According to DAE's own seismic network at Gauribidanur in Karnataka, the tests released energy equivalent to 55,000 tonne of TNT, or about five times that of the nuclear test in 1974.

However, what is puzzling is that while the 1974 blast produced a seismic signal of 5.0 on the Richter scale, the latest blasts produced a signal of only 4.7 in the same scale as recorded by the British Geological Survey and the National Geophysical Research Institute (NGRI) based in Hyderabad.

"Considering that the Monday blasts were five times more powerful, the seismic magnitude should have been higher than 5.0," says JG Negi, scientist emeritus at the NGRI.

The Richter scale is so calibrated that even an increase of one unit in that scale corresponds to an increase in the amount of energy released by a factor of about 30.

Accordingto Negi, it is somewhat puzzling that the latest tests with a reported yield of 55 kt produced a seismic signal lower in magnitude than that of the 1974 test.

The NGRI scientist, however, said that lower seismic magnitude could be explained if the energy released by the thermonuclear device was mostly in the form of heat and radiation than elastic shock waves.

Another puzzling factor is that only one of the five blasts was detected by seismometers around the world although the global system created for monitoring nuclear blasts is capable of detecting any nuclear explosion larger than 1000 tonne anywhere in the world.

So, the system might be expected to catch three of India's five blasts, the other two being smaller than 1000 tonne and hence undetectable.

However, defence scientists connected with this week's nuclear tests told PTI that the explosives were set off simultaneously. They said the signals can be resolved to show separate peaks.

It is believed that two of the three devices wereplaced in the same shaft at two levels and the thermonuclear device was exploded at a depth of 300 metres about one-and-half kilometres away from the first hole.

The explosions were carried out simultaneously apparently to avoid the effect of one interfering with another. According to scientists proper instrumentation took care of interference.