By Simon Fraser BBC News Online

Earthquakes are common in Afghanistan and particularly in the Hindu Kush.

The mountain range lies near the boundary of the Eurasian and Indo-Australian tectonic plates, where the greatest continental collision on Earth is currently taking place.

There are on average four earthquakes a year measuring five or more on the Richter scale, with the epicentres occurring within 60 kilometres (37 miles) of the Hindu Kush.

The depth and magnitude of this event preclude any connection with human activity, including the recent bombing

US Earthquake Info Center

"The Indian continent is moving north, and it is colliding with the Eurasian continent, and that results in the subsequent uplift of the Himalayan mountains and the Tibetan plateau," Dr Brian Baptie of the British Geological Survey told the BBC.

"It's this collision that is the cause of all the seismic activity that is going on in this area."

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

Until about 100 million years ago, the Indo-Australian and Eurasian plates were partly separated by the so-called Tethys Sea.

They first collided about 60 million years ago.

The collision is a slow-motion one - the two plates are converging on each other at a rate of about 4.4 centimetres (2 inches) a year - but the energies are colossal.

As a result, the Himalayas are still rising. Satellite measurements put their ascent at about five millimetres per year.

US bombing

Afghanistan's latest earthquake occurred at a depth of about eight km.

Near-surface earthquakes have been caused in the past by human activity such as mining.

Some e-mails to BBC News Online have suggested that United States bombing, and in particular the recent use of thermobaric shockwave bombs, may have contributed to the disaster.

But the US National Earthquake Information Center rules this out as a possible cause.

"The depth and magnitude of this event preclude any connection with human activity, including the recent bombing," it says in a bulletin posted on its website.

"The faulting that produced the earthquake reflects internal deformation of the subducted Eurasian plate rather than slip on the boundary between the Eurasian and Indian plates."