Decoding the dynamics of friction
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, was 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 <g data-gr-id="203">softsediment</g> 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.
Pushed around 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 <g data-gr-id="224">exact</g> 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, <g data-gr-id="230">geologist</g> at Hyderabad-based National Geophysical Research Institute.
This theory of strain build-up and release during an earthquake has been known for some <g data-gr-id="229">time</g> 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.
Seismic gaps 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 <g data-gr-id="217">seismic</g> gap. 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 <g data-gr-id="243">m,</g> 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, <g data-gr-id="238">professor</g> at the department of earth sciences, University of Hong Kong. There is no evidence that the energy is being dissipated through <g data-gr-id="202">aseismic</g> means (mechanisms other than earthquakes) and earthquakes will eventually recur in such areas, he says.
Uttarakhand in <g data-gr-id="303">seismic</g> 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,” he adds.
H N Srivastava, emeritus scientist at the Council for Scientific and Industrial Research and former additional director general of the Indian Meteorology Department, also puts Uttarakhand in the high-risk category, but says that instead of a single 700 km seismic gap, the Himalayan arc can be divided into 10 seismic gaps. In a paper published in Geomatics, Natural Hazards and Risk in 2013, the team made two categories of seismic gaps on the basis of the historical seismicity of the area. Category 1 seismic gaps are classified as those where earthquakes of magnitude 8 or greater occurred and can recur, while category 2 seismic gaps could experience earthquakes of magnitude less than 8. But the key puzzle of when will these earthquakes occur remains unsolved.
Key questions The Nepal earthquake has actually raised several important questions. For example, if the region to the west of the earthquake has a potential slip accumulation of around 9 m, while the region that witnessed the quake has a potential slip accumulation of only around 3.3 m, why did the earthquake not occur in the region where the slip accumulation was more? asks Mitra. He says that to answer such questions we need to know how faults cause an earthquake, how active faults interact and whether an earthquake in the Himalayas can trigger an adjacent fault to cause another earthquake. These questions are topics of ongoing research and we do not have conclusive answers, he says. According to Gaur, we need to improve our knowledge of earthquake cycles by understanding major earthquakes that have taken place in the past. There is also a need for more precise delineation of the locked regions in different segments of the Himalayas, beginning with the already identified seismic gaps in Uttarakhand, Kashmir and Bhutan. It is also crucial to use high-resolution seismology to create images of the slipping tectonic plates along these segments. But none of this is being done, he says.
“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, <g data-gr-id="360">seismologist</g> 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.
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.