Why the May 12 Nepal quake is called an aftershock

The May 12, 2015 earthquake of 7.3 magnitude that struck 68 km west of Namche Bazar, close to Mount Everest is an aftershock of the April 25 Nepal quake of 7.9 magnitude that has till date killed over 8,000 people and injured many thousands and renderedseveral thousands more homeless.

The magnitude of aftershocks will always be lower than the main event. The higher the magnitude of the main event, the higher will be the magnitude of aftershocks. The magnitude of the aftershocks will generally show a decreasing trend with time. Such a reduction in the aftershock magnitude was witnessed in Nepal after the April 25 event.

But the May 12 Nepal aftershock suddenly showed a spike in magnitude. So why is the May 12 event called an aftershock despite its magnitude suddenly spiking in intensity three weeks after the main event? “Aftershocks will generally follow an exponential decay pattern. But in some cases there can be outliers; some anomalous things can happen,” said Dr. R.K. Chadha, Chief Scientist at the Hyderabad-based National Geophysical Research Institute (NGRI).

For a higher magnitude earthquake, the phenomenon of aftershock magnitude decay is not always a linear process, he said. Such anomalous events had been observed in the past across the world.

After the December 2004 9.1 magnitude earthquake off the coast of Sumatra that caused the killer tsunami waves, there were two aftershocks of 7.2 magnitude that did not follow a linear pattern of magnitude decay. Similarly, there was an earthquake on April 11, 2012 of 8.6 magnitude in the Bay of Bengal. The very same day, an aftershock of 8.2 magnitude was registered in the same fault.

But most importantly, the May 12 Nepal earthquake occurred in the same fault some 160 km east of the April 25 quake. The fault is in between the two major fault systems in the Himalayas — the Main Boundary Thrust and the Main Central Thrust. The two fault systems are 100-120 km apart and run as a linear belt all along the Himalayas from Kashmir to Arunachal Pradesh. “In the Himalayan region, 90 per cent of the earthquakes occur between these two fault systems,” he said.

According to Dr. Chadha, seven to eight earthquakes that rocked the region on May 12 after the 7.3 event are aftershocks of the April 25 quake and not of the May 12 event. “If you remove the 7.3 quake aftershock, the other earthquakes are following the pattern of magnitude decay of the April 25 main event,” he said.

The only silver lining is that despite the May 12 Nepal event, there has not been any extension or migration of the fault. “That means as on May 13, whatever energy is leftover from the April 25 event is being released within the 170-180 km initial fault length.

Cautioning on the vulnerability of buildings, he said that people should not reside in houses that are partially collapsed or have already developed cracks. They should be retrofitted before people can move in, he said. For a nearly 8 magnitude earthquake, aftershocks can continue for up to three months and in some cases up to six months.

Published in The Hindu on May 13, 2015

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