The Japan Meteorological Agency observed class 3 long-period seismic motion following a magnitude 7.7 [1] earthquake off the coast of Miyagi on April 26, 2026 [2].
This specific type of seismic activity is critical because it poses a significant risk to high-rise buildings and furniture. While standard shaking affects lower floors, long-period waves can cause severe swaying in taller structures, making it difficult for occupants to stand [3].
The earthquake occurred at approximately 16:53 JST [4]. The agency said class 3 motion was specifically observed in northern Miyagi Prefecture and inland southern Akita Prefecture [5]. Other reports indicate that long-period seismic motion was observed across 16 prefectures nationwide [6].
Reports on the maximum seismic intensity varied among sources. One report listed the maximum intensity as shindo 5 weak [7], while another recorded it as shindo 5 strong [8].
"Long-period seismic motion refers to large shaking where the 'period'—the time it takes for one oscillation—is long," the Japan Meteorological Agency said [9].
The agency said the class 3 designation indicates a level of shaking where standing becomes difficult [3]. This phenomenon is often more pronounced in urban centers with dense concentrations of skyscrapers, as the buildings can resonate with the long-period waves generated by large offshore earthquakes.
Officials confirmed the epicenter was located off the coast of Miyagi [10]. The agency continues to monitor the region for aftershocks and further seismic instability following the initial event.
“Class 3 indicates difficulty standing and poses risk to high-rise buildings.”
The observation of class 3 long-period seismic motion highlights a specific vulnerability in modern urban infrastructure. Unlike short-burst tremors that damage older, low-rise masonry, these waves resonate with the natural frequency of skyscrapers, potentially causing significant non-structural damage and panic even if the building's frame remains intact.
