How earthquakes happen

New Delhi, April 26
Most earthquakes originate from compressional or tensional stresses built up at the margins of the huge moving plates that make up the earth's surface.

The immediate cause of most shallow earthquakes like in Nepal now, occuring within 70 km of the surface, is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another.

The focus of an earthquake is the point where it originates within the earth. The earthquake epicentre is the point on the earth's surface directly above the focus.

The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves, which depends on many factors such as the magnitude, distance from the epicentre, depth of focus, topography, and local ground conditions.

The area of Saturday's earthquake in Nepal is the so-called Indus-Yarlung suture zone, where the Indian subcontinent collided 40 million to 50 million years ago with the Eurasian plate, creating the Himalayan mountain ranges which are still rising by around one centimetre a year as an ongoing consequence.

In areas underlain by water-saturated sediments, large earthquakes, usually magnitude 6.0 or greater, may cause liquefaction.

Beneath the Kathmandu Valley is a 300-metre deep layer of black clay, the remnants of a prehistoric lake, which amplifies the damage caused by severe earthquakes. Studies have established that this region is prone to soil liquefaction in strong earthquakes, when vibrations can cause solid ground to collapse, swallowing buildings in the process.

The Indian subcontinent has a history of devastating earthquakes. The earthquake zoning map of India divides the country into four seismic zones - 2, 3, 4 and 5 - in which Zone 5 expects the highest level of seismicity and is referred to as the Very High Damage Risk Zone. Kashmir, the western and central Himalayas, the northeast region and the Rann of Kutch fall in this zone.