For a higher magnitude earthquake, Decay is not always a linear process.

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.

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.

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.


Ref - The Hindu
"For a higher magnitude earthquake, the phenomenon of aftershock magnitude decay is not always a linear process"