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
November 5th, 2009 at 6:24 pm
Tom Parsone of the USGS is perfectly right. You cant forecast mid-continent earthquakes. The truth is that you cant forcast earthquakes. Earthquakes are unpredictable by their very nature. They are a manifestation of expansion of the planet on which we live. As such they occur anywhere, everywhere and anytime on the face of the earth as long as the earth continues to expand. We have to realize that river valleys are earthquake expansion points or faults or rifts: call it what you may. The New Madrid Fault line is by no means inactive just take a look at the plethora of annual tremors on the USGS maps and see for yourself. So when we say that earthquakes along the New Madrid fault are aftershocks of the 1811-1812 earthquake we miss the point. What we should be saying is that the fault is still actively expanding. That is what causes the quakes. Engineers dont know much about earth expansion even though bridges across the Mississippi keep falling into the river with loss of lives. We have to realize that the earth is enigmatic and hides many secrets from us.
My concern is that we dont understand why earthquakes occur because we dont understand that the planet is expanding. Planet earth was once the size of the Moon but it is today 7800 miles across and growing. It will continue to grow to perhaps twice that diameter. We keep missing the point because we stick to old concepts. We have been taught these concepts and they are all wrong. e.g.
Darwin studied raised beaches all across the Pacific and on the South American Continent. He was intrigued by how high the beaches had raised out of the sea. A obvious observation but his interpretation of that observation was wrong. It would never have occured to him that the opposite was true: the sea had retreated from the land leaving the beach behind. Darwins observation still rules earth science today. Other theories have been based on that observational mis-interpretation namely “Isostatic Rebound” The moot question therefore is; did the land rise from the sea or did the sea recede from the land? Geologist would welcome acceptance of Earth Expansion it would solve many puzzles; no doubt because they have been taught flawed theories. Once we accept earth expansion we will understand earthquakes; Once we get to that stage we will have to look at the receding seas for the final peg in the coffin of the “Isostatic Rebound” theory. Our seas have been receding as long as the earth has been expanding. Your guess of that time I leave to you. You can read more at: http://www.widemargin2000.com or you can email me at: richard_guy72@yahoo.com. You can also see my videos on http://www.youtube.com and my articles on Google under the key words. “Receding Seas”
November 6th, 2009 at 2:13 am
Expanding Earth theorist! Huzzah! Another checkbox in my collection of woo. The usual collection of creationists and alt-med people turned up ages ago but this is special…
November 6th, 2009 at 12:23 pm
If the earth is expanding, then the rest of the universe must be contracting?
November 6th, 2009 at 12:43 pm
Um, dick guy, sea level is rising, not falling. Does that mean the Earth is shrinking?
“Current sea level rise has occurred at a mean rate of 1.8 mm per year for the past century,[1][2] and more recently at rates estimated near 2.8 ± 0.4[3] to 3.1 ± 0.7[4] mm per year (1993-2003). ” (from teh wiki)
November 6th, 2009 at 4:38 pm
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November 7th, 2009 at 11:45 am
I see that Mr Ed Yong gives short shrift to the postulations put forth by Mr Richard Guy.
Yet the global warming “crisis” is based on much flimsier evidence than Mr Guy presents and the same lock-step thinkers who pooh-pooh Mr Guy will be hooraying Mr Gore and his band of no-nothing thieves
November 8th, 2009 at 7:12 am
oes that mean the Earth
November 9th, 2009 at 9:16 am
great article, but really Richard Guy? Did we not pay attention to the lesson on subduction?
and @jay, he’s being mocked because it’s a completely stupid theory that looks to be completely made up. There’s absolutely no evidence for it period. Not sure where and how you made a connection to global warming, but how nice for you.
November 9th, 2009 at 2:56 pm
You have it wrong there gecengece. There is no evidence of subduction. Subduction is like oil on water and you say the oil will go under the water. Have a look at Neal Adams’s videos on the subject and you will see it makes much more sense than the continental drift story. Expansion is happening on all the planets and their moons.
I have a theory about the moons origin that is linked the the expansion theory and it looks like we will have a second moon soon.
November 9th, 2009 at 6:35 pm
So, how does one distinguish between an earthquake and an aftershock?