As monitored from Seismic Network Operations, IC MDJ, Mudanjiang, Heilongjiang Province, China, this is an earthquake signature characteristic of nuclear testing. Or so that's the working assumption: North Korea is doing nuclear testing.
What follows will be links, because I don't know any more about this than what I find out from the media.
Nuclear Test Facility, Google Plus. This pinpoints the test facility, and gives its address as Nuclear Test Rd, North Hamgyong, North Korea.
New York Times/AP: Earthquake Detected in North Korea. "There is a high possibility that North Korea has conducted a nuclear test," said Chi Heoncheol, an earthquake specialist at the government-funded Korea Institute of Geoscience and Mineral Resources. Chi said a magnitude 3.9 magnitude earthquake and a magnitude 4.5 earthquake were detected in the North's 2006 and 2009 nuclear tests."
NHK World Radio Japan news in English, monitored online at 11:39 p.m. EST, February 11, 2013, has no news. The news of the earthquake and testing is top story in the 12:00 a.m. EST, February 12, 2013 edition.
BBC News. "We believe that there is a possibility that North Korea carried out a nuclear test, looking at past cases," said Chief Cabinet Secretary Yoshihide Suga – an opinion echoed by South Korea's defence ministry.
NK News is live blogging. Update 1142EST: Steve Herman at Voice of America reports that a source in Yanji, China, reported that a swaying motion was felt for 10 seconds around the time of the blast. Yonhap add that 163 nationals residing in the Kaesong Industrial Complex are reported to be safe.