SEOUL (Reuters) - A small natural earthquake detected in North Korea on Wednesday was likely a result of seismic instability lingering in the area since North Korea conducted its sixth and largest nuclear test in 2017, the South Korean government said.

A magnitude 2.5 earthquake was detected at 9:33 a.m. (0033 GMT) in Hamgyong Province, the location of North Korea’s shuttered Punggye-ri Nuclear Test Site, according to South Korea’s Meteorological Administration.

“It was a natural earthquake, presumably caused by the sixth nuclear test,” the administration said in a statement on its website. “The area is about 3 kilometres (1.86 miles) southeast of the sixth nuclear test site.”

Punggye-ri is the only known site in North Korea used to test nuclear weapons. At least six tests were conducted there between October 2006 and September 2017.

In early 2018, North Korea said it would close the site, saying its nuclear force was complete.

The entrances to tunnels at the site were blown up in front of a small group of foreign media invited to view the demolition, but North Korea rejected calls for international experts to inspect the closure.

Frustrated at what it sees as a lack of reciprocal concessions by the United States in denuclearisation talks, North Korea now says it is no longer bound by its self-imposed moratorium on test firing nuclear weapons and long-range ballistic missiles, though it has not conducted new tests.

The 2017 nuclear test, which North Korea said was a thermonuclear weapon, appeared to be several times larger than previous blasts, according to monitoring organisations at the time.

In the weeks after the sixth explosion, experts pointed to a series of tremors and landslides near the nuclear test base as a sign the large blast had destabilised the region.

Wednesday’s quake is the latest confirmation that the nuclear explosion had permanently changed the geology of the area, said Woo Nam-chul, an earthquake analyst at KMA.

“The terrain of the area was solid enough to have no natural earthquakes before the sixth nuclear test in September 2017.”