The Ground Beneath Our Feet

It is surprising isn’t it, that with all the advances in science, we still haven’t come up with a way to predict natural disasters like earthquakes and volcanic eruptions. The best thing that we can do is to make an informed guess about approximately when an earthquake might occur. And then too, it’s only mathematical probability. So all we know is that an earthquake may (or may not) occur at around so and so time at a given place. To complicate matters further, these predictions are really only applicable at places which have a history of earthquakes. Not the most comforting of thoughts, is it?!

But to be fair, earthquakes rarely strike at completely unexpected places. There is a lot that goes on before an earthquake strikes. And geologists can usually sense these going-ons well in advance. By this, I mean that the chance of an earthquake occuring at a place that has absolutely no history (and I mean really age old history) of quakes is very remote.

I do not want to sound too technical, but earthquakes can be of different types and the one that rocked Southeastern Asia on Boxing Day was of the most common variety – the tectonic earthquake. The earth can be visualized as a fractured sphere fitted together by a jigsaw-puzzle-like pieces of crust known as plates (or the tectonic plates). Underneath these plates (i.e. underneath the crust) is the hot, molten magma. When you take into consideration the size of the earth, the crust is just a wafer-thin layer of solid rock. So the plates lie atop the magma and when the magma moves, the plates move alongwith it. Picture a sheet of metal moving along over a conveyor belt. It’s something like that!

When the plates move, they could press against each other. So when an ocean plate (the oceanic Indo-Australian plate in the case of the 26th December earthquake) pushes and grinds up against a continental plate (the huge continental Eurasian plate in this case) one plate wins the battle (usually the continental plate wins this battle). And the plate that loses gets pushed beneath the other plate. The constant pushing and grinding leads to tremors within the earth’s crust. These tremors are what we feel as earthquakes. Usually, the process of one plate getting pushed beneath the other, known as subduction, is a painstaking process and the resulting tremors are low intensity. But sometimes, the pressures on the plate boundaries gradually build up to such an extent that suddenly one plate gives way. When this happens, we get the whopper earthquakes like the one that occured on Monday, 26th December, off the coast of Indonesia.

Taking the above example ahead, when the ocean plate (which is obviously at the bottom of the ocean) suddenly sinks beneath the continental plate, a huge column of water above that spot (where the subduction occurs) is suddenly pulled downwards and then is pushed upwards with an unbelieavble amount of force. This action sets off what are referred to as Tsunamis. To understand this, simply visualize that happens when u throw a pebble in a pond. Ripples start propagating outwards from the spot where the pebble struck the water. This is about what happens on a much much larger scale in case of Tsunamis. Huge waves start propagating outwards from the spot where the subduction occured. These waves move outwards with speed and when they encounter a shoreline, rush onto land like a huge huge tide… flooding everything in the path. It is worth noting here that inspite of popular conceptions that Tsunamis involve gigantic waves crashing down upon the shores, the fact is that Tsunamis usually hit the shores like a sudden and exceptionally huge tide. As they reach inland, the flooding tide loses force and suddenly starts receding dragging anything and everything in its path towards the sea. It is this strong current, trying to return back to the sea, that causes the most destruction of life as the water carries hapless victims with it into the sea.

Of course, the dynamics of a Tsunami are much more complicated than this. But I’ve just tried to give a rough idea of what led to Monday’s events. For further reading, there are a number of sites all over the Net where you can get a lot of detailed information on this subject.

Earthquakes have been a way of life for earth. Earth’s history is violent. In the past, entire species have been wiped away by the fury of nature. The ones that survive live to experience Earth’s dangerous beauty with the knowledge that they are but transient inhabitants of the Blue Planet.