South Korea thinks so, via AP:

North Korea is believed to have conducted a widely anticipated nuclear test Tuesday, following an earthquake that monitoring agencies around the globe said appeared to be unnatural.

The South Korean Defense Ministry, which raised its military alert level after the quake, said it was trying to determine whether it was a test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.

A world nuclear test monitoring organization detected what it called an “unusual seismic event” in North Korea.

“The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK nuclear tests,” said Tibor Tóth, the executive secretary of the Comprehensive Nuclear Test Ban Treaty Organization. The DPRK refers to North Korea’s formal name, the Democratic People’s Republic of Korea.

The NY Times further reports:

It may take days or weeks to determine if the test, if that is what it proves to be, was successful. But American officials will also be looking for signs of whether the North, for the first time, conducted a test of a uranium weapon, based on a uranium enrichment capability it has been pursuing for a decade. The past two tests used plutonium, reprocessed from one of the country’s now-defunct nuclear reactors. While the country only has enough plutonium for a half-dozen or so bombs, it can produce enriched uranium well into the future.

Iowahawk is all over this one:

Update: The Norks confirm:

North Korea confirmed on Tuesday that it had conducted its third, long-threatened nuclear test, according to the official K.C.N.A. news service, posing a new challenge for the Obama administration in its effort to keep the country from becoming a full-fledged nuclear power.

The K.C.N.A. said the North used a “miniaturized and lighter nuclear device with greater explosive force than previously” and that the test “did not pose any negative impact on the surrounding ecological environment.”