November 12, 2019
Climate change and the hurried retreat of glaciers could add to the seismic risk in the Himalayas, one the most complex earthquake systems in the world
Seismologists and geologists studying the Himalayas have been trying update the seismic map of the mountain belt for several years now. Their urgency comes from the expectation of one or more high-magnitude earthquakes in the region in the 21st century.
If the task of assessing the risk to life and property in the Himalayas was not tough enough, it has been further complicated by climate change.
“Climate change may facilitate not only rainfall, not only GLOFs [glacial lake outburst floods], not only drought, not only heavy rain and flash floods but also the modulation of seismicity which is an important phenomenon,” says O.P. Mishra, geophysicist and seismologist at the Geological Survey of India and former head of the geological disaster division at the SAARC Disaster Management Centre.
For more than a decade, scientists have warned that the Himalayas are primed for a great earthquake, that is an earthquake that measures 8 or above on the Richter scale. The last great earthquakes occurred in the first half of the 20th century and a long-standing theory has been that, since that period, large strains are likely to have built up in the system again.
“From our deformation studies we have found that the whole Himalayan belt has accumulated enough stress to trigger great earthquakes anytime, anywhere in the belt,” says J.R. Kayal, former deputy director general and head of geophysics at the Geological Survey of India.
It has been thought until recently that all the earthquakes in the Himalayas were caused by the thrust mechanism of the Indian plate moving below and pushing up the Eurasian plate. But new evidence suggests that the picture is more complicated and far from complete.
Malay Mukul, associate professor in the Department of Earth Sciences at the Indian Institute of Technology Bombay, has been studying the Sikkim Himalayas since the late 1990s. Mukul found that the Sikkim Himalayas experience strike-slip earthquakes, where two parts of the earth’s crust move sideways against each other. The Sikkim earthquake of 2011 was once such strike-slip earthquake.
The presence of strike-slip earthquakes calls for an adjustment in calculations of seismicity in the Himalayas, as Mukul explains. “The strain in a mountain belt is like putting water into a beaker. You keep filling it up, once it fills up and you get an earthquake. Then it’s like emptying the beaker. If you have a strike-slip earthquake also going that might be actually telling you that the beaker is leaky. It is not filling up at a constant rate. You have a leak somewhere and things [strain] are getting used up in a different way. In that case, if that’s true, then it should also reduce the hazard.”
Strike-slip earthquakes also show that one model cannot explain the seismicity for the Himalayas as a whole. Mukul’s and other research shows that earthquakes in the Sikkim Himalayas also occur at a greater depth than the thrust earthquakes.
Mishra calls the earth’s crust at the Himalayas a ‘factory of faults’ where many faults which are known to be seismogenic (earthquake generating) and several unknown faults are also probably seismogenic but have not been investigated thoroughly.
American and Canadian researchers recently published their findings from Nepal of a fault 63 kilometre long in the higher reaches of the central Himalayas. They suggest that this previously unknown fault is an active fault and that the discovery calls for changes to current seismic assessments.
Melting glaciers exacerbate earthquake risk
Climate change provides a twist in this already complex mosaic of factors. A warming Himalayan climate could witness more rain. More rain can hasten the melting of glaciers. “If there is climate change and if there is a soaring of temperatures going on and if the melting of the glaciers is facilitated, then the ice sheet melting leads to the loosening of the lithostatic pressure (vertical pressure on the underlying crust),” Mishra says. As a glacier retreats and its weight eases, the earth could show a tendency to bounce back up in the form of a moderate or even a strong earthquake.
Such earthquakes are more likely in the Arctic and Antarctica where enormous chunks of glacial ice are lost. In the Himalayas, glacial melt could be a contributing factor in tipping over in an already stressed system.
A new study from the Divecha Centre for Climate Change at the Indian Institute of Science says that in the last four decades the Himalayas have lost anywhere between 300 and 600 gigatonnes of glacial mass. The total glacial mass of the Himalayas is between 3,600 and 4,400 gigatonnes. The authors Anil V Kulkarni and Yogesh Karyakarte analysed 11,000 square kilometres of glacial area in the Indian Himalayas, compiling data from satellite images, aerial photographs, topographic maps and field studies from agencies like the Geological Survey of India, the International Centre for Integrated Mountain Development and Indian Space Research Organization. The data shows that 13% of glacial area has been lost.
Mishra is certain that if one were to plot the effects of climate change against seismicity one would see a strong correlation between the retreat of ice sheets and increased seismic activity. “Since 1954 you can see the rate of seismicity drastically increased,” he says. “It may be well corroborated that the ice sheets are melting and the zones are becoming seismically active.”
Can earthquakes trigger climate change?
A warmer climate has the capacity to trigger earthquakes and the converse, Mishra says. It is also conceivable that an earthquake could trigger climate change. Great earthquakes can disturb the axis of rotation of the earth. Since the earth’s climate and seasons are determined by this axis of rotation, these would be disrupted as well.
To have a better idea about these risks and how imminent they may be, Mishra calls for an integrated approach in all branches of research of the Himalayas – glaciology, seismology and geology.
Researchers studying the Himalayas may differ on the mechanisms of earthquake generation, or about whether the beaker of seismicity is half full or half empty. But they all agree on two things.
First, more research is required to understand how, why, when and where earthquakes may occur. Second, the best way to protect the lives and property of the 50 million people living in the region is to enforce building codes. “With the advancement in our knowledge western countries have reduced their casualties to zero but in India our casualties have grown ten-fold because of the population growth. So we haven’t really learnt our lessons,” says Kayal.