Last week we explored the formation of the Himalayas. Their birth is a remarkable story. Scientists now think, and have evidence to prove, that the Indian landmass was in fact not connected to the Asian continent may millions of years ago. It was part of a giant super continent which broke apart and started drifting many millions of years ago. Over time the Indian landmass drifted slowly towards Asia and finally collided into it, occupying a space that was previously the domain of a great sea called the Tethys. As a result of this collision, between India and Asia, the Himalayas slowly arose over the course of millions of years. The great pressure and heat generated by such a grid metamorphosed earlier rocks into newer ones. Large parts of the Tethys sea were trapped and arose with the Himalayas – their presence is alluded to even today by the many saltwater lakes in the Himalayas as well as the marine fossils that are found routinely in these great mountains.It so happens that the Himalayas are still rising. They are still being born as the collision between India and Asia continues. So fierce is this pressure the Indian landmass is slowly being pushed below the Asia landmass. So the Himalayas continue to rise but with a small catch. Imagine two large and perfectly homogeneous blocks of stone sliding on a table and colliding into each other. Perhaps moldable clay putty blocks are easier to imagine. Either way the the collision front will rise up on impact. Furthermore they will continue to rise up as you push the blocks against each other slowly. Now imagine that these blocks are not uniform but have various irregularities, hard stone and soft soil, coupled with a layered structure of the earth in some places. It is quite possible that as you keep pushing the blocks together different parts of the blocks are internally moving at different rates in places because some areas get compressed easily and others resist pressure better. But this process cannot go on for ever. The built up pressure must release itself somehow and it often does by one portion suddenly slipping and moving against the other.
These phenomena, when they happen deep inside the earths crust, suddenly release a lot of energy sending shock waves to the surface. These are the terrifying vibrations that we perceive as earthquakes.
As the Indian landmass presses against the Asian one, tremendous pressure is generated. All this energy has to somehow either be stored or be released. Imagine a spring connected to a wall. If you push against the spring and into the wall it will compress more and more – storing a lot of energy which can be released by the spring flying out if you let go. But what happens if you don’t let go and keep on pushing. Now if you push hard enough the wall will give way suddenly. Similar phenomena happen deep inside the earths crust where rock faces can slide against each other. They stay put upto a threshold of pressure after which a sudden slip occurs, one way sudden shock waves – earthquakes – can originate inside the earth.
These waves travel to the surface of the earth often causing enormous damage. In fact such shocks can be strong enough to even trigger tsunamis. In the Himalayas they routinely cause great destruction of life and property with landslides and house collapses being triggered by such earthquakes. Responding to earthquakes as a civilization poses a curious challenge. On the one hand earthquakes can cause great damage. On the other hand they are really rare occurrences planning for which may be difficult to justify financially as the earthquake you built for never shows up. Himalayan civilizations would have faced similar challenges over millenia. They would have wondered how to build so that their cities and villages would be protected against earthquakes. Kings and Queens might have wondered about building temples that would immortalize them. Surely such temples would have to be made earthquake proof. But how ?
Imagine you are holding a long thin wooden ruler – about a meter in length – gripped firmly at the base. What would happen if you shake the base back and forth vigorously. The ruler would flex one way and then another , bending as you shake the base. This is our home simulation of a tower in an earthquake. One way to make such a building earthquake safe is to make it stiff. So stiff that no matter what the force shaking it, the building would not bend or flex appreciably. This can work but at the very least it is an expensive solution. More concrete – more cement and more steel – equals more money. The Himlayan builders, perhaps in collective moments of inspiration, chose a different method. Their premise could be surmised as such. If a building wants to move , then make sure it can move without cracking up and breaking. They built in a manner that combined just enough flexibility with just enough rigidity using ideas that were so elegant that modern engineers have begun using them in only the last few decades. We will explore the methods in the next article but let me leave you with one of their insights.
Our shaking the base of a building transfers force into it. The building flexes and stresses as a result. But what if we put the heavy structural elements in a building on wheels – allowing the top structure to slide over the lower structure. Let it move, avoid stresses but just make sure that the entire building does not come apart. These ideas led to the development of building methods where no joint is nailed down and move – a bit – to dissipate the shaking of the ground. For details on exactly how, do tune in next week.