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,” explainsStephen 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.
