Megaquake aftershocks pose lasting danger
Tokyo, Santiago at risk after megaquakes 400 kilometres away
Scientists are warning residents of Vancouver and other cities in earthquake zones to take heed of new evidence that megaquake aftershocks can pose a great danger for years, even hundreds of kilometres from the epicentre of the original earthquake.
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.
They also warned that other cities within reach of earthquakes — such as Vancouver, Taipei, Manila, Lima and Jakarta — could suffer a similar fate.
"Mine is a humbling science. We cannot predict earthquakes and we cannot tell the public what they really deserve to know — which is how big, when and where," Stein told CBC News.
This is particularly true for the West Coast of Canada...so be prepared
"This is particularly true for the West Coast of Canada with the great subduction earthquake off somewhere in its future, something along the same scale as what Chile and Tokyo have just experienced. And so the best we can do is say there is a modest probability, over let's say the life of your home mortgage, of experiencing that earthquake, so be prepared."
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.
"Although we cannot predict earthquakes, we are better at predicting aftershocks and earthquake sequences and that information allows us to take hazards more seriously," Stein told CBC News.
Without this knowledge, Stein said, governments will be reluctant to act.
Carlos Ventura, a professor of civil engineering at the University of British Columbia, said if a megaquake were to strike California, its aftershocks could be felt in Vancouver years later.
"They're going to last longer and there's going to be more of those," Ventura said, adding that while B.C.'s monitoring system is a good start, officials should modernize building codes to account for aftershocks.
Ventura also said international data is difficult to import because monitoring models around the world are not standardized.
"We say earthquakes do not recognize borders, but our models do," he said.