Japan shifted eastward within 15 minutes of quake

About 15 minutes after the magnitude-9 Tohoku earthquake on 11 March 2011, almost the whole of Japan jumped half a centimetre east. Researchers say the nationwide lurch—measured across GPS stations from landward regions and extending at least across Japan into

At about 15 minutes after the magnitude-9 Tohoku earthquake struck on 11 March 2011, Japan’s ground didn’t just keep shaking. It stepped.

Nearly the whole country jumped half a centimetre to the east as an immensely powerful seismic wave. after travelling 5. 800 kilometres to Earth’s core. bounced back toward the surface. In the shadow of the disaster that followed—including localized land movements of many metres and 40-metre tsunami waves that contributed to the meltdown of three reactors at the Fukushima Daiichi nuclear plant—5 millimetres can sound small.

But the scale and the timing were what made the moment hard to ignore: the shift unfolded over a distance of 3,000 kilometres—nearly seven times longer than the earthquake’s main rupture line, and longer than any slip ever recorded.

What also set the phenomenon apart was its near-simultaneous pattern. “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. ” said Sunyoung Park. a researcher at the University of Chicago.

Park and her colleagues reached that conclusion by analysing extensive GPS and seismic data collected during the catastrophe. The finding is striking in practice because it suggests the motion wasn’t confined to a narrow boundary. “It is not just a narrow ‘edge’ that moved,” Park said. “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.”.

In other words, Japan didn’t merely move. It moved in a way that appeared to span the width of the country and reach beyond it—into the ocean.

Earthquakes can send waves deep through Earth, where they reflect off the core and return toward the surface. Usually. by the time that core-reflected wave comes back. it has weakened enough that it doesn’t produce a comparable nationwide step. The Tohoku event was different because the main shock was so large that. even after weakening on its journey. the returning wave still carried enough force to produce the countrywide lurch.

Park tied it to the way tectonic plates behaved during the event. “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 said. The picture that emerges is of four adjoining tectonic plates moving in unison as the reflected wave arrived.

The implications extend beyond a single historic event. Park said the episode demonstrates that there are previously unrecognized mechanisms of destruction that can follow earthquakes. “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.”.

For Robin Lee at the University of Canterbury in New Zealand, the message is equally direct: “It shows that large earthquakes can trigger widespread, delayed fault motion minutes later, and over much larger regions than expected.”

Understanding how to apply those lessons will take work. More research is needed, Lee said, to clarify what the same kind of delayed, large-area movement could mean for other parts of the world with similar faults.

Tohoku earthquake Japan shifted eastward GPS measurements seismic wave Earth’s core Fukushima Daiichi tectonic plates delayed fault motion Sunyoung Park University of Chicago