Photo: Reuters
Photo: Reuters

Waves below our feet

Earthquakes are among our most familiar natural phenomena. So why can't we predict them and save lives?

When the earth started shaking in Nepal in April, killing thousands by the time it was done, I found myself agonizing over an old question. It springs from this sobering factoid: there are over a million earthquakes on our planet every year. That’s two every minute, somewhere in the world. You could say that quakes are among our most familiar natural phenomena.

So why can’t we predict them and save lives?

The bald truth is, we may never be able to predict them with any accuracy. That’s the nature of quakes. But studying them has taught us a lot about the world we inhabit. Quakes set off shock waves that are felt—or at least, that can be measured—all over the world. Amazingly, it’s these waves that have taught us much of what we know about the interior of our planet.

When we invented and deployed seismographs—instruments that record movements in the Earth—one early finding was that the shock waves from earthquakes are of three kinds.

Primary (P) and secondary (S) waves travel through the Earth, while surface waves travel along the surface, or crust of the planet. (There are actually different kinds of surface waves too, but I’ll let that be.)

Of the three, surface waves are the slowest. S waves are faster, and while they can travel through solid matter, they vanish in liquids and gases. This is because S waves have a transverse motion, but I’ll let that be as well: it’s not as important here.

P waves are the fastest, zipping seemingly unhindered through the Earth’s dense interior.

So when an earthquake occurs, the P waves it sets off are the first to arrive at a given seismic station. A while later, S waves arrive. Later still, seismographs record the slow surface waves. From considering these gaps in the times, seismologists can calculate how far away the quake was. And by comparing data from seismographs at different places, they can pinpoint exactly where the earthquake happened.

So if you’ve ever wondered, “How on Earth do they know where the epicentre was?", you have your answer.

There’s more to learn from the P and S waves, in particular. Since they spread in all directions, seismographs all over the world receive and record them. Or so you’d think.

Oddly, some stations receive them diminished, and other stations seem to be in a shadow zone where the tremors are felt only faintly, if at all.

A quake in Peru, for example, might not be recorded in India. Even more interesting, S waves don’t even turn up in some places that record the arrival of P waves. Is there some huge obstacle somewhere far below our feet that stops the waves?

Not quite.

Scientists eventually figured out the explanation: inside the Earth, the waves pass through different kinds of materials. Much like water refracts light, these materials refract and diminish the waves. They get diverted from their path, arriving in unexpected places—or sometimes, they disappear altogether. Indeed, if the S waves disappear, that must be because there is a liquid or gaseous part inside the very Earth that feels so solid to us. After all, S waves can only move through solid matter.

The detective work goes on.

The P and S travel times, recorded at different places, allowed scientists to infer their speeds at different depths. Broadly, the speeds increase as the waves approach the centre of the Earth. But at several precisely known depths, the speeds change suddenly.

About 60km below the surface, S and P wave speeds switch abruptly from about 4 and 7km per second (kmps), respectively, to about 5 and 8.5kmps. For the next 3,000km downward, the speeds increase steadily to about 7.5 and 14kmps.

Then, P waves suddenly slow to 8.5kmps, and their direction of travel changes sharply. And S waves? They disappear altogether.

All of which suggest that there are major changes in the properties of matter at those depths. This is how we know that the Earth has a crust that is rarely over 60km thick.

Get below that, and there’s a doughy, plasticky mantle layer for the next 3,000km down. Then we reach an outer core of molten iron and nickel—the liquid that stops S waves—for about 2,000km. Finally, there’s a dense, solid inner core with a radius of about 1,500km.

Remember that the deepest we’ve penetrated the Earth is a few kilometres into its crust, via probes. Yet we know plenty about what lies below, all deduced using a vast amount of earthquake data gathered over many years. And this teaches us about quakes themselves. For example, while we know that most quakes originate in the crust, some occur in the mantle, as much as 750km down.

None of this is consolation to the victims of the great tragedies of Nepal, Kutch, Latur and Kobe. But understanding quakes, and this planet itself, holds at least the promise that future quakes may not prove as tragic.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at dilip@livemint.com. To read Dilip D’Souza’s previous columns, go to www.livemint.com/dilipdsouza

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