Last Friday, as the first of a series of strong earthquakes hit the northern tip of Ishikawa Prefecture, the Meteorological Agency issued its first early warning for a lesser-known type of quake: "long-period ground motion."

The agency issues advance warnings for powerful earthquakes based on waveform data observed by seismographs near the epicenter. In February, it started to include expected shakes from slow ground movements as part of its early-warning system, to inform people mostly on the upper floors of tall buildings of the imminent danger of furniture and equipment moving or falling.

The initial jolt in Ishikawa was the first time an early warning for such slow shaking was issued.