Before North Korea announced they had conducted the test, seismic activity had already been detected in North Korea by the United States Geological Survey (USGS) and the seismic network operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Preparatory Commission, near the site of previous nuclear tests at Mantapsan in Kilju County. A large tremor, first estimated at magnitude of 4.9, was detected in North Korea, and governments in the region were trying to determine whether it was a third nuclear test. The USGS upgraded the magnitude of the possibly nuclear tremor from 4.9 to 5.1, located east-northeast of Sungjibaegam, North Korea. The tremor occurred at 11:57am KST (2:57am UTC), and the USGS said the hypocenter of the event was only one kilometer deep.