SEISMIC ENGINEERS said this week they believed the earthquake damage in the Gujarat region of India was worsened by the fact it was struck by two increasingly strong tremors within seconds of each other.
Engineers at London's Imperial College said this week that the first earthquake, measuring 7.6 on the Richter Scale, was quickly followed by another three seconds later, measuring 7.9.
The quake struck two weeks ago, severely damaging structures as far as 200km from the epicentre at Bhuj.
Efforts to rescue survivors were still under way early this week.
Many buildings were severely damaged by the first earthquake and were then flattened by the second one.
'One earthquake is bad enough but then to have an even larger one seconds later is really incredible and very rare, ' said head of seismic engineering at Imperial College professor Amr Elnashai. He is due to lead a mission to Gujarat in the next few days to do a detailed inspection of the building collapses.
Hard sandstone at the epicentre led to greater frequency of vibrations he said.
Eyewitnesses said that strong vibrations were felt for up to seven minutes. The low water table in the region increased the violence of the shaking which flattened 90% of the buildings at the epicentre and produced 80% of the fatalities in some areas.
A high water table in other earthquakes such as the Turkish earthquake (NCE September 16 1999) caused the ground to liquify during the quake. Buildings tended to sink or tilt rather than collapse.
Building permits for the Gujarat region included earthquake codes but are said to have been ignored as construction boomed in the economically strong region.
It is thought many flattened buildings were already badly weakened by having columns knocked out on the first floor to clear space for shops and restaurants. Initial reports state that many of the buildings suffered a first floor collapse.
The Gujarat earthquake was bigger than the Turkish earthquake of 1999 (7.6), the Los Angeles 1994 quake (7.1) and the San Francisco quake of 1989 (6.9). It was the first major earthquake in India since 1819.