How has our understanding about seismicity in the Himalayas evolved? Can you share with us some new findings from the region that can explain the cause of recent intense earthquakes in this region? Can you share some of your own analysis and reading in the light of the recent earthquake?
Our understanding of Himalayan earthquakes has significantly improved over the past 20 years through major advances in three fields of study: (a) Seismology observations and modelling of Himalayan earthquakes; (b) Geological observations of fault zones and timing the rupture of these faults using Isotope dating techniques; and (c) Global Positioning System-based Geodesy measuring the deformation of the surface at very high precision (millimetre scale resolution) using satellite data.
From geological and geophysical evidences, we know that 50 million years ago, the Indian plate collided with the Eurasian plate and initiated the formation of the Himalayan Mountain Belt. This process has continued over the last 50 million years with underthrusting (which is similar to diving underneath) of the northern edge of the Indian plate beneath the Himalayan mountains and the Tibetan plateau.
In the early 1990s, Vinod Gaur of CSIR Centre for Mathematical Modelling and Computer Simulation and Roger Bilham of University of Colorado, Boulder, USA, initiated the study of GPS Geodesy in India to quantify the convergence of the Indian plate with Tibet and the deformation associated with it.
It led to the first basic knowledge that the Indian plate, in the present, is converging with the Eurasian plate at a rate of ~50 mm/yr, of which a third of the convergence (~18 mm/yr) is being accommodated within the narrow belt of the Himalaya. This ~18 mm/yr convergence was modelled to be accommodated along the interface which separated the diving Indian plate from the overriding Himalayan mountains. This interface is known as (in geological terms) the basal decollement or detachment and is also referred to as the Main Himalayan Thrust (MHT).
It was also understood from geological observations and laboratory measurements of rock behaviour, under different temperature (T) and pressure (P) conditions (which mimic the interior of the earth), that rocks behave in a brittle manner at low T and P (close to the surface) and would be ductile at greater T and P (at depth). Using this understanding, we could then predict that the MHT at shallow depths ( 20 km) would be a brittle interface with frictional locking behaviour and would be ductile at greater depths, undergoing creep (a ductile deformation process).
From seismological studies, we knew that earthquakes always occur in brittle rocks and rupture by frictional failure. Combining this knowledge with the GPS geodetic information and behaviour of rocks at depths, we understood that the MHT at a shallow depth is frictionally locked and accumulates elastic strain energy in response to Indian's convergence with Tibet. Although the rate is very slow (~18 mm/year in Nepal Himalayas), it occurs steadily over centuries.
This results in build-up of energy which can be enormous if stored over several decades. A direct evidence of the presence of this locked zone and its width (~100 km in Nepal Himalayas) comes from precise location of small-to-micro-earthquakes (magnitude 4), which cluster along the downdip end of this locked zone.
This understanding has been a result of the dense network of local seismograph stations in the Nepal Himalayas and advancement in the seismological algorithms for location of earthquakes, which happened mainly in the last two decades. Once the frictional strength of the material in the locked zone is overcome by convergence (at strain exceeding 10-4), rocks on either side of the interface (MHT) slide past each other in a flash producing a big earthquake. The size of the fault which is broken in the process, and the amount of relative slip on the fault, determines the magnitude of the earthquake. Earthquakes, which would break a fault ~150 km long (along the arc) and the entire locked MHT (~100 km across), would result in magnitude 8 earthquake.
This model of strain buildup along the MHT and then release in major earthquakes had been hypothesised for the Himalayas from the combined knowledge from seismology and GPS Geodesy. If this were true, then major historical Himalayan earthquakes, which we have meagre knowledge of, should have produced breaks in the MHT, reaching very close to the surface near the Himalayan foothills.
From excavation of this region, by trenching, geologists found trace of the MHT in the front of the Nepal Himalayas. From the presence of organic matter in the sediments of the fault zone (MHT), they could use radiocarbon and cosmogenic isotope dating to ascertain the age of the fault when it moved last time.
They found out that such faults rupture in a cycle of centuries. Therefore, our hypothesis that convergence is accommodated on the MHT, shallow part of which accumulates elastic strain energy and slips in a few 100 years producing mega-quakes all made scientific sense. What was missing in the jigsaw was the real evidence that such events do occur.
The Nepal event, albeit the destruction and death it caused, emphatically confirmed the scientific understanding acquired by the advancement and convergence of knowledge from the three fields mentioned in the beginning.
In order to develop similar understanding in the Kashmir Himalaya, IISER Kolkata in collaboration with University of Cambridge and Jammu and Kashmir's Sri Mata Vaishno Devi University have undertaken a seismological study since mid-2013. We have deployed 12 seismograph systems in Jammu and Kashmir and are in the process of expanding this network to about 25 stations. This will provide invaluable data for the first time from this segment of the Himalayas.
Seismologists often talk about a seismic gap running a length of 700 kilometres through Himalayas. Many have stated that the devastating earthquake in Nepal is perhaps not what they were expecting. The big earthquake is expected to be of a much greater magnitude. How far do you agree with this theory?
With calculations, and using the last mega-quake occurrence along the Himalayan front, it is known that there are a number of seismic gaps in the Himalaya with a potential to slip and produce greater than 7.5 magnitude earthquakes.
To the west of the present Nepal event, the last known historical earthquake occurred in 1505. This would mean that this region has accumulated a potential slip of ~9 metres. Similarly in the region east of this earthquake, the last known mega-quake occurred in 1934. Therefore, this region has accumulated a potential slip of ~1.5 metres. Similarly, the region in Jammu and Kashmir Himalayas, between the Kangra (1905) and Kashmir (2005), is known to have accumulated a potential slip of ~7 metres.
Scientists have failed to find any evidence so far of historical earthquakes in the front of the Sikkim Bhutan Himalayas and, therefore, we do not know how much potential slip has accumulated. But it is evident that the Indian plate is converging at a similar rate there too. Hence, all these regions which have accumulated potential slip are known as seismic gaps and are sufficiently strained to drive future great earthquakes.
What advancements have we made in predicting these earthquakes? What are the new challenges if any? Is there any mechanism to overcome these challenges?
I have strong reservations about pursuing earthquake prediction as a scientific goal for seismologists and/or scientists. Japan had spend trillions of dollars in the past trying to predict an earthquake and failed. This is when they realised that it is far more important to be able to design and build structures which will survive earthquakes. In this, they have had phenomenal success and are world leaders in earthquake-resistant designs. I think this is the model to follow for the entire world and definitely for India.
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 the neighbourhood?
India is surrounded by active plate boundaries to the north and east. Moreover, the Indian landmass is billions of years old and has pre-existing structures or faults like Bhuj, central India and northeast India. This makes India very vulnerable to earthquake hazards.
Due to the convergence of Indian with Eurasia, stress from the plate boundary (in the north) gets transmitted to within the Indian subcontinent and reactivates faults. These faults have caused several large earthquakes in the recent past within the Indian subcontinent, such as Latur (1993), Jabalpur (1997) and Bhuj (2001).
It is not really true that earthquakes have increased since the Bhuj earthquake. The time window we are looking at is minuscule when compared to the history of the Earth and of continental India. However, with advances in geological and geophysical techniques we have been able to better map surface and subsurface active faults and ascertain their earthquake potential using surface deformation measurements.
Almost all of India is crisscrossed by faults, having variable earthquake recurrence intervals. It is very difficult to ascertain if they are accumulating elastic strain now and how close they are to failure. But it is known from several studies that India has a colder than average continental crust and, therefore, the entire crust is seismogenic. This means that if there is an earthquake within the Indian plate, this will rupture bigger faults going to greater depths, producing larger magnitude events. This information should be incorporated while designing building codes and then applied across the country.