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The Nepal Earthquake: Why did it happen and why now?

  • Earthquakes in Nepal above 4 magnitude in the past week.
    Earthquakes in Nepal above 4 magnitude in the past week.

Earthquakes in Nepal above 4 magnitude in the past week. Map: Gubbi Labs.

More than 3000 people have been reported dead, after a massive earthquake hit Nepal on Saturday. This is the major earthquake tragedy in the region since the Bihar earthquake in 1934, which measured 8.2 on the Richter scale and caused over 10,000 fatalities. After the initial hit, aftershocks have been felt across the area, adding to the damages. As the earth’s crust readjusts itself after the main earthquake, aftershocks are felt in the same region.

Why did this quake happen, and why now? In a paper published in February, a team of scientists from the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) and Indian Institute of Science (IISc) Bangalore, discussed why the region is prone to earthquakes and warned about the possibility of an imminent event.

The earth’s crust is broken into distinct regions, called “tectonic plates”. Millions of years ago, the Himalayas formed when the plate containing the Indian subcontinent pushed against the Eurasian plate, causing the land to buck and fold and rise into the world’s tallest mountain range. What is lesser known is that the Indian plate is still moving toward Tibet – at the rate of 20±3 mm/year.

A “fault” is abreak in the earth's crust, along which movement can take place, causing an earthquake. One section of the crust along the fault moves, or “slips”, while the other is stationary, causing the earth’s crust to rupture.

“The shallow nature of rupture (around 15 km at the epicenter) and the large magnitude of the slip (>3 m) explains the devastation it wrecked”, said Prof. Rajendran from JNCASR, when contacted.

With the Indian plate pushing against the Eurasian plate, the southern part of Tibet that interfaces with India is absorbing about 80% of this convergence. This means that these regions are accumulating a large amount of strain, which they release in the form of high magnitudeearthquakes (greater than magnitude 8).

“We find that some of the Himalayan segments show seismic quiescence for relatively long time, which means that strain that was accumulated there for several centuries has not been released.  The central seismic gap [the Central Himalayas] is one such segment, which is matured to generate major earthquakes”, added Prof. Rajendran.

The central Himalaya is considered to be historically a “seismic gap”. Excavation sites near Ramnagar in Central Himalaya suggested that there have been successive occurrences of two great earthquakes in the region in 13th and 14th centuries AD but none since then. This means that the strain is accumulating and the region is currently locked, only to get released anytime in a great earthquake. Longer the relapse time since the last earthquake, larger will the expected dimensions of the potential future slip and greater would be its damage potential.

“In fact the present earthquake has released only a century of strain build up”, says Prof. Rajendran. “This region continues to be a zone that needs to be flagged for major seismic activity in the future”.

About the experts:

Dr. C. P. Rajendran is a Senior Associate with the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore. https://sites.google.com/site/cprajendran/. Dr. Kusala Rajendran is an Associate Professor at the Centre for Earth Sciences (CEaS), Indian Institute of Science, Bangalore. http://ceas.iisc.ernet.in/~kusala/

CP Rajendran: rajendran@jncasr.ac.in; cprajendran@gmail.com

Kusala Rajendran: 080-2293-2633; kusala@ceas.iisc.ernet.in

With inputs from a recent paper published in The Journal of Geophysical Research, titled “Medieval pulse of great earthquakes in the central Himalaya: Viewing past activities on the frontal thrust”http://onlinelibrary.wiley.com/doi/10.1002/2014JB011015/abstract