On December 26, a massive earthquake measuring 8.9 on the Richter Scale struck off the coast of Sumatra, northern Indonesia. It is the biggest one recorded in 2004 and possibly in four decades.
Sumatra is about 2,500km from the Indian coast and in the normal course, the latter would not have felt anything more than a slight tremor, which it did. But the quake exposed India to the phenomenon and destructive capability of tsunamis, which are triggered by quakes.
Tsunamis devastated Tamil Nadu and Andhra Pradesh in India and the neighbouring country of Sri Lanka.
Dr V Rajamani is a professor at the School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, specializing in the field of geo-chemistry. He explained the factors behind the earthquake that hit northern Indonesia and its implications to Deputy Managing Editor Amberish K Diwanji. Excerpts from the interview:
8.9 on the Richter Scale. That is a massive earthquake. How often do such earthquakes occur?
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.
This was a submarine earthquake. Would you explain this phenomenon and what happens?
Such quakes occur under the sea or ocean. If they occur 30 kilometres or more below land, 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 below ground level, 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 is 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. Tsunamis are caused by earthquakes.
When a 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. 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 height goes up to a few metres.
There have been tsunamis as high as 30-40 metres, but the tsunamis that hit India were about 3-4 metres high.
Yet, no one can recall tsunamis crashing into India?
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).
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.
So in the present case, the deaths are more due to human failure?
I have come back from Chennai just 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. So when the tsunamis came, they no doubt killed many of the poor people who lived 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 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 politicians have encouraged it.
-
They had no warning at all: Dayanidhi Maran
The tsunamis have also caused flash floods that claimed lives.
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 River in Delhi.
India is a considered a seismic active region. Would you throw some light on this?
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 years or so, 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 Earth.
The problem is that even after we know that India is 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. 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 United States and the death toll was negligible. The reason was that they are ready for quakes. We are not.
-
Also See:
Tsunami hits south India; over 1,800 dead
Tsunamis claim over 6,000 lives in Asia
What is a tsunami?
Tidal waves lash Tamil Nadu coast, several dead
Tidal waves hit Andhra Pradesh coast, several dead
Earthquake in Andaman and Nicobar Islands
Quake in eastern India, one dead in WB
Tidal waves pound Sri Lanka, hundreds dead
