Kate Ravilious
BBC News
May 13, 2015
Even when the aftershocks subside, the population will need to be alert to monsoon-triggered problems
The magnitude-7.3 earthquake that shook Nepal on Tuesday was an aftershock that the country could really have done without.
To make matters even worse, the epicentre of this quake lies right under one of the most landslide-prone parts of the country, making fresh landslides and flooding a real worry.
Triggered by the stress changes caused by the magnitude-7.8 earthquake just over two weeks ago, this large aftershock occurred at the eastern end of the fault segment that slipped previously.
As aftershocks go, it was atypical (the US Geological Survey had estimated a 1-in-200 chance of a M7-7.8 event occurring this week), but not entirely unexpected.
“A seismological law (Bath’s law) states that the largest aftershock is normally around one unit of magnitude smaller than the mainshock, so the large size of this aftershock is slightly surprising, but could never have been ruled out,” explains Stephen Hicks, a seismologist at Liverpool University.
However, its location is truly unfortunate. “It has happened directly beneath the area of greatest landslide density and impact caused by the 25 April earthquake, and my guess is that this quake will cause similar levels of landsliding as in the 25 April earthquake,” says Alex Densmore, a landslides expert at Durham University and part of Earthquakes Without Frontiers, a project to increase resilience to earthquakes.
In particular, Densmore is concerned about the stability of the Sun Koshi Dam, which impounds a 1.2km-long lake, created when monsoon-rains triggered a landslide that blocked the Sun Koshi River in August 2014.
Lying partway along the exceedingly precipitous Arniko Highway (which connects Kathmandu with Kodari on the Nepal-China border), this unstable temporary dam perches above the highway, whilst a number of villages lie further downstream.
Thankfully, the magnitude 7.8 earthquake on 25 April did not appear to affect the dam or reservoir, but Tuesday’s large aftershock will have shaken it considerably.
“It was close enough to the dam to potentially weaken it, or weaken the hillsides above the dam, leading to further landslides on to the dam.
“It needs to be checked as soon as possible,” says Densmore.
Right now, Densmore and his colleagues are waiting to analyse satellite images taken after Tuesday’s quake, to see if the lake level appears to have changed.
“If the lake level starts to go down without any corresponding change in the flow into the lake at the upstream end, then that would be a good sign that the dam has been destabilised.
“It would also be possible to check downstream to see if the water is more sediment-rich than normal, which would also indicate that water is flowing through the remnants of the dam,” explains Densmore.
With luck, the dam will hold for now, but the shaking from Tuesday’s large aftershock, plus the added surge of water from monsoon rains in a few weeks, could cause failure of the dam, bringing flash floods downstream.
“The floods would not necessarily be catastrophic, but the water level could rise rapidly by several metres or more, so depending on the layout of the villages and the highway, some form of evacuation might be appropriate in that case,” the Durham researcher told BBC News.
Landslides in this mountainous country are a major hazard following the two big quakes
Once they get the satellite images, Densmore and his colleagues will also be thinking about how the landscape is likely to change.
Wide-open valleys may look like the safest places to be right now, but once the monsoon rains arrive, excess sediment is likely to be washed down off the hills, burying the valleys under a deluge of mud.
“After the 2008 Wenchuan earthquake in China, some of the most productive farmland was buried under metres of sand, making the valleys uninhabitable, and survivors were moved to new towns,” says Densmore.
Right now Nepal is jittery again, and everyone is waiting with trepidation for the next aftershock.
“The probability of similar-size aftershocks is low, especially as time goes on. But again, we cannot completely rule out similar-sized or larger aftershocks,” says Hicks.
As Tuesday’s magnitude 7.3 aftershock shows, earthquakes do not take much notice of predictions.
