Viewpoint: Nepal hit by second earthquake measuring 7.3 magnitude

Stephen Hicks, who is studying for a PhD in Earthquake Seismology at the University’s School of Environmental Sciences, comments on the second major earthquake to strike Nepal just a fortnight after the first:

“Aftershocks from earthquakes can sometimes be just as, if not more, devastating than the initial mainshock. Today’s magnitude 7.3 earthquake occurred at the eastern edge of the magnitude 7.8 mainshock rupture on 25th April 2015.

Stresses from the mainshock were transferred to the eastern edge of the fault, bringing it closer to failure and causing today’s large aftershock.

On average, we expect the largest aftershock of an earthquake to have a magnitude of around 1­ – 1.2 units smaller than the mainshock. So in the case of Nepal, magnitude 6.6 – 6.8 aftershocks were plausible.

An aftershock as strong as magnitude 7.3 is somewhat surprising. However, earthquake processes tend not to conform to laws of averages, and can often take us by surprise. Today’s earthquake will have its own aftershock sequence. 

In terms of energy released, today’s earthquake was around 6 times smaller than April’s earthquake, but is likely to cause just as much damage at the surface.

Further damage

Manmade structures and mountainous slopes that were already weakened by April’s earthquake will be further damaged, leading to more devastating building collapses and landslides, especially in rural areas.

Like last month’s earthquake, it is possible that this event has further caused some Himalayan mountains to shrink slightly, including Mount Everest. Further analysis of data from this earthquake is needed to quantify this effect.

However, the focus on all our efforts in the short-term must be on the aid operation and to better understand earthquake hazard in Nepal and along the Himalayan region as a whole.”

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