Some of the recent Indian and Pakistani nuclear tests might have been "muffs" that were either extremely weak or failed to explode as designed, a top expert says.
India's government said it exploded five nuclear devices and the Pakistanis six in underground tests between May 11 and 30. Seismic waves from those explosions rippled through the Earth's crust, and some were detected by seismographs hundreds and thousands of miles away.However, Western seismologists are investigating the possibility that India and Pakistan successfully detonated fewer bombs than claimed, according to Thorne Lay of the University of California at Santa Cruz, an adviser to the U.S. government on monitoring of nuclear tests.
The tests "may have included `muffs' that failed to achieve the anticipated (explosive) yield," said Lay, a geophysicist who chaired two National Research Council studies on nuclear-test monitoring.
While some bombs unquestionably detonated with full force, "we're puzzled by the very weak signals or non-detection of (seismic) signals" from others, Lay said.
India's underground tests are conducted in a region with "very stable continental rocks, which should be very efficient at transmitting (seismic) signals."
Lay said some explosions may have fallen short of critical mass. In a normal atomic bomb, the fissionable material is compressed extremely quickly into a super-dense state called critical mass.
This unleashes a flood of subatomic particles called neutrons, which split enough atoms fast enough to trigger an atomic explosion.
In 1945, an atomic bomb incinerated Hiroshima with an explosive force of 14,000 tons of TNT. By contrast, a subcritical explosion might equal 10 to 20 tons of TNT, the nuclear version of a wet firecracker.
One possible explanation, Lay said, is that the Indians and Pakistanis might have detonated some bombs simultaneously. This might have made it difficult for Western seismologists to assess the true magnitude of individual blasts.
Another possibility is that India and Pakistan may have deliberately conducted subcritical blasts to test parts of nuclear devices, rather than full-fledged bombs.
However, Lay said, it would be hard to reconcile this with the fact that India and Pakistan have referred to all the blasts as nuclear explosions.
Other U.S. officials and test-monitoring experts confirmed that seismographs failed to detect proof that all 11 blasts occurred as claimed.
India's claims for the May 13 blasts are "a puzzle, because we saw absolutely no signals" from the alleged explosions that day, said Paul Richards of the Lamont-Doherty Earth Observatory at Columbia University in New York.
Thomas Cochran, a physicist and critic of U.S. nuclear weapons policies at the Natural Resources Defense Council, said: "It's clear that we didn't detect all the blasts that were alleged to have occurred. Some of that is explainable and some of it is not."
Cochran recalled seeing a photo of a surface crater formed by one of India's explosions. The crater looked unusually small for something allegedly generated by a nuclear blast of the advertised size, he said.
Cochran said certainly one possibility is that Indian and Pakistani officials overstated the size of the blasts. Another possibility is that some tests were conducted in geological terrain that might have muffled the blasts.
Yet another possibility is that the seismic network itself failed. But Western experts reject this explanation.
"I really see no failure of the monitoring system at all," Lay said. If anything should be questioned, he said, it is the claims made by the Indian and Pakistani governments.