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The Killer Plates

Earthquakes can be explained but not predicted.

india Updated: Mar 13, 2003 12:09 IST

The Friday fury that the people of Gujarat faced is only a chapter in the bulky volume of Nature's anomalies. Consider these statistics: Every day, there are about 1,000 very small earthquakes in various parts of the world. That roughly translates to one every 87 seconds. And every year there are 800 moderate earthquakes, capable of causing damage, and some 18 high-intensity ones that can wreak the sort of havoc witnessed in Gujarat.
What causes an earthquake?

Simply put, earthquakes are caused by 'faulting' - a sudden lateral or vertical movement of rock along a ruptured (broken) surface. Dr Harsh Gupta of the National Geophysical Research Institute, Hyderabad explains: "The surface of the Earth is in continuous slow motion. This is plate tectonics - the motion of the seven massive rigid plates forming the surface of the Earth.

Each of these plates is about 100-150 kms thick. The Indian plate extends from the Himalayas beyond the Andamans and Java Sumatra to Australia, where it touches the Pacific plate. The rate of movement of the plates varies from 2 to 12 cm per year. So slow, you really cannot be blamed for not noticing it.

Since all the plates are moving, they meet up occasionally, rubbing, colliding or sliding against each other (technically called "strain"). As the motion continues, the strain builds to the point where the rocks cannot withstand any more and, with a lurch, they give in breaking up. An earthquake follows, the grinding rippling up through the earth's crust to the surface.
Light brush - small quake. A real collision - Bhuj happens.

Motion effect
Any of the following three things can happen when the plates come up against each other, says Gupta.
Scenario A: One of the plates sinks under the other. Million of years ago, the Indian plate, which was initially attached to the South Pole, started moving towards Eurasia: this resulted in the Tethys sea - north of the plate - getting subducted or sanking beneath the Eurasian plate.

Scenario B: Collision. The Indian plate collided against the Eurasian plate, giving birth to the Himalayas some 40 or 50 million years ago. Incidentally, the Indian plate continues to push against the Eurasian plate, which is the reason for most of the seismic activity in this region.

Scenario C: The two plates slide past each other. There is no sinking but the sliding itself causes a lot of strain. The most notable example of this is the San Andreas fault in the USA, which has caused most of the quakes there.

Intra-plate stress
The above were examples of inter-plates stress. Intra-plate stress too can cause massive earthquakes. Intra-plate stress is due to the fact that not every part of the plate is similar and there are weak zones and faults, this leads to stress. This is known as stable continental region earthquake. This type of earthquakes is rare - Latur belonged to this category.

Preliminary findings are that Bhuj too is of this type.

Gupta says, "Paleo-seismological studies at Latur revealed that such an earthquake had occurred 2000 years ago. Studies of the 1897 Shillong plateau earthquake showed that three such earthquakes had occurred in the last 1500 years."

Vulnerable near the faults
Experts say earthquakes are common near faults. The Bhuj earthquake is very near the Allabund fault, which was caused by the 1819 earthquake which rocked Kutch. It is a scarp (a steep slope) about 100 km long and 6 metres high; it's visible in satellite pictures.

The Geological Survey of India has divided the country into different seismic zones, but, experts say, it should not be taken as the Bible because both Koyna and Latur were mapped in zones of negligible probability of occurrence of earthquakes, yet both were rocked badly; Koyna in 1967 and Latur in 1993.

The main problem is that with the present state of scientific knowledge, it is not possible to predict earthquakes and certainly not possible to specify in advance their exact date, time or location.

In short, you never know if it can hit you - and when it will - till it does.

First Published: Mar 06, 2003 12:58 IST