TOKYO - A magnitude 7.2 earthquake that struck off the Pacific coast of Iwate Prefecture in northeastern Japan on Thursday triggered widespread perceptible tremors in the country's eastern half, from as far as Kanagawa Prefecture, south of Tokyo, to the northern main island of Hokkaido.
This was due to long-period ground motion, which travels over long distances and can cause buildings to sway even hundreds of kilometers from the epicenter. The weather agency reported intensity levels 1 and 2 on the four-tier scale for long-period ground motion for these areas.
Level 2, at which people indoors feel strong shaking and find it difficult to walk without holding onto something, was observed in five prefectures including Aomori, Miyagi and Hokkaido. Level 1, which causes most people indoors to feel shaking, was recorded in the other areas.
Experts said the quake occurred on a plate boundary and is likely part of a series of major quakes since late 2025, warning that the unusually frequent seismic activity may not be over and could pose a tsunami risk.
In Iwate and neighboring Aomori prefectures, strong quakes have continued, including an M7.5 quake last December and an M7.7 in April.
Shinichi Sakai, a University of Tokyo professor of seismology, said the latest quake occurred on a plate boundary just like the M7-class events since late last year, seeing it as part of the same activity.
But this may not be a normal situation, he warned, saying he has never seen quakes with such frequency before. If a temblor causes seafloor deformation, there would be a risk of tsunami, he said.
The concern echoes a lesson from 2011, when an M7.3 struck the area on March 9 -- two days before the M9.0 Great East Japan Earthquake and tsunami led to the loss of more than 22,000 lives. An "Off the Coast of Hokkaido and Sanriku Subsequent Earthquake Advisory" has since been created but the agency did not issue it this time because the temblor did not meet the criteria for activation.
Masanao Shinohara, a University of Tokyo Earthquake Research Institute professor of seafloor seismology, said the latest quake was "a typical plate boundary earthquake."
Shinohara said the earlier quakes since late last year broke a relatively shallow offshore area, but that this one likely occurred at the deepest part of a zone where two tectonic plates are locked together.
The focus close to land apparently caused the quake as deep as 44 km to produce strong shaking, but similar earthquakes have occurred in the past and this event was not especially new, he added.
The Japan Meteorological Agency says long-period ground motion, which takes long for shaking to complete one full back-and-forth cycle, occurs in major earthquakes and can cause high-rise buildings to sway for long periods and/or trigger elevator malfunctions even hundreds of kilometers away.
The highest level 4 on its scale means people cannot remain standing and must crawl to move, while level 3 makes it difficult to stand.
