core-reflected seismic – About 15 minutes after the magnitude-9 Tohoku earthquake on 11 March 2011, almost all of Japan shifted about 5 millimetres east. Researchers say the motion was driven by an immensely powerful seismic wave that traveled 5,800 kilometres to Earth’s core and boun
Around 15 minutes after the magnitude-9 Tohoku earthquake struck on 11 March 2011, Japan didn’t just shake—it stepped. Almost the whole country jumped half a centimetre to the east. a sudden lurch attributed to a seismic wave that raced 5. 800 kilometres into Earth’s interior. struck the core. and then rebounded toward the surface.
In a catastrophe that included localized land movements of many metres and 40-metre tsunami waves—and the meltdown of three reactors at the Fukushima Daiichi nuclear plant—5 millimetres can sound small. But this wasn’t a local wobble. The movement unfolded across about 3. 000 kilometres. nearly seven times longer than the length of the earthquake’s main rupture line and longer than any slip ever recorded.
What makes the event stand out is its timing and how consistently the shift appeared across the country. Sunyoung Park of the University of Chicago says the pattern was not what scientists typically expect to see from an ordinary earthquake at that exact moment. “We see a small 5-millimetre eastward step that happens nearly simultaneously and with similar size across almost all of Japan. without any ordinary earthquake at that exact time.”.
The eastward step also wasn’t confined to a narrow boundary. Park describes it as a broad movement: “It is not just a narrow ‘edge’ that moved,” she says. “The eastward step extends at least across the whole of Japan where we have GPS stations. If we had similarly dense instruments on the seafloor. we could say more precisely how far offshore this motion extends. but on land. the shift is observed pretty much everywhere across Japan.”.
By working through extensive GPS and seismic data recorded during the disaster. Park and her colleagues traced how such a vast motion could be triggered—and why it arrived precisely 15 minutes after the main shock. Earthquakes do generate waves that dive deep, reflect off the core, and return upward. But they usually weaken substantially by the time they reach the planet’s centre and then travel back.
Tohōku was different. The main shock was so large that even after weakening on its way to the core. the returning wave remained strong enough to produce the nationwide lurch. Park says the broad movement was enabled by the way tectonic plates shifted: “We think the vigorous shaking from the original Tohoku earthquake might have already weakened the plate boundaries. making them more susceptible to be moved when the core-reflected wave came by. ” she says. “So in four adjoining tectonic plates, they moved in unison.”.
In that view, the second surge wasn’t an unrelated aftershock. It was a delayed punch landing after the core-reflected wave returned—one that helped drive fault motion over an unusually large region.
The researchers argue the episode also changes how scientists may need to think about earthquake consequences. Park says the event demonstrates previously unrecognized mechanisms of destruction that can follow major quakes. “It shows that. after a big earthquake. we might also need to be aware of potential seismic hazards due to such deep-travelling wave arrivals that can trigger more events. and over very large distances.”.
Robin Lee of the University of Canterbury in New Zealand cautions that the wider meaning is not limited to Japan. More work is needed to understand what such deep-travelling arrivals could do elsewhere. “It shows that large earthquakes can trigger widespread. delayed fault motion minutes later. and over much larger regions than expected. ” Lee says.
For now, the Tohoku rupture remains a warning that shaking can have a second chapter—arriving not with the main quake, but minutes later, after a wave’s long journey to the core and back, moving the ground across distances that were never part of the usual script.
Tohoku earthquake 2011 earthquake seismic waves Earth’s core GPS tectonic plates Fukushima Daiichi Japan displacement deep-travelling waves seismic hazards