North Korea ? North Korea announced today that it successfully carried out an underground nuclear test, weeks after threatening to restart its rogue atomic program.

The country’s official Korean Central News Agency called today’s test “part of measures to bolster its nuclear deterrent for self-defense.”

South Korean President Lee Myung-bak convened an emergency security session. His spokesman, Lee Dong-kwan, confirmed that a nuclear test may have been carried out in the North.

Seismologists from the U.S., South Korea and Japan reported earthquakes in an northeastern area, where North Korea conducted a nuclear test in 2006.

A 4.7-magnitude earthquake was registered in northeastern North Korea at 9:54 a.m., the U.S. Geological Survey said.

The quake, measured at a depth of 6 miles underground, occurred 40 miles northwest of the city of Kimchaek, the USGS said.