“In future, if any earthquake were to happen along the Himalayas, it may happen further west of the focus of the main April 25 quake,” said Dr. R.K. Chadha, Chief Scientist at the Hyderabad-based National Geophysical Research Institute.
That is because the 7.8 magnitude April 25 quake was caused by a unidirectional fracture. From the focus of the earthquake, the fault propagated in a nearly west to east direction thereby releasing the energy that was stored for many decades.
While stress unloading has taken place from nearly west to east of the focus of the main quake, stress loading could have occurred further west of the focus of the main quake.
A case in point
The December 24, 2004 Sumatra earthquake that caused the killer tsunami waves was also caused by a unidirectional fracture that propagated in a south to north direction. As a result of the unidirectional fault propagation, stress was unloaded in the northern direction and stress loading occurred south of the focus of the main Sumatra earthquake. “This caused the March 28, 2005 earthquake of 8.7 magnitude in the Nias Island, some 300 km south of Sumatra,” he said.
The April 25, 2015 quake was caused by thrust faulting “on or near the main frontal thrust” where the Indian plate is pushed under the Eurasian plate. According to the U.S. Geological Survey, from the epicentre of the earthquake, nearly 80 km to the northwest of the Nepalese capital of Kathmandu, the India plate is converging with the Eurasian plate at a rate of 4.5-5 cm per year.
About 3 cm of the Indian plate movement is accommodated in the Himalayan collision zone and the remaining is accommodated along the east-west strike-slip faults in the Tibet region. As a result, earthquakes caused by thrust fault in the Himalayan region occur along the strike-slip fault in an east-west direction.
Any quake will cause displacement either in a vertical or horizontal direction. “We don’t know for sure the amount of displacement caused by this earthquake,” said Dr. Chadha, “but for an earthquake of 7.8 magnitude [revised value], the displacement can be 1 to 2 metres along the fault. The quake was on a thrust fault and so most of the displacement will be in a vertical direction.”
According to Colin Stark at the Lamont-Doherty Earth Observatory, Columbia, 1-10 feet of India had slipped northward underneath Nepal.
The U.S. Geological Survey has revised the depth of the focus to around 15 km. Explaining why the focus of the earthquake cannot be a couple of kilometres deep as initially put out, he said the magnitude of an earthquake has a relationship with the volume of crust. Hence, the focus of the quake has to be 10-15 km deep as a large volume of crust is needed for a 7.8 magnitude quake.
Amplification of waves
Nepal is situated on an ancient lakebed and hence the soil at Kathmandu valley is soft and “liquefies easily.” Unlike a rocky terrain where the seismic waves travelling with great velocity can have very good transmission, a terrain with sediments can amplify the seismic waves, thereby amplifying the ground motion.
A continental-continental plate collision will necessarily produce shallow-focus earthquakes (less than 70 km) as continental plates are less dense than oceanic plates and both the plates have the same density. However, when two ocean plates collide, both the plates will go down to form a trench, like in the case of the Mariana Trench in the Pacific Ocean.
“Both the ocean plates will go down because of the role of gravity,” Dr. Chadha said. In the case of two colliding continental plates, gravity is not that intense.
In the case of a continental plate colliding with an oceanic plate, the oceanic plate subducts under the continental plate as it is heavier than the continental plate. In general, the focus of most of the earthquakes in subduction zones is intermediate (70-300 km) to deep (300-600 km).
“In future, if any earthquake were to h