100 Year Old Aftershocks?

Great now we get to be freaked out not only by recent earthquakes, but also by earthquakes from 100s of years ago. Thanks a lot science.

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. (Not Rocket Science)

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