Nepal: One year after the quake
Last year on April 25, Nepal witnessed the most devastating earthquake in its recent history. One year down the line seismologists have made a significant contribution in mapping the region to understand its seismicity.
Since the 7.8-magnitude quake shook Nepal, the country has shrunk in size and a number of physical changes have taken place. Following the temblor, some of the country’s physical features have become elevated, and at the same time Mount Everest, the tallest peak in the world, has decreased in height by a few millimetres.
Mapping physical changes
A number of studies have confirmed the lateral movement of the Himalayan region, including Kathmandu, towards the Indian sub-continent after the Nepal quake. The Indian Institute of Science Education and Research (IISER), Kolkata, has estimated the magnitude of this shift to be about 4.8 metres with a margin of error of plus or minus 1.2 metres. The study collected primary and secondary seismic wave data from a number of seismographs, including the one situated on the institute campus.
A complete picture of the physical changes in Nepal resulting from the quake is provided by geodesy. Geodesic assessment of ALOS-2 (Japanese satellite) images has confirmed that there was an elevation of Kathmandu and the surrounding region as well as a lateral movement of the entire area towards the Indian subcontinent. The images show that the land around Kathmandu moved towards the satellite by roughly 1.5 metres. On the other hand, the land lying northwards to Kathmandu moved away from the satellite. The study was carried out by scientists from the California Institute of Technology. Scientists involved in Advanced Rapid Imaging and Analysis, a NASA Jet Propulsion Laboratory initiative, were also involved.
According to the US-based National Oceanic and Atmospheric Administration, geodesy is the science of accurately measuring and understanding the Earth’s geometric shape, orientation in space and gravity field. Post the earthquake, there was a movement of landforms in all three dimensions. “...try holding your hands in a namaste (folded palms) position inclined at a small angle. Your right hand, which is under the left, is the Indian plate, the left hand over it is the Eurasian plate and the interface between the two represents the Main Himalayan Thrust (the interface between the Indian and the Eurasian plates) which has a shallow inclination of about 10 degrees. Now, if you slide your left hand over the right, you will notice your fingers curl upwards, showing how elevation and lateral movement can be simultaneous,” Revathy Parameswaran, a scientist at the Indian Institute of Science (IISc) in Bangalore said.
IISc’s analysis of Nepal was based on a field study and data collected from seismic stations across the world. These stations pick up seismic waves and invert the waveform to see what kind of motion occurs at the hypocenter and epicenter of a quake.
As capital Kathmandu and its surrounding area became elevated, the region lying at the back of the city decreased in height. According to a study by the UK-based Centre for the Observation and Modeling of Earthquakes, Volcanoes, and Tectonics, the highest peaks in the Himalayas decreased by about 0.6 metres in the first few seconds of the quake.
Narrowing of Nepal
With Kathmandu and the region north of it shifting after the quake and the southern region remaining intact, Nepal experienced narrowing. An interesting thing is that the southern part of the country did not experience any slip even when the entire country is part of one fault plane.
C P Rajendran of the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, said this indicated some sort of heterogeneity between the northern and southern parts of Nepal. “There can be differences in the rheology, geology and geometry of these two regions,” Rajendran added. For instance, there can be more rocks in the south (geological differences) which blocked the advancement of the rupture.
Differences in the frictional properties of these two regions (rheological differences) may have restricted the southern region’s slip.
Experts suggest that changes in the physical entity of the region are not permanent in nature. To understand the situation, consider a slice of bread squeezed by external pressure. Constriction in the Kathmandu region and the force causing it is resulting from the collision of the Eurasian and the Indian plates.
This has forced Kathmandu to slip from its original position. But the squeezing of the bread slice ceases to exist when external pressure is removed. “Similarly, seismic stress will be relieved and Kathmandu will come back to its original position. The mountains will also build up,” Parameswaran added. However, in Nepal’s case, the post-seismic relaxation has not occurred as yet, Supriyo Mitra of IISER said.
Liquefaction of soil and sand blows
A significant physical change that resulted from the Nepal earthquake is the liquefaction of the region’s soil. Liquefaction occurs when soil particles loosen up and tend to flow like a liquid. This spells trouble for structures as well as people.
While Parameswaran said such a thing has persevered over time, Mitra said this was a temporary aspect. The Himalayan Monitoring and Adaptation Programme’s (HiMAP) post-quake assessment of glacial lakes in the region revealed that instances of soil liquefaction are “not really a cause for concern”.
The Nepal earthquake has forced us to rethink the seismicity of the region. According to our present understanding, earthquakes in the Himalayan region result from the collision between the Indian and the Eurasian plates at the rate of 18 millimetres per year.
“This is comparable to the rate at which your fingernails grow. You don’t see them growing every day but cumulative growth becomes noticeable after certain weeks or months,” Parameswaran added.
Rocks on the upper surface (15-20 km) of the Main Himalayan Thrust (MHT) are cold and brittle. This shallow segment accumulates strain. Further down the zone, the MHT shifts into a warmer region with temperatures more than 350 degrees Celsius. When the strain in the lower zone exceeds a particular limit (2 cm per year), the segment ruptures releasing energy in the form of an earthquake.
Post-quake analysis of Nepal has added new dimensions to this understanding. “This earthquake tells us that Himalayan events can occur over a very wide range of sizes. Not all Himalayan earthquakes will break the whole fault interface between India and Tibet. If the residual strain is stored in sections of the fault interface, then observations of displacement from the historical and paleoseismic record may be misleading about the magnitude, frequency and recurrence interval for quakes,” Rebecca Bendeck of the University of Montana, US, said.
“The characteristics of shaking in the Nepal quake were unusual, with little high-frequency energy that may have reduced the amount of destruction. This was very lucky for the people, but we do not know if future earthquakes will happen in the same way.”
Understanding the recurrence of quakes
Scientists are trying to improve the prediction of the recurrence of earthquakes. The recurrence interval between two quakes in a particular region depends on the magnitude of the last temblor and the rate of collision of two continental plates that led to the event.
Scientists have tried to accurately measure this recurrence interval in a number of ways. One of the most widely used ways is the analysis of the organic material found in the sediments of the Main Frontal Thrust, a region where the MHT meets the Earth’s surface. It is then followed by carbon dating to find the age of the fault. This gives us a measure of the last time the fault was seismically active. An analysis of a number of faults in a region can give us the recurrence interval between two quakes.
Using the technique seismologists have predicted the recurrence interval of quakes in different regions of the world. For instance, the recurrence interval for Himalayan quakes, such as the one in Nepal, is about 100-200 years. The recurrence interval for quakes in the Hindu Kush-Himalayan Region (HKH), which are a result of subduction of the oceanic part of the Indian plate under the Eurasian plate, is about 50-100 years, Mitra said.
He, however, pointed out the loopholes involved in this method. For quakes that do not cause surface ruptures, an analysis of the fault cannot happen as a segment of it that originally moved lies underground. Then again while the last seismic activity of a fault can be predicted, to predict the recurrence interval requires analyses of a number of such faults in a particular region. There may be faults in a region that we are unaware of.
Lastly, averaging is a contentious step. “If we simply take the known events and average the time interval among them, then indeed we get an average recurrence interval. However, this leaves out all the complicated interactions of earthquakes and other geophysical phenomena. We know this because if we look at earthquake sequences in places where we know about multiple events, the variation in the time interval among them can be much larger than the average interval,” Bendeck added.
Sequences often include several events relatively close together in time and then long gaps. The average does not really represent the likelihood of an event if incidents are irregular.
Predictions and warnings
New insights into the Nepal region offer us a better understanding of the vulnerability factor.
As mentioned before, the area south of Kathmandu was crumpled by the quake. So, there is a greater stress in the zone towards Terai and lesser stress north of Kathmandu. Some seismologists have attributed the stress in the south to the slowing down of the rupture under the capital.
“Based on our past experience of earthquakes in the region, we expect earthquakes in the areas northwest and southeast of the rupture zone,” Mitra told Down To Earth.
(Views expressed are strictly those of Down to Earth)