India's seismic suffering

A continental collision is the cause

By BBC News Online science editor Dr David Whitehouse

The earthquake that rocked north-western India is classified as a major event. Only about 18 quakes of this strength occur each year. It was a rare large quake in a region that is frequently shaken by smaller ones.

The last comparable quake in the region occurred on 16 June, 1819, killing 2,000 people. That quake was estimated to be about magnitude 7.7 on the impulse scale, slightly less than the current quake.

This quake occurred at 0316 GMT (0846 local time). Seismometers at the British Geological Survey's (BGS) headquarters in Edinburgh detected the passing shock waves from the quake some 10 minutes after it occurred.

Their initial estimates suggest that it was a relatively shallow one that delivered much destructive energy to the surface.

Slow-motion crush

India is no stranger to earthquakes as it lies next to the region where the greatest continental collision on Earth is taking place.

The surface of the Earth is divided into seven major moving tectonic plates.

Seismic signals detected by the BGS in the uk

About a hundred million years ago, when dinosaurs roamed the continental masses, the so-called Tethys Sea lay partly between the Indo-Australian and the Eurasian plate.

But about 60 million years ago, the Indo-Australian plate collided with the Eurasian plate - and neither would give way.

The collision is a slow-motion one, the closing speed is only 2 cm (1 inch) a year but the energies are colossal. The interface between the two plates has crumpled and been pushed upwards forming the Himalayas, the greatest mountain range on the planet.

The impact continues to this day and the Himalayas are still rising. Satellite measurements puts their ascent at about 5 millimetres a year.

Although an earthquake of magnitude 7.9 is rare, it is by no means unprecedented in a region that has been subjected to much seismic suffering.