NAGANO — A large underground display screen on a mountain in the city of Nagano reflected an abnormal seismic wave with an alarm at 9:54:40 a.m. May 25.
Matsushiro Seismological Observatory officials studied the earthquake wave, pinpointed its direction and distance and immediately reported to the Meteorological Agency in Tokyo that they "observed a shock caused by unnatural shaking of the ground in the northeastern part of North Korea."
The display device that sensed Pyongyang's nuclear test is part of the observatory's detailed quake monitoring system comprising eight of the newest and most advanced seismometers, and is the only one of its kind in Japan.