Even though Greater Tokyo is located 400 kilometres from the epicentre of the devastating magnitude 9.0 earthquake that struck Japan’s east coast in March 2011, the incidence of smaller earthquakes in the city of 36 million people jumped 10-fold after the megaquake, say U.S. and Japanese seismologists in an article in the journal Science Thursday.
Today, two years later, the rate of earthquakes in Tokyo is three times higher than it was before the megaquake, likely because of the stress caused by the relatively distant quakes on the fault located beneath Tokyo.
Santiago, Chile, saw a similar increase in earthquakes after the February 2010 magnitude 8.8 megaquake, which was also 400 kilometres away.
“Aftershocks just one magnitude smaller than their main shock are common, and there is a small probability that an aftershock will be larger than its main shock,” wrote Ross S. Stein of the U.S. Geological Survey and Shinji Toda of Tohoku University. ”Thus, they cannot be dismissed as harmless.”
The authors noted that Tokyo and Santiago have each been nearly destroyed twice by earthquakes since being founded around 1600.
“Other great cities within reach of earthquakes, such as Vancouver, Taipei, Manila, Lima, and Jakarta, could suffer a similar fate,” they warned.
Megaquakes assumed to reduce subsequent risk
The recent findings about aftershocks haven’t yet been incorporated into national assessments of earthquake risk, Stein and Toda note.
In fact, models used to assess earthquake risk usually assume that a megaquake reduces the risk of subsequent earthquakes by relieving accumulated stress on the fault at the epicentre, they said.
They hope that will change with the launch in 2014 of an open model of earthquake risks and consequences called the Global Earthquake Model.
The researchers suggested that better post-earthquake monitoring and modelling based on the results is needed around the world in order to help scientists understand whether the increased rate of earthquakes in cities like Tokyo after a megaquake some distance away means that a large quake is more likely to hit the city itself.
“Without this knowledge,” they added, “governments will be reluctant to act.”
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