Waves reflecting off Earth’s core shifted Japan after 2011 earthquake

Around 15 minutes after the magnitude-9 Tohoku earthquake on 11 March 2011, almost the whole of Japan jumped half a centimetre to the east. This lurch resulted from an immensely powerful seismic wave that travelled 5800 kilometres to the planet’s core and then bounced back towards the surface.

In the context of the devastation caused by the earthquake, including localised land movements of many metres and 40-metre tsunami waves that led to the meltdown of three reactors at the Fukushima Daiichi nuclear plant, 5 millimetres may seem insignificant.

But this movement took place over a distance of 3000 kilometres, nearly seven times longer than the length of the earthquake’s main rupture line and longer than any slip ever recorded.

What also makes the case unusual is the timing and the pattern, says Sunyoung Park at the University of Chicago. “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.”

Not only was the shift immense in its north-south extent, but its width encompassed all of Japan and beyond, into the ocean.

“It is not just a narrow ‘edge’ that moved,” says Park. “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 analysing extensive GPS and seismic data recorded during the catastrophe, Park and her colleagues have figured out how such a phenomenally vast movement was triggered and why the rupture took place 15 minutes after the main quake.

Earthquakes often generate waves that travel deep into Earth’s interior and reflect off the core, but they usually become quite weak by the time they have travelled to the planet’s centre and then back up.

In Tohoku’s case, the main shock was so large that the original wave, though weakened, remained powerful enough on its return to the surface to cause the nationwide lurch, as four adjoining tectonic plates moved in unison.

“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,” says Park.

The event demonstrates there are previously unrecognised mechanisms of destruction that can follow earthquakes, says Park. “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.”

More research is now needed to understand the implications of this kind of movement for other parts of the world with similar faults, says Robin Lee at the University of Canterbury, New Zealand.

“It shows that large earthquakes can trigger widespread, delayed fault motion minutes later, and over much larger regions than expected,” says Lee.

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