Equipment at the Montana Bureau of Mines and Geology here detected North Korea’s purported nuclear testing over the weekend. “It could be easily overlooked, unless you knew where to look,” said Mike Stickney, a seismologist who heads the bureau’s earthquake studies office. A handful of the bureau’s monitoring stations registered a faint record of the underground explosion Sunday evening, Stickney said. Typically, equipment at the bureau picks up seismic events that register at magnitude 5.5 on a global stage, but some did pick up the explosion, which was about a 4. 2. “That’s not very large seismologically speaking,” he said. The Montana Bureau of Mines and Geology is a non-regulatory agency that conducts research and assists in the development of the state’s mineral and water resources. It also provides advisory, technical and informational services on Montana’s geologic, mineral, energy and water resources.