DEVASTATION IN the Gujarat region of north west India that left up to 100,000 dead was caused not by one but two earthquakes, according to seismic engineers at Imperial College, London.
The first event, at 8.45am on 26 January, measured 7.6 on the Richter Scale and was followed three seconds later by one measuring 7.9.
'One earthquake is bad enough but then to have an even larger one seconds later is really incredible and very rare, ' said Imperial's head of engineering seismology and earthquake engineering, Amr Elnashai.
Many buildings were damaged by the first event and destroyed by the second. About 90% of buildings were flattened at the epicentre and there were 80% fatalities in some areas.
The extreme destruction of the huge earthquake, comparable in scale to the San Francisco event in 1906, is being blamed on the fact that its hypocentre was only 9.6km below ground.
Gujarat stands on a layer of hard sandstone resting on softer alluvial deposits.As the quake struck, the alluvial layer flexed upwards, forcing the sandstone to break up along a 70km long fault line in the Rann of Kutch. This created dramatic 7m wide cracks in the ground at the epicentre in a salt marsh about 40km north of the town of Bhuj.
The hard sandstone led to greater vibration frequency, Elnashai said. Eyewitnesses said they felt the ground shaking for up to seven minutes.The low water table in the region increased the violence of the shaking and the damage.
Shock waves travelled through the alluvium, causing damage as far away as the region's biggest city Ahmedebad, some 400km distant, where nearly all domestic and commercial buildings over three storeys collapsed.
Building permits for the region include earthquake codes but are said to have been ignored as construction boomed in the economically strong region.It is thought that many buildings were already weakened by having columns knocked out on the first floor to make space for shops and restaurants.