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How has our understanding about seismicity in the Himalayas evolved?
The Himalayan arc has evolved due to India-Eurasia convergence. In this process, the Indian plate thrusts beneath the Eurasian plate and about 2 cm of the convergence is accommodated in the Himalaya. The contact surface between the two plates, which is a northward gentle dipping plane, is referred to as the detachment and it is at this surface where great and major Himalayan earthquakes occur. Generally, the detachment under the southern 100 km of the Himalayan mountain range is seismically active and produces earthquakes. Further north, beyond this distance, the depth of detachment becomes more than 20-25 km where the rocks are ductile, and cannot store strain and hence we do not have large magnitude earthquakes in that region. Geodetic investigations (using GPS) of crustal deformation indicate that the frontal Himalayan region is accumulating strain in response to the India Eurasia convergence. If there was no friction between the two plates, the plates will slip over the detachment very smoothly but since there is friction on this surface, the two plates stick with each other and the downgoing Indian plate tries to drag the overlying plate. This happens because of friction at this contact.
GPS measurements provide unequivocal evidence for this. After some time (say some tens or hundreds of years), when the stress due to this process exceeds the frictional strength, the two plates suddenly get unlocked, and that's it, and that's when you have major earthquake. This has been seen at several places.
Many scientists have said the devastating earthquake in Nepal is not the real big earthquake expected in the region. How far do you agree with this theory? Should we prepare for a bigger earthquake?
The seismic gap is a region where a great or major earthquake has not occurred in the past 100-200 years. The Himalayan arc has experienced at least four major (7 M 8) or great (M 8) earthquakes in the last 200 years the earthquakes of Kashmir (2005), Kangra (1905), Nepal-Bihar (1934) and Assam (actually Arunachal, 1950). The regions of the Himalayan arc between these points are referred to as seismic gaps. The Nepal region falls between the 1905 Kangra and 1934 Nepal-Bihar earthquake points. The region has not experienced a great earthquake in past more than 400 years and hence a great earthquake is expected to occur there. Now how big it is going to be, and when it is going to occur, is a big question. But the fact is that if the process of strain accumulation is uniform over past 400 years, then there is enough strain in this region to cause a great earthquake. We need to create awareness among people and encourage them to follow building codes. Ultimately, it is the buildings which kill people and not earthquakes.
Geologists have also pointed out that soil liquefaction during the April 25 earthquake has caused large-scale devastation in and around the Kathmandu valley. Can you shed some light on this aspect?
Soil liquefaction is a process in which the water trapped in the sand molecules comes out due to shaking and the sand loses its strength. It's like when you thump the sand on the beach with your hand, in the beginning you do not see the water and everything looks quite solid and dry but after some more thumping, you start seeing the water and the material loses strength. This is soil liquefaction. So the places where you have sediments and the water table is quite shallow or the sediments contain water molecules the soil liquefies when the earth shakes and as a result loses strength. This is what happened in the Kathmandu valley and also in the Bihar plains during the 1934 Nepal-Bihar earthquake. There are ways to lay foundations of big structures so that they do not suffer damage during this process.
The Indian subcontinent at large has been experiencing extreme weather events such as heavy rainfall and snowfall. Do these events contribute to increased seismicity?
Weather, rainfall, snowfall do not seem to contribute towards the occurrence of great earthquakes. But they do cause annual variations in the plate movement and have also been seen to influence small magnitude seismicity but not the large one.
What advancements have we made in predicting earthquakes? What are the new challenges, if any?
Earthquake prediction is still not possible. Though we are trying to understand the precursory signals, if any, in various physical quantities through field observations, we are not able to progress much in this direction. The process of earthquake occurrence is complex and non-linear. There are several parameters which contribute and govern earthquake occurrence. I guess that with the more observations with high precision, there will be a day when we will be able to completely understand the process and we will be able to predict the earthquakes.
South India, since the Bhuj Earthquake of 2001, has been experiencing increased seismicity with scientists discovering new faults. Can you point out if there are any new areas which are becoming seismically vulnerable for human habitation in India and its neighbourhood?
It is not correct to state that the seismicity level has increased. People are looking at the short period, and in that short period an apparent increase in the seismicity will be averaged out over a long period. We should not forget that the earthquake detection level has increased in past few decades and the reporting of earthquake occurrences has also increased. We need to ascertain whether every newly discovered fault is seismically active in current times. Generally, it is considered that the region of south India (barring Kachchh and regions close to Narmada-Tapti regions) is safer than the regions in the Himalaya and north-eastern India.
What is the biggest gap in our understanding about earthquakes in Himalayas?
What kind of research is being carried out to find the answers to these questions?
Most of the research is focused on seismological and geodetic research. The seismological research helps in analysing earthquakes, their location, characteristics and also in deciphering the subsurface structures which are responsible for earthquake and geodynamic processes. Geodetic research deals with estimating strain rate the faults on which the strain for future earthquake is accumulating. They also help us in understanding the overall geodynamic processes that take place in the Himalayan arc. Besides these, there are some other observations which help us in monitoring the possible earthquake precursory processes. But such observations are very limited.
Paleoseismology, which deals with identification of past earthquakes, is also pursued in quite a significant way.
What is your research team studying at present?
We, at NGRI, are dealing with all the above studies (seismological, geodetic and paleoseismology). We have several field observatories in the Himalayan region. The broad theme of all our studies is to understand the earthquake occurrence processes in the Himalaya. From the geodetic observations, we try to estimate: rate of strain accumulation for future earthquakes in various Himalayan segments; size of the region where strain accumulation is under way; and the faults or other structures where strain accumulation is taking place.
From seismological studies, we are trying to find out the subsurface structures which can produce earthquakes, or which participate in the geodynamic processes, to identify the region of large damage due to future earthquake due to local site conditions, and the possible regions where large earthquakes are likely to occur.
Paleoseismological work in Himalaya is helping us in identifying the past major and great earthquakes, their size, and region of occurrence. Putting all these research outputs together helps us in assessing the seismic hazard of a region.