From Bloomberg (italics mine):

"Tremors Detected

The U.S. Geological Survey detected a tremor of magnitude 4.2, which is within the 'range' of previous underground nuclear tests by others, said Bill Leith, a seismologist at the agency. Amy Vaughan, a geophysicist at the National Earthquake Information Center, said the tremor occurred in the northeastern part of the country. South Korean officials said their instruments detected tremor of magnitude 3.58 to 3.7. [the Richter scale is logarithmic, meaning each whole number increase equals ten times the previous force...so that's a significant difference]

Bush and other administration officials said the U.S. still was analyzing the data, and some experts said they remained skeptical about North Korea's claim.

'There are lots of questions about this test,' said Jim Walsh, a nuclear weapons expert at the Massachusetts Institute of Technology who visited North Korea last year. While the tremors detected were within the range of nuclear tests, he said, they were on the lower end. [the ? is...which number is he talking about?]

'Unusual' Test

'It's very unusual for countries that are testing for the first time to have such a small test,' Walsh said. 'People are going to ask whether this was a failed test -- in other words, a fizzle -- of a much larger bomb that only half went off, or a fake.'"