THE MASSIVE scale of the Gujarat earthquake is being blamed on the fact that it started just 9.6km below ground. The point underground where rock begins to break up at weak seams or fault lines, triggering the shock waves, can be up to 720km deep but it is usually less than 72km.
Gujarat stands on a layer of hard sandstone rock resting on softer alluvial deposits.
As the quake struck, the alluvial layer flexed upwards, forcing the surface sandstone to break up along a 70km fault line in the Rann of Kutch. This created dramatic 7m wide cracks in the ground at the quake's epicentre in a salt marsh about 40km north of Bhuj.
Powerful shock waves travelled much further through the Kutch region, transmitted through a layer of alluvial deposits.
Waves of energy travelled far beyond the 70km fault line, as far down as Gujarat's biggest city Ahmedebad 400km away, where nearly 100 buildings collapsed. The 500km radius of the quake compares to the Mexico earthquake of 1985, which had its epicentre in the Pacific Ocean. The quake that hit Turkey 18 months ago affected an area only 100km across (NCE 16 Sept 1999).
The enormous energy of the quake comes from the Indian tectonic plate rubbing against the Eurasian plate. The movement of the two plates originally created the Himalayas and the Indian plate is still moving northwards by 10mm a year.
The Gujarat quake is said to be the first major one in the region since 1819, although a smaller earthquake hit the town of Anjar in 1956.
At 7.9 on the Richter scale, the Gujarat quake compares in size to the San Francisco earthquake of 1906. The 1989 San Francisco earthquake was 7.1, that of Los Angeles North Ridge in 1994 was 6.9 and Japan's 1995 Kobe disaster was 7.2.