Aftershocks Could Raise Earthquake Risks for Vulnerable Cities

Tokyo escaped damage in the 2011 earthquake and tsunami. But a new study indicates that aftershocks from the quake may have increased the seismic danger the Japanese megacity faces

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Devastation from the 1923 quake that leveled Tokyo. A similar earthquake today could cause $1 trillion in damages

As catastrophic as the 2011 northern Japan quake and tsunami was—killing more than 15,000 people and causing more than $200 billion in economic damages—in a way, the country was lucky. Tokyo, a city with a metro population of more than 35 million, was 250 miles away from the center of the quake, so it largely escaped any direct consequences. A quake closer to Tokyo would have been unimaginably devastating—and it’s happened before, with the city being virtually destroyed by quakes in 1855 and 1923. That last temblor killed as many as 140,000 people. The death toll wouldn’t be as high today, thanks to Japan’s strict building codes and earthquake preparation, but the price tag could be as high as $1 trillion. I lived in Tokyo in 2006 and 2007, and I remember waking up some mornings to a slight swaying from a small shock. Every time I descended into the city’s labyrinthine metro system, I couldn’t help wondering if today would be the day the big one hit.

But here’s a scary thought: it’s possible that the 2011 quake, though it was far from Tokyo, may have made that big one more likely. In a new article in the journal Science, researchers found that in the wake of the 2011 Japan quake, the rate of small shocks beneath Tokyo increased by a factor of 10. Today the quake rate in Tokyo is three times what it was before the 2011 event. (Something similar happened to the Chile capital of Santiago in the aftermath of a major 2010 quake in that country.) It seems that large quakes can increase seismicity risks well beyond the immediately impacted zone—and that could mean that the risks of a large earthquake striking those capital cities could be increasing in ways scientists don’t yet understand.

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Study authors Ross Stein of the U.S. Geological Survey and Shinji Toda of Japan’s Tohoku University calculate that the stress imparted by the earthquakes near both Santiago and Tokyo seem to have brought faults that run beneath those cities closer to failure. That’s worrying, especially given the fact that a 2012 study estimated that Tokyo has a 70% chance of being hit by a magnitude 7-plus quake by 2016, about 25 years earlier than the Japanese government had predicted.

But then, earthquake prediction is still extremely inexact, hampered in part by insufficient data on seismic risks in much of the world. Stein and Toda note that individual countries often employ independent approaches to earthquake data collection and classification—even the U.S. and Canada have different systems—which hampers efforts to create an accurate global assessment of risk. Fortunately, that may be changing—the Global Earthquake Model (GEM) was launched in 2009 to produce what will be the authoritative standard for assessing and communicating quake risk. In 2014 GEM will launch a model that will include not just the seismic hazard, but also calculations of the lives and money that could be saved if cities improve earthquake resilience. As I’ve written before, prevention can be the difference between a quake that inflicts fatalities and one that just inflicts financial losses. But that will only work if prediction can get ahead of risk.

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