Most earthquakes originate from the compression and tension that build 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, occurring 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 epicenter 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 epicenter, depth of focus, topography, and local ground conditions.
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.