SEOUL, South Korea (AP) — South Korea has detected a fourth small earthquake near North Korea's main nuclear test site after the country's sixth and most powerful nuclear test explosion last month.

South Korea's weather agency says the four quakes likely occurred because the underground nuclear explosion Sept. 3 weakened or affected the tectonic plate structures in the area.

No such quakes were detected after North Korea's five previous, and much smaller, nuclear tests since 2006.

The agency says Friday's magnitude 2.7 quake was 54 kilometers (0.62 miles) northwest of the town of Kilju in northeastern North Korea.