Wednesday, October 5, 2011

From the Bengal Post - The Earth shook & all hell broke loose

The Sikkim Earthquake is a prelude to bigger disasters. Constructing even concrete houses in the region is fraught with risks.

The Darjeeling-Sikkim Himalaya sits in a seismically high-risk zone. This was a fact that almost everyone in the hills has known fora while. However, there is no record of any great earthquake (less than7.0 Richter scale magnitude) in the history of the Darjeeling-SikkimHimalaya. This led to a sense ofcomplacency in the hills and urbanisation and development mushroomed. The “Ikra” Assam type houses gave way to “plainstype” flat- roofed multi-storeyed concrete houses that were more often than not built by slicing-up hill-slopes like “tosh-roti” (bread) or on precariously balanced stilts on hill slopes. Those who were aware shivered at the very thought of a great earthquake occurring in the Darjeeling-Sikkim Himalaya and collectively yelled from every available roof-top about the earthquake hazard and the associated disaster we were like to face.
Nevertheless, dams are being built on the Teesta, tunnels are being driven deep into the hills of Darjeeling-Sikkim based on the fact that the largest earthquake that the Darjeeling-Sikkim Himalaya recorded before September 18, 2011 was of a magnitude of 6.2 earthquake on November 19, 1980 which did not cause much damage. As Seismic Zonation is done on the basis of the highest magnitude earthquake recorded in a region, the Darjeeling-Sikkim Himalaya was put at IV rather than V.
This made sense till the mid 1990s when Global Positioning System (GPS) came along and changed the whole game. The Global Positioning System allows us to measure positions on the surface of the earth very accurately and also determine how much the point is getting displaced over time. This information gets translated into surface velocities that give us an insight into how the ground is moving even when earthquakes are not being recorded. GPS measurements tell us that India is moving like a car towards Northeast at about 5 cm/year and slamming into a wall-like Tibet in extreme slow-motion along the Himalayan boundary that decelerates it from 5 cm/year at Bangalore to 4.7 cm/year in North Sikkim; the Himalaya is the front end of this colliding and continuously crumpling car. The difference of 0.3 cm of motion is being absorbed into the Darjeeling-Sikkim Himalaya as strain energy every year since millions of years. The accumulated strain keeps getting released as earthquakes when it crosses a particular threshold determined by the strength of the rocks. Given this fact, not experiencing a great earthquake is not good news. It merely points to the fact that the strain that has gone into the Darjeeling-Sikkim Himalaya has not been released in a significant manner.
The front-end of the colliding car has not finished crumpling! Every new crumple results in a major earthquake. This is the backdrop against which the September 18, 2011 earthquake occurred.
The good:
There were a number of good things about this earthquake. First, its epicentre was located in the Kanchenjunga Range far away from populated regions and not in Mangan. So, the location of the epicentre was fortuitous. Second, the earthquake was a moderate 6.9. Not quite a great earthquake but, as Bharat Mani Pradhan put it, “Powerful enough to shake us and hopefully stir us into some action”. The third good thing about this earthquake was that it was a strike-slip earthquake which is basically similar to the motion of rubbing your hands when held vertically. In terms of the colliding car, it was not a new crumple developing in its front end but just a big scrape on its side during the process of frontal head-on collision.
Would the scrape on its side slow its forward motion and prevent new crumple zones from developing? That is a question that future GPS measurements will be able to address. For now, we have to view this earthquake as a wake-up call and get our act straight.
The bad: It is obviously the confirmation of our worst fears and the resultant death and destruction that was caused by the earthquake.
Reinforced cement concrete (RCC) buildings are bad in seismically active zones. The risk of having concrete over our head was established beyond doubt for Darjeeling- Sikkim during this earthquake. Earthquakes also trigger landslides and the double whammy of earthquakes and landslides is something we have to be aware of, expect and plan for in Darjeeling-Sikkim. The less we mess around with intact rock in widening roads or tunnelling deep into the mountains, the better. The fact that tunnels collapsed in Chungthang and multiple landslides impeded the ability to transport relief and rescue teams has to be viewed with extreme seriousness. An honest evaluation of the performance of the dams during the earthquake must be carried out to assess how they will behave when earthquakes of higher magnitude strike the Darjeeling-Sikkim hills. Will the dams increase the speed of our colliding car and cause it to crumple sooner? That is again a question that needs to be revisited in the aftermath of September 18, 2011. The earthquake has forced us to address some tough question sooner rather than later.
The ugly!: This has to be the realisation that we are grossly under prepared to tackle a calamity that would descend upon us if a great earthquake with an epicentre in the Teesta Valley would strike us. Also, it confirms what GPS results are telling; the Darjeeling-Sikkim Himalaya is continuously getting strained and it is only a matter of time before we get a great earthquake. The ugly truth of the situation is that we cannot predict earthquakes in human time-scales because the processes in the earth operate in a super-slow mode spread over millions of years. However, science is letting us know that we are overdue for a great earthquake in our space. The other ugly part of the situation is that a vast majority of us have already committed our lifetime’s savings into constructing the spacious and “pucca” RCC dream houses. Can we financially afford to abandon the houses that we have built even after the realisation that we may be living in buildings that may end up being tombs? Like it or not, we have to make very tough personal decisions. We have to decide if we want to risk it all and continue to live in our RCC houses and hope that we can find a “triangular space of life” in our collapsed concrete structures when the big ones strikes. A real “Ram Bharose” or “whatever will be, will be” existence as Wing Commander Prafulla Rao put it. The other choice is to cut our losses and go back to not having concrete over our heads. That is the tough and ugly personal choice each and every one of us will have to make. The least we can do is to not build any new concrete over our heads and keep the earthquake hazard in mind when making decisions on all future developmental activities in the Darjeeling-Sikkim Himalaya. We owe it to each and every person who had to sacrifice their lives on September 18, 2011 to give us this wake-up call and this lesson. This has to be the lasting legacy of this tragic event and the best way to pay our respects to those who were taken away from us on September 18, 2011.

- Published in the Editorial of Bengal Post (04Oct2011)

Dr Malay Mukul is an ex-student of
Dr Graham’s Homes, Kalimpong & is presently
Associate Professor,
Department of Earth Sciences,
IIT (Mumbai)
Praful Rao

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