The magnitude 7.9 Nepal earthquake of April 25 was the worst Himalayan quake since the 1934 magnitude 8.2 Nepal-Bihar quake that devastated the northern Bihar plains. Mahatma Gandhi, who visited Bihar after the 1934 quake wrote that it was “providential retribution for India’s failure to eradicate untouchability”, prompting Rabindranath Tagore to retort in a letter to Gandhiji’s Harijan that “physical catastrophies must have origin in physical facts”.
Like other Himalayan quakes, the Nepal temblor is a dramatic manifestation of the Indo-Australian and Asian tectonic plates moving closer, an inexorable process over the last 50 million years, which is the cause for the rise of the Himalayas and due to which the mountains bear immense tectonic stresses that are released through temblors.
Four great earthquakes have struck the Himalayas during the last 200 years, and it was getting ready for another, as a paper that I co-authored in February had forewarned. The Nepal quake is also a painful lesson in what is in store for the communities that live in the Himalayas and the adjoining Indo-Gangetic Plains. Sadly, the lessons are forgotten soon after such events.
The shallowness of the source (15 km deep and located 70 km north-west of Kathmandu), the magnitude of the slip (2-3 m), and the fact that the fault plane extended under a densely populated Kathmandu, which itself is built on a primitive sediment filled lake that amplifies seismic wave energy, made this a major earthquake disaster. With the tectonic plate convergence across this segment of the Himalaya estimated at about 2 cm/year, the slip released in this earthquake is only equal to a century of strain build-up (estimated convergence of India and Asian plates is about 2-3 cm per year).
Research shows that this segment has not seen any great earthquakes in the last 700 years, and the unspent, accumulated slip over several hundred years needs to be released through future large earthquakes. In our paper in the February issue of the Journal of Geophysical Research, we said that “this region is due for great earthquakes”. This also means that the segment, which includes parts of Uttarakhand Himalaya is still capable of generating damaging earthquakes. That said, it should be noted that the seismological community the world over is still grappling with the ability to make short-term predictions in terms of time, location and magnitude.
That’s why every time an earthquake occurs, the scientific community is asked why it can’t predict earthquakes, as compared to weather forecasters. The reason is that while all the parameters of weather-related processes can be monitored and quantified (although their complex interaction may be beyond numerical modeling, which explains our bad predictions about monsoons – leading one modeler to quip, “the model did not fail, the monsoon failed”), in contrast, the processes leading to an earthquake happen tens of kilometres below the surface of the earth. None of the parameters involved in the deeply-buried earthquake source can be measured directly and any model that attempts to forecast deformation rate that leads to ultimate failure assumes the properties of the rocks, which again are not open to direct observations.
But some recent advances in satellite-based observations and seismicity precursory observations like silent earthquakes before a great earthquake, at least along some of the subduction zones, augur well for earthquake prediction science, although we need to cover an arduously long distance to get there. With each new earthquake, researchers are gaining valuable new insights.
(C.P. Rajendran is senior associate at Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru)
