Scientists have discovered that the devastating 9.0-magnitude earthquake that struck Japan on March 11, 2011, triggered a previously unknown seismic event that permanently shifted nearly the entire country eastward.
According to a new study led by geophysicist Sunyoung Park, GPS measurements recorded that about 15 minutes after the earthquake began, most of mainland Japan moved east by approximately 5 to 6 millimetres. Although the displacement was small, it was permanent and affected a vast area stretching from Hokkaido to Kyushu.
Researchers initially considered the movement a possible data error because the shift occurred uniformly across a region nearly 3,000 kilometres long. However, years of analysis revealed that the movement was caused by seismic waves from the earthquake travelling deep into the Earth, reaching the planet’s core, and then rebounding back to the surface.
Scientists had long known that earthquake waves can travel to the Earth’s liquid outer core and reflect back. What surprised researchers was the discovery that these returning waves retained enough energy to trigger large-scale movement across multiple tectonic plate boundaries. The event displaced sections of four major tectonic plates, creating what researchers describe as an entirely new type of seismic phenomenon.
Unlike the violent shaking and tsunami generated by the initial earthquake, this secondary event spread its energy over an enormous area, making it less noticeable despite releasing energy comparable to a magnitude 7.5 earthquake. Because the energy was distributed broadly, the resulting ground movement caused far less concentrated damage than a typical earthquake of similar strength.
The 2011 earthquake remains the most powerful ever recorded in Japan. It struck northeast of Tokyo, triggered a massive tsunami, caused the nuclear disaster at Fukushima Daiichi Nuclear Power Plant, and resulted in nearly 20,000 deaths.
Researchers say the discovery has important implications for future earthquake monitoring and hazard assessment. Unlike aftershocks, which are difficult to predict, seismic waves travelling to the Earth’s core and back take roughly 15 minutes to complete the journey. This means scientists may be able to anticipate similar events following extremely large earthquakes.
Experts believe the phenomenon may occur elsewhere around the world, but it has likely gone undetected because few countries possess monitoring networks as extensive and sensitive as Japan’s. The findings could reshape scientific understanding of how powerful earthquakes influence the Earth’s crust and tectonic systems long after the initial shaking has ended.
The study also highlights how deep-Earth processes can have measurable effects on the planet’s surface, revealing a previously hidden source of seismic activity that could become an important consideration in future earthquake risk assessments.
