Dr V Rajamani is a professor at the School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, specialising in the field of geo-chemistry. He explained the factors behind Sunday's earthquake and its implications to Deputy Managing Editor Amberish K Diwanji.

It is a massive earthquake, especially if we remember that the Gujarat earthquake that caused so much death and destruction measured 7.3 on the Richter scale. Earthquakes of such magnitude are very rare, perhaps occurring once every few hundred years. But earthquakes of lesser magnitude are more frequent; thus we can say less the magnitude, more the frequency.

'Earthquakes do not give prior warning'

Such quakes occur in the earth under the sea or ocean. If they occur 30 kilometres or more deep in the earth, they are called deep earthquakes and their devastation is much less since the earth is able to absorb the energy released by the quake.

In this case, the epicentre was just 10 kilometres deep in the ground, so it was a shallow earthquake, which is much more dangerous. Also, the depth of water makes a difference. The more the water depth, more of the energy unleashed by a quake can be absorbed, and vice-versa.

When such a submarine quake occurs, it generates seismic sea waves, otherwise known as tsunamis. I have heard some reports term them as tidal waves; they are not tidal waves which are caused by the tides due to the moon's gravity pull; these are tsunamis that are caused by earthquakes.

When the quake occurs, tsunamis of small amplitude are caused on the surface (that is, waves that range from centimetres to half a metre in height). These tsunamis travel very fast, up to 800 kilometres per hour, which means that in the present case, the quake occurred at 6.28 am, and the tsunamis hit Tamil Nadu three hours later.

The thing is, as the tsunamis travel, they become larger, especially as they enter shallow waters, which means a beach or when approaching land and here the high goes up to a few metres. There have been tsunamis as high as 30-40 metres, but the tsunamis that hit India were about 3 metres to 4 metres high.

That is true but the fact is that earthquakes must have occurred and such tsunamis must have occurred. In fact, one of the greatest earthquakes also occurred in the Indonesian islands when the Krakatoa volcano blew up (in 1883, considered the greatest volcanic explosion in recorded history).

'India not prepared to tackle tsunamis'

Every volcanic explosion causes an earthquake, though not vice-versa. So when Krakatoa blew up, it must have caused tsunamis but no one remembers them because it must not have caused much damage at all.

Earthquakes by themselves don't kill; what kills is human error.

I have just come back from Chennai a couple of days ago and I had noticed that all along the beach there are huge settlements. It is common knowledge that staying alongside the beach is fraught with risk and is never to be done. So when the tsunamis came, they no doubt killed many of the poor people who live along the beach.

In fact, two reasons why the death toll in villages is less is, one, of course the density of the settlements is much less than say in Chennai; second, because in the villages, the villagers don't live on the beach but behind the beach. A beach is like a ridge that prevents the sea from entering the village. So the death toll in Chennai and towns is high because without regard to elementary rules, people have been settling down on the beach and this has been encouraged by the politicians.

While a beach or seafront creates a natural ridge to prevent the tsunamis from going deep inside, the exception is at the mouth of a river where no such natural formation occurs. Here, tsunamis force their way up the river from the mouth and thus cause massive flash floods. But here again, it is the poor people who will die because they live along the banks of the river when they should never have been there in the first place.

Every time there is a flood in India, the death toll is among those who live on the banks. This happens every year along the Yamuna in Delhi.

This earthquake -- and all the earthquakes that happen in India -- occurred because the Indo-Australian tectonic plate is grinding into the Asian plate, and sinking under it. Where the two plates meet is an arc that runs from the Himalayas down to the Indonesian islands past Australia. So every decade or so there is an earthquake, every 50 or so years there is a massive earthquake, and every few hundred years there is an earthquake of this very huge magnitude. This is all part of the tectonic plates shifting and there is nothing we can do to stop that.

Also earthquakes have their advantages: they push land further up, thus helping vegetation flourish. So it is part of the shaping of the earth.

The problem is even after we know that India is in the active quake region, we have done so little to prepare for earthquakes. For instance, Delhi is in the high seismic zone, yet unscrupulous builders keep making their houses taller. So the next time there is a tremor, these houses are going to tumble and cause huge death and destruction.

We cannot predict where and when the next earthquake will strike. What we can predict is that it will happen, sooner of later, though now we can say that the magnitude is likely to be less. What we can do is prepare for it so that the death toll is low. Please remember, after the Gujarat earthquake, there was a massive quake in the western part of the US and the death toll was negligible. The reason was that they are ready for quakes. We are not.