The South Korean government has convened an emergency meeting over the issue, YTN television reported.
South Korea confirmed that North Korea may announce a nuclear test in 06.30 MSK, the Yonhap agency reported.
According to the agency, Pyongyang said that an "important statement" will be made at noon (12.30 p.m. Seoul time or 04.30 GMT).
The US Department of defense is investigating the earthquake data.
Tokyo admits the likelihood that North Korea might have conducted a nuclear test on Wednesday, which caused an earthquake in the country, Yoshihide Suga, Japan's Chief Cabinet Secretary said.
"Judging by the past experience, we are now considering the possibility of North Korea's nuclear test," Suga told reporters.
South Korea's weather service reported that the earthquake was of an "artificial nature." The USGS say that the earthquake was of 5.1 magnitude with the epicenter located in Pekam county, in Yangkang province.
South Korean sources reported that the earthquake occurred near the town of Kilchu.
South Korean intelligence claims that North Korea did not notify the United States and China about their plans to conduct a nuclear test, the Yonhap agency reports, citing a representative of South Korean intelligence service.
"North not previously notified the United States and China on the nuclear test."
In 2003, Pyongyang withdrew from the international Nuclear Non-Proliferation Treaty (NPT), a deal that was intended to prevent the spread of nuclear weapons manufacture and possible use.
In 2005, North Korea declared itself a nuclear power, and conducted several nuclear weapons tests, sparking concerns in the international community, particularly in neighboring South Korea. Almost four years later, Pyongyang tested additional nuclear weapons.
The official data on the country's nuclear program has not been released, and research on the issue is based on external observations and reports made by North Korean authorities.