u.s. geological survey say saturday could conclusively confirm whether magnitude 3.5 earthquake detect north korea manmade natural usgs put epicenter quake 22 km 14 miles east-northeast sungjibaegam north korea near country main nuclear test site chinese earthquake officials early say tremor suspect explosion south korean officials ctbto nuclear proliferation watchdog say believe natural quake depth poorly constrain hold km seismologist usgs say air force technical applications center aftac sole organization u.s. federal government whose mission detect report technical data foreign nuclear explosions usgs say confirm north korea quake natural manmade