How well do scientists understand the seismicity of the Himalayas?
What makes the Himalayan region a hotbed of seismic activities? The answer lies in the processes which led to the formation of the mountain range. The youngest range in the world, the Himalayas were formed due to the collision of the Indian plate with the Eurasian plate 40-50 million years ago. The Indian plate has been sliding under the Eurasian plate ever since. And this process is not over. The contact surface between the two plates is where pressure builds up and causes major earthquakes.
This is exactly where Nepal is located. But why was the earthquake so destructive this time? There are two reasons for this. First, the epicentre was just 15 km below the surface and this amplified the impact. Some previous earthquakes in the same area have had epicentres as deep as 200 km.
Second is the location of Kathmandu, one of the worst-affected areas. Just 140 km from the epicentre, Kathmandu sits atop a lake basin. Over the ages, the basin got filled with more than 600 m of soft sediment. When a seismic wave passes through a layer of sediment, it makes the sediment behave like jelly. The process is called soil liquefaction. Earthquake waves travel at a high velocity through the stiff, crystalline rock of the crust but slow down dramatically when they enter the basin. This increases their amplitude and causes stronger tremors. In addition, the sharp contrast in the densities of the softsediment in the basin and the rocks that surround it can cause the waves to reflect, trapping energy in the basin for a longer period. This extends the duration of shaking.
Though the Himalayas are prone to seismic activity, there is very little information on earthquakes that occur in the region. Most of what is known has emerged in the past 20 years, after the development of GPS technology which made exact measurement of plate movement possible.
It is now known that the plates move around 45 mm a year. Of this, around 18-20 mm shift is accommodated by the thrusting of the Indian plate beneath the Himalayan belt. Due to this, the Himalayas advance over India by about 2 m each century and the Indian plate disappears by an equal distance beneath Tibet. “There is friction between these two plates and they stick to each other. The down-going Indian plate tries to drag the overlying plate and after some time, say tens or hundreds of years, when the stress due to the movement of plates exceeds the frictional strength, the two plates suddenly get unlocked. That’s when you have major earthquakes,” explains Vineet K Gahalaut, geologist at Hyderabad-based National Geophysical Research Institute.
This theory of strain build-up and release during an earthquake has been known for some time but there was little evidence to substantiate it. This missing piece of evidence in the jigsaw has been provided by the Nepal earthquake, says Supriyo Mitra, associate professor at the department of earth sciences, Indian Institute of Science Education and Research, Kolkata. “We now know that the Nepal earthquake ruptured an approximately 150 km by 70 km area of the locked surface of the Himalayan front, lurching the whole block forward by over 10 m on the Indian plane, with Kathmandu sitting on top of it. This knowledge could be useful for further studies as a bigger earthquake is likely in the area,” says V K Gaur, honorary scientist at the Council for Scientific and Industrial Research’s Fourth Paradigm Institute, Bengaluru.
The magnitude of the earthquake depends on the amount of the slip the plates undergo. Till a major earthquake actually occurs, scientists refer to regions of accumulated potential slip as seismic gap (see ‘Mapping the gaps’). Seismic gaps are prone to earthquakes because the accumulated strain beneath the surface has not been released.
“We know that there are a number of seismic gaps, each spanning 200 km or more, in the Himalayas which can produce earthquakes of greater than 7.5 magnitude,” says Gaur. For example, the region to the west of the present Nepal earthquake has an accumulated potential slip of approximately 9 m, while the accumulated potential slip in the region to its east is approximately 1.5 m. Though we do not know when these earthquakes will occur, we can say that if it were to happen today, and if it released the entire stored energy, it will be an earthquake of magnitude more than 8 on the Richter scale.
“We have a fairly accurate knowledge of the current level of pent up energy along these seismic gaps, but we do not know the strength of the frictional locking at the Himalaya-Indian plate because it is very variable. It is this strength that decides how much strain a given segment can bear without breaking,” says Gaur. Therefore, it is difficult to predict an earthquake.
According to Eric Kirby, geologist at Oregon State University, USA, GPS monitoring and geological studies suggest it would take scores of magnitude 7 quakes to accommodate all of the plate motion, but only a handful of mid-size, magnitude 8 quakes, or just one of magnitude 9. The energy released by a quake increases by a factor of 30 with each additional point in magnitude and would lead to great devastation. People residing in seismically active areas should always be prepared for major earthquakes, says L S Chan, professor at the department of earth sciences, University of Hong Kong. There is no evidence that the energy is being dissipated through aseismic means (mechanisms other than earthquakes) and earthquakes will eventually recur in such areas, he says.
Uttarakhand in seismic gap
The most prominent segment of the Himalayan front that has not witnessed any major earthquake in the past 200–500 years is a stretch on which Uttarakhand lies. The state has a population of over 10 million. It is crucial to understand that a big earthquake is overdue in the region, says C P Rajendran, professor, geodynamics unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru. “The Nepal quake has released only a small fraction of the accumulated strain,”
| Decoding earthquakes
From seismological studies, we know that earthquakes always occur in brittle rocks and rupture due to frictional failure.
From geological and geophysical evidences we know that 40-50 million years ago, the Indian plate collided with the Eurasian plate and initiated the formation of the Himalayas. These studies also helped in understanding the behaviour of rocks under different temperature and pressure conditions.
The study of GPS geodesy in India began in the 1990s to quantify the convergence of the Indian plate with Tibet and the deformation associated with it.
“In fact, Indian seismologists have shown a tendency to opt for less challenging problems and distract attention from zones prone to major earthquakes to zones which witness moderate earthquakes. The Indian government has been persuaded to sink hundreds of crores of rupees in drilling a deep hole at Koyna in Maharashtra which is the site of moderate seismic hazard.
The Koyna experiment is unlikely to shed light on the mechanism of the Himalayan earthquakes,” Gaur says.
According to Harsh Gupta, seismologist and former secretary at the Ministry of Earth Sciences, the problem of understanding earthquakes is not limited to India. Even Japan could not accurately measure the susceptibility of the region which witnessed the Fukushima earthquake in 2011. The maximum magnitude predicted for an earthquake in the region was 8 on the Richter scale, but the Fukushima quake which caused the unprecedented tsunami measured 9.
Gahalaut says we only have a broad understanding of earthquakes. It is not understood what happens just before an earthquake. Developing such understanding will help in identifying precursors to an earthquake and formulate methods to predict earthquakes. Another issue is lack of historical records of earthquakes. “We want to know when was the last major or great earthquake in this region, its size, its rupture,” he says. There is also a need to increase regional cooperation among the countries in the Himalayan region—India, Pakistan, China, Bhutan and Nepal. “Earthquakes do not respect international borders,” he adds.
Human-made factors at play?
Apart from geological theories, some scientists also hold anthropogenic factors responsible for earthquakes. Bill McGuire, professor emeritus, geophysical and climate hazards, University College London, is of the view that climate change can affect the structure of the earth. Evidence from the Ice Age indicates that seismic faults are sensitive to small changes in pressure caused by the weight of ice and water on land. This makes it likely that effects of climate change such as melting of ice, rise in sea-level and floods would change the distribution of weight on the planet. McGuire has written a book, Waking the Giant: How a Changing Climate Triggers Earthquakes, Tsunamis and Volcanoes, on the topic, which was published in 2012.
A 2008 research led by French geologist Pierre Bettinelli explains how this could be working. His team used a decade of data from GPS receivers and satellite measurements of land-water storage to connect the winter season with the frequency of earthquakes along the Himalayan front. The team then analysed around 10,000 earthquakes in the Himalayas and found that there were twice as many earthquakes during the winter months—December through February—as during the summer. In the Himalayas, monsoon rains swell the rivers of the Ganga basin, increasing the pressure bearing down on the land. After the rains, the riverwater soaks into the ground and the built-up load eases outwards with the flow of the rivers/streams, towards the front of the range. This outward redistribution of stress triggers earthquakes in winters. The findings were published in 2008 in the journal Earth and Planetary Science Letters.
There is another theory that links dams to earthquakes. Globally, there are over 100 identified cases of earthquakes that scientists say were triggered by reservoirs, according to a report on the website of Rivers International, a global network of ngos and experts. The most severe one may be the 7.9 magnitude Sichuan earthquake in May 2008 which killed 80,000 people and has been linked to the construction of the Zipingpu dam on the Min river. Experts explain that the extra pressure created on micro-cracks and fissures in the ground under and near a reservoir can lead to earthquakes. The dam was just 500 m from the fault that failed and 5.5 km from the quake’s epicentre.
However, most experts disagree with theories that blame human-made factors for inducing earthquakes.“Weather, rain or snowfall do not seem to contribute towards the occurrence of great earthquakes. They do cause annual variations in the plate movement and have been seen to cause small magnitude seismicity but not a large one,” Gahalaut says.
As researchers work to understand earthquakes and develop capacity to make accurate predictions, there is a lot that can be done to minimise the destruction. Japan, which has the most advanced earthquake prediction system in the world, can predict quakes only few seconds before they arrive. The Japanese, therefore, construct buildings which are resistant to earthquakes. This is the only way to prevent damage to life and property.
| Rudimentary research
At a time when there is a concentrated global effort to better understand the movements of the plates, and accumulation and release of strains, such incidents show governmental apathy, says V K Gaur, honorary scientist at Bengaluru's CSIR Fourth Paradigm Institute.
However, Harsh K Gupta, former secretary, MoES, says that the ministry has several projects in the Himalayan region. It has set-up multi-parametric laboratories in critical areas in the Himalayas to observe geophysical parameters and precursors to earthquakes. Additionally, under a very popular programme, many schools in the Himalayan region have been provided with seismological labs so that students can see earthquakes being recorded and analysed. MoES also plans to install Earthquake Early Warning system in a critical location in the Kumaun region, he adds.
There are other government institutes in the country where earthquake research is being done. In Hyderabad-based National Geophysical Research Institute, researchers are doing different seismological, geodetic and paleoseismological studies to assess the seismic hazard of the region. "We have several observatories in the Himalayan region," says V K Gahalaut, geologist at the institute.
At CSIR Fourth Paradigm Institute, scientists are engaged in evaluating hazard potential of habitats located on softer soils, such as the Kashmir valley. There are also plans to study the accumulation of strains in the Himalayas, says Gaur. However, the quality of these research projects will be proved only when the results are released.
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