Kathmandu lies in the southern foothills of the Himalayas, where the Indian and Eurasian tectonic plates collide: earthquakes are a fact of life there.
But a team of French and Nepali geologists working in the country have discovered data in recent years that leaves them unusually concerned.
By studying historical records and old seismic scars, they found that major 8.0-plus-magnitude earthquakes struck in 700-year cycles as the plates below the mountains slipped in a devastating release of energy.
And, says tectonics researcher Paul Tapponnier, “it became clear that the region between Kathmandu and Pokhara had not seen a very large earthquake since 1344” — just over 670 years ago.
On Saturday, a 7.8-magnitude quake rocked central Nepal exactly where the team described, killing more than 4,700 people. The researchers had presented their findings at the Nepal Geological Congress just three weeks ago.
Their story conveys how difficult it can be to translate the science of seismology, which deals in yawning geologic timescales, into evidence that can affect human lives, which flit out after mere decades.
“If you have 10 (earthquake) events over 10,000 years, you can average it out. But that doesn’t mean each cycle will happen on that year,” said Laurent Godin, a structural geology and tectonics professor at Queen’s University and a Himalayas expert. “It’s not that precise, unfortunately.”
That uncertainty feels even more painful in light of early measurements of Saturday’s quake: it may not have been the “Big One” scientists predicted. Tapponnier and his colleagues say Saturday’s quake probably only released a fraction of its potential energy.
A much larger 8.1-8.3 magnitude quake, which struck eastern Nepal in 1934 and killed more than 10,000 people, shows what the region is capable of.
“There are other segments locked to the West and probably to the South of Saturday’s rupture,” said Laurent Bollinger, also a member of the French-Nepali team and a scientist at the CEA, France’s energy institute.
“However, we are unable to say whether these segments will rupture within the next days or decades.”
Courts have not always been sympathetic toward the uncertainties of seismology. After a 2009 earthquake in L’Aquila, Italy, six seismologists were convicted of manslaughter for failing to adequately warn the public of risks. The verdicts drew sharp outcries from the scientific community and the sentences were overturned last year.
Godin hopes that better forecasting tools will one day be available: colleagues of his are studying chemical signatures in underground water to see if they betray when the earth becomes unstable.
“Can we predict earthquakes? The answer is no,” said Peggy Hellweg, operations manager at the University of California’s Berkeley Seismological Laboratory. “But the other problem with earthquakes — and society, essentially — is that people forget that they need to be prepared.”