Scientists offer new post-quake analyses
Last year on April 25, Nepal witnessed the most devastating earthquake in its recent history.
One year down the line seismologists have made 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. The 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 sandblows
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 everyday but cumulative growth becomes noticeable after certain weeks or months,” Parameswaran added.
Rocks in 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 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.”
Is there a ramp under MHT or new kind of ruptures?
Seismologists believe that the Nepal earthquake did not result in surface rupture. However, Parameswaran and her team observed fissures on the ground in Phujel (a town in Nepal’s Gorkha district). These were observed in the east-west direction cutting across terraces as well as in the north-south direction.
Locals said that when the cracks formed, they were much deeper and they could feel their depths with poles more than a metre high. Now, all such fissures have been filled up due to either ploughing or rainfall. “We consider these fissures to be potential surface ruptures,” Parameswaran said.
Seismologists think the absence of a surface rupture is due to the fact that it slowed down under Kathmandu.
Mitra said that in the first 20 seconds of the quake, the rupture moved fast. By 30-35 seconds, it slowed down underneath Kathmandu and by 50 seconds it had split into two, never managing to reach the surface.
The question is what does the slowing down of the rupture mean. Mitra’s theory of the rupture missing the surface has been supported by a number of national and international seismologists such as Roger Bilham of the University of Colorado and Michael E Jackson of the National Science Foundation (USA). Their theory is that a gradual six-degree ramp on the MHT slowed down the rupture.
Another theory is that the rupture slowed down as there was not enough stress driving it to the surface. It has happened in the past also. “Previous models of large Himalayan quakes predicted that the whole fault would slip up to the surface, releasing all of the stored energy of the collision between India and Tibet. However, after the Nepal event, many scientists looked back into the catalogue of known historic Himalayan quakes and found that similar incomplete ruptures appear to have also happened in the past,” Bendeck said.
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 upon 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 mostly 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 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.
In the region lying south-east of Kathmandu, there has not been a quake since 1934. The region has accumulated 1.5 metres of slip which can result in a magnitude-7 earthquake on the Richter scale. The region northwest of Kathmandu seems more vulnerable. Though no earthquake has occurred here in the past 500 years, the region has acquired a nine-metre potential slip which can result in a magnitude-9 quake.
HiMAP has predicted an increase in the number and frequency of glacial lake outburst floods (GLOFs) after the Nepal earthquake. An assessment of Imja, Tsho Rolpa and Thulagi glacial lakes revealed that the quake resulted in the loss of morainal material, meaning that the mass of rocks or sediments brought down by glaciers has decreased significantly.
This means heightened chances of rocks falling and frequent avalanches as lateral moraines (parallel ridges of debris along the sides of a glacier) act as buffer against such events.
Scientists have noticed that the outlet ponds of glacial lakes widened and deepened after the quake. As they continue to become wider and deeper, these ponds may result in catastrophic floods downstream.
HiMAP chose to study these three lakes as a significant number of people live downstream and can be greatly affected by floods.
In the absence of a surface rupture, the physical damage from the Nepal earthquake was observed largely in the form of secondary effects like landslides. The chances of landslides occurring get compounded by the liquefaction of soil.
Ever since the earthquake, a number of organisations have tried to map the landslide risk in a number of regions in and around Nepal. The most recent among these is the Yale Himalayan Initiative, an undertaking by the US-based Yale School of Forestry and Environment Studies.
The initiative has produced a map that divides Nepal into four zones: low risk, moderate risk, high risk and very high risk. The study model takes into account elevation, slope, roads, population centres, rainfall pattern and drainage system of the country. Surprisingly, Yale’s report shows that geological fault lines are not as important as several other factors in predicting landslides.