madrid afer the 1811 quake
Large earthquakes are often followed by aftershocks, the result of changes in the surrounding crust brought about by the initial shock. Aftershocks are most common immediately after the main quake. As time passes and the fault recovers, they become increasingly rare. This pattern of decay in seismic activity is described by Omori's Law but Stein and Liu found that the pace of the decay is a matter of location.
At the boundaries between tectonic plates, any changes wreaked by a big quake are completely overwhelmed by the movements of the plates themselves. At around a centimetre per year, they are regular geological Ferraris. They soon "reload" the fault, dampen the aftershocks, and return the status quo within 10 years. In the middle of continents, faults move at less than a millimetre every year. In this slow lane, things can take a century or more to return to normal after a big quake, and aftershocks stick around for that duration.
Stein and Liu's study could help scientists to more accurately predict the risk of future earthquakes, especially in unexpected areas. If they're right, then it would be positively misleading to base such assessments on small quakes that could sometimes be aftershocks of historical events. In the longer term, Stein and Liu predict that such approaches will "overestimate the hazard in some places and lead to surprises elsewhere". The disastrous earthquake that hit China's Sichuan province in May 2008 highlights the catastrophic impact that unexpected mid-continent quakes can have.
To begin with, we need to better understand the network of faults that criss-crosses continents. Fortunately, such work is already underway. Palaeoseismology - a field of research that reads traces left by prehistoric earthquakes - is providing a much longer history of tremors than our pitifully short records do. Meanwhile, GPS mapping can reveal places where plates are being deformed. These are the sorts of data that will allow us to separate the aftershocks of earthquakes past from indicators of future quakes.
Again, New Madrid proves the principle - a cluster of large earthquakes hit the area in the past thousand years, but the crust shows no sign of recent deformation according to two decades of GPS measurements. It seems that recent activity really is the legacy of centuries-old quakes, a threat that has since shut down.
Reference: Nature doi:10.1038/nature08502
