K-NET is the largest strong motion network with more than 1,000 stations operated by National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan. Each station has a three-component accelerograph on the ground. An acceleration record of each component is stored in a file with a file extension .ns, .ew or .ud. Further information and observed records are available at http://www.k-net.bosai.go.jp/.