Sunday, May 10, 2015

Nepal 7.9R Earthquake (25Apr2015) : 96 Aftershocks (in two weeks) and still counting...

Placed above is a record of aftershocks (source) following the 7.9R earthquake which devastated large parts of Nepal on 25Apr2015. There have been 96 aftershocks till date (including the induced 5.1R quake which was epicentred around Mirik.
In order to generate awareness about quakes especially amongst the people of the Himalayas, I posed several questions to experts.
The questions and answers (in this case given by Dr Malay Mukul, of IIT Mumbai, Dept of Earth Sciences) are placed below :-
Q. Regarding the Nepal quake, why are there so many aftershocks WITHIN a short time frame - 96 within a time span of 2 weeks? The 6.8R (18Sep2011) Sikkim earthquake which we experienced never generated so many aftershocks.
A. This is normal. If you just see the aftershock records for the Bhuj earthquake,you will find that they continued for months after the main shock. You would see is a gradual decrease in the aftershock magnitude as well as number of events. Now we are mostly getting 3s and 4s.
The aftershock activity is also a function of the magnitude and depth of the main earthquake. Shallower and large magnitude main shocks create more intense aftershock activity.
The Sikkim earthquake was much lower in magnitude as well as deeper.
Q. Are the continuous rattling (aftershocks) dangerous for installations and structures already weakened by previous tremors? I know that 2 hydel projects in Darjeeling district suffered some damage during the recent quake.
A. Yes! Aftershocks tend to add to the damage created by the main shock and in many cases topple structures that were still standing at the end of the main shock. So if your building has suffered structural damage you need to make sure you either fix it or stay away from it.


Comment by Praful Rao
My thanks to Dr Malay Mukul for his prompt answers. 

Two major earthquakes have shaken in this region within the last 4 years, (6.8RSikkim and the recent Nepal one), yet we continue to flout all regulations regarding building codes and continue to construct thousands of 8-9storey buildings on these fragile mountains. I also wonder whether the numerous dams that we have constructed or are constructing  are safe?

Praful Rao,

Dist Darjeeling

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