The Japan Meteorological Agency reported long-period ground motion classified as class 3 following a strong earthquake off the coast of Miyagi Prefecture [1, 2].
This classification is significant because long-period seismic waves can cause prolonged shaking that affects tall buildings and infrastructure, potentially increasing the risk of damage even far from the epicenter.
The earthquake occurred at 16:53 JST on April 20, 2026 [4], at a depth of approximately 10 kilometers [5]. The seismic activity was centered offshore Sanriku, affecting the Miyagi-prefecture coast. The JMA observed class 3 long-period ground motion in northern Miyagi, as well as in northern Akita and southern Yamagata [1, 2, 3].
Reports on the maximum observed intensity vary among sources. Some data indicates a maximum intensity of shindo 5-weak [1], while other reports cite a higher intensity of shindo 5-strong [2]. The JMA said the class 3 designation for long-period ground motion specifically indicates that the shaking was intense enough to make standing difficult [1, 2].
The agency's monitoring system tracks these specific wave patterns to help engineers and city planners improve the seismic resilience of high-rise structures. Because these waves oscillate slowly, they can resonate with the natural frequency of tall buildings, causing them to sway more violently than shorter structures would during the same event [1].
“The JMA observed class 3 long-period ground motion in northern Miyagi, as well as in northern Akita and southern Yamagata.”
The occurrence of class 3 long-period ground motion highlights a specific vulnerability in urban infrastructure. Unlike standard seismic intensity, which measures peak acceleration, long-period motion focuses on the duration and sway of the earth. This event serves as a critical data point for the Japan Meteorological Agency to refine early warning systems and building codes for skyscrapers in the Tohoku region.
