Lee Ji-eun, AP
A South Korean official explains seismic waves of North Korea's nuclear test which were measured in South Korea, at the Korea Meteorological Administration in Seoul, South Korea, Tuesday, Feb. 12, 2013. North Korea said it successfully detonated a miniaturized nuclear device at a northeastern test site Tuesday, defying U.N. Security Council orders to shut down atomic activity or face more sanctions and international isolation. (AP Photo/Yonhap, Lee Ji-eun) KOREA OUT

SEOUL, South Korea — South Korean officials detected an "artificial earthquake" near North Korea's main nuclear test site Wednesday, a strong indication that nuclear-armed Pyongyang had conducted its fourth atomic test. North Korea said it planned an "important announcement" later Wednesday.

A confirmed test would mark another big step toward Pyongyang's goal of building a warhead that can be mounted on a missile capable of reaching the U.S. mainland.

The U.S. Geological Survey measured the magnitude of the seismic activity at 5.1 on its website. An official from the Korea Metrological Administration, South Korea's weather agency, said it believed the earthquake was caused artificially based on their analysis of the seismic waves and that it originated 49 kilometers (30 miles) north of Kilju, the northeastern area where North Korea's main nuclear test site is located. The country conducted all three previous atomic detonations there.

South Korean government officials couldn't immediately confirm whether a nuclear blast or natural earthquake had taken place.

North Korea conducted its third nuclear test in February 2013.

Another test would further North Korea's international isolation by prompting a push for new, tougher sanctions at the United Nations and worsening Pyongyang's already bad ties with Washington and its neighbors.

Pyongyang is thought to have a handful of crude nuclear weapons. The United States and its allies worry about North Korean nuclear tests because each new blast brings the country closer to perfecting its nuclear arsenal.

Since the elevation of young leader Kim Jong Un in 2011, North Korea has ramped up angry rhetoric against the leaders of allies Washington and Seoul and the U.S.-South Korean annual military drills it considers invasion preparation.

AP writer Hyung-jin Kim contributed to this story from Seoul.