Kathmandu earthquake nightmare not yet over

The devastating magnitude 7.8 earthquake that rocked Nepal on Saturday could be repeated within the next decade or two, geologists say. The enormous quake, which struck an area in central Nepal between the capital Kathmandu and the city of Pokhara, has had a far-reaching impact. Over 3000 people are already known to have lost their lives, with victims as far afield as Bangladesh, India, Tibet, and Mount Everest where avalanches were triggered. And the possibility of aftershocks, in addition to the risk of landslides on slopes made unstable by the quake, mean the danger is far from passed.

Last year, reporting for Cosmos, I travelled to Nepal to meet with the geologists working on the ground there trying to identify where the next big earthquake would strike. They feared that a major quake was imminent [see Kathmandu’s earthquake nightmare].

With tragically prescient timing, members of that team working in the region last month identified a historical earthquake pattern and anticipated a large earthquake in precisely the location where Saturday’s quake occurred. Laurent Bollinger from the CEA research agency in France and his colleagues believe that this week’s  earthquake was primed over eighty years ago by Nepal’s last massive earthquake, a magnitude 8.2 quake that flattened around a quarter of Kathmandu in 1934, killing over 17,000 people.

Earlier this year Bollinger and his colleagues were working in the jungle in central southern Nepal, digging trenches across Nepal’s main earthquake fault (which runs for over 1000km from west to east) and using charcoal fragments caught up in the fault to carbon date when the fault last moved. Using the same technique in a region to the south-east of Kathmandu, the team had already shown this segment of fault had produced major earthquakes in 1255 AD and then again more recently in 1934.

But until this year the team had never managed to pin down the location of the rupture in the segment of fault lying to the south-west of Kathmandu. Bashing through dense jungle and trekking up dry river beds they eventually found the tell-tale offset of rock layers in the landscape and set about dating when it had last moved. “We showed that this fault was not responsible for the great earthquakes of 1505 and 1833, and that the last time it moved was most likely 1344,” says Bollinger, who presented his findings to the Nepal Geological Society  two weeks ago.

Immediately this rang alarm bells for the scientists. Large earthquakes have a habit of passing strain along the earthquake fault, like slowly toppling dominos, so that one earthquake primes the next. In this case they could see a worrying historical pattern. The 1255 earthquake, that ruptured the eastern segment of the fault, appeared to have transferred strain westwards, priming the major earthquake that occurred eighty-nine years later, in 1344, on the western segment of fault.

The eastern section ruptured again in 1934, and Bollinger and his colleagues could visualise history repeating itself.  “We could see that both Kathmandu and Pokhara would now be particularly exposed to earthquakes rupturing the main fault, where it likely last did in 1344, between the two cities,” explains Paul Tapponnier from the Earth Observatory of Singapore, who was working with Bollinger. And indeed on Saturday history did repeat itself, releasing the strain 81 years after being primed by the 1934 earthquake.

However, early indications suggest not all of the strain has been released from this segment yet. “Early calculations suggest that this magnitude 7.8 earthquake is probably not big enough to rupture all the way to the surface, so there is still likely to be more strain stored and we should probably expect another big earthquake to the west and south of this one in the coming decades,” says Bollinger.

Also of interest: Kathmandu’s earthquake nightmare