The heightened alert is in response to growing threats from the North over expanded sanctions approved last week by the U.N. Security Council for its long-range rocket test last month in violation of earlier U.N. sanctions.
Yonhap, quoting sources at the Korea Institute of Geoscience and Mineral Resources, said a situation room set up at the agency's national data center in Daejeon city in the middle of South Korea will receive and process seismic information.
"Our job is to detect the blast and figure out its location and size," Chi Heon-cheol, head of the institute's seismic research office, told Yonhap. The report said it was the institute's seismic array network that detected the North's May 2009 nuclear test 40 second after it occurred. The North's first nuclear test was in 2006.
North Korea has been issuing a number of threats since the UNSC resolution, blaming its "sworn enemy" the United States for it.
The North has said it would target the United States with rocket launches and a "higher-level" nuclear test, a threat the White House dismissed as being "needlessly provocative." Any new test would be the North's third.
Last weekend, the North's official media quoted its leader Kim Jong Un as expressing his resolve to take serious action.
Though it is not clear when a third nuclear test might occur, South Korean experts have said the North may be able to do it at short notice once the go ahead it given.
Separately, Yonhap quoted South Korea's chairman of the Joint Chiefs of Staff, Gen. Jung Seung-jo that North Korea is ready for a nuclear test "anytime" for political purposes and urged his military to stay vigilant.
"North Korea's nuclear program development poses a direct threat to our people. In addition, it could launch various provocations to cause fatal damage or casualties on our side," Jung said during a visit to military units north of Seoul. "Lately, North Korea has increased live-fire drills by three times compared to the past."
The general told Yonhap South Korean and U.S. forces were closely monitoring North Korea's nuclear test preparations. The United States currently has about 28,500 of its soldiers stationed in South Korea.