IS THE BIG ONE COMING?
I shall endeavour to explain Bilham's hypothesis in the simplest language possible. The earth's surface consists of a number of large, rigid plates that move relative to one another and interact at their boundaries. Some billions of years ago the northward moving Indian plate slipped underneath the Asian plate pushing it both horizontally northwards and upwards. This led to the upheavals forming the Himalaya. The movement still continues and the convergence rate of India towards Tibet is 2cm a year and that is compensated by the Indian plate slipping under the Asian plate. However, if the slip does not occur it would mean that 2cm of convergence remains stored in the rocks as elastic strain. If no slip has occurred in 100 years the stored strain would be 2cm X 100 or 2m, in 200 years 4m and should the slip occur after 500 years the movement would be 10m resulting in a mega-quake of around M=8 (or a reading of 8 on the Richter Scale).
Any major compensatory movement requires an event exceeding M=7 and Bilham's observation is that in the Himalayan segment between Kathmandu and Dehradun has not had that for several thousands of years. The records show only two large earthquakes, 1803 and 1833, and Bilham does not believe they were great earthquakes else they would have caused massive devastations and the British administration would have records of the widespread destruction and the efforts taken for reconstruction. It therefore appears that the chance of a great earthquake is real since sometime in the future the plates must make an adjustment. Of course a pent up force of several thousands of years never undergoes adjustment in just one big shake-up but the alarming arithmetic is that even if 500 years were to be adjusted the movement would be 10m and such an event would measure Magnitude Eight on the Richter Scale.
To understand the effects of an M=8 plus event we can study the Great Bihar Earthquake of 1934, which did not evidently cross M=8, but came close to it. At 2:13 pm on 15 January two million square miles of northern India and western Nepal shook violently for 5 minutes. It took another 15 minutes for ceiling lamps to stop swinging in Calcutta and many more days for the dust to settle from the landslides in the mountains of Nepal. At Darjeeling a number of badly constructed houses totally collapsed, in many buildings cracks appeared or walls fell out and lots of bungalows were damaged by the fall of masonry chimneys crashing through the roofs. A ground fissure, over 300 yards long, appeared below the station yard in Tindharia and numerous breaches were seen all along the cart road. Should a Nepal-Dehradun great earthquake occur then north Bengal is assured of some consequences just as in the case of Bihar Earthquake. The point is not when it will come, since earthquakes are inevitable and necessary features of adjustment between the tectonic plates, but of our preparedness. Matters like maintenance of essential supplies, handling fire hazards, law and order problems, outbreak of diseases, post shock syndrome counseling and the eventual reconstruction are but a few of the many things that will need a look into. However, the greatest tragedy and hurt will be the loss of lives and that can never be restored but, and it is ,an important but, it can be prevented. Studies of earthquakes have shown that the major cause of deaths has always been due to poorly constructed buildings. Over the past few decades buildings have mushroomed all over the hills of Darjeeling, Kalimpong, Kurseong and Sikkim and are these structures built according to any studied and proven system of seismic resistant building codes or have they been built under outdated or whimsical codes will need to be considered seriously. That even these codes can be bypassed was proven a large building in Gangtok going down recently without the violence of an earthquake and such a vulnerable building could not be an isolated case. It is never too early to begin the process to prevent loss of lives but I could be talking to the wall, for all you know or care.
comment by praful rao
In this connection I am reproducing an extract from a recent article "How prepared are we for Disaster?" by Patricia Mukhim, editor Shillong Times:-
What is the use of having a department that is creating more man-made disasters than solving them? Here I am talking of the Urban Affairs Department and its other arms as well as the State PWD. The latter as I have always maintained have no pride in their work. The former specialises flouting the rules it makes. Can the PWD point to one building which will withstand an earthquake? If there is one such we would like to know which one. Whenever a techno-legal regime becomes operational in Meghalaya can the government bring down all those badly constructed buildings that are not earthquake proof?? No it cannot. So they will remain towers of corruption for generations."