A new study in Science says the 2011 Tohoku earthquake may have done more than shake Honshu, Japan, for about six minutes. It may also have sent a seismic wave deep into Earth, off the core and back to the surface, shifting the entire country eastward by up to 6 millimeters.
That finding matters because the shift showed up about 15 minutes after the main shock, long after the worst shaking and the massive tsunami that followed the magnitude 9 quake in March 2011. The disaster killed more than 18,000 people, but the GPS record now points to a quieter aftereffect: a countrywide nudge toward the Japan trench.
Sunyoung Park and her colleagues said they first noticed a step-like offset in the GPS data and could not match it to any known earthquake at that time. Park said the team was puzzled because these kinds of offset signals usually appear when an earthquake has just happened, yet there was no known quake that fit the timing. The wave they identified was an ScS wave, a type of seismic wave that plunges below the surface, reflects off Earth’s core and rises back up. In this case, the wave appears to have survived a 3,600-mile, or 5,800-kilometer, round trip and reached the surface about 13 minutes after the main shock.
The strongest part of the result is not just that the wave returned. It is that the eastward shift appeared uniformly across the entire country and at multiple plate boundaries, not just near the rupture zone. Researchers considered whether the movement came directly from the main shock, from a longer release of energy, or from a submarine landslide. Those ideas could explain motion near the earthquake’s source, but not a clean offset across all of Japan. The ScS wave was detected at stations across Japan before the displacement, which is why the team says the deep-bouncing wave is the best fit for the signal they saw.
There is still a limit to what the data can prove. The study lacked offshore GPS coverage, so Park said it is possible the eastward shift extended beyond the length of Japan. That leaves a bigger question hanging over the finding: if a single megathrust earthquake can send a wave down to Earth’s core and back with enough force to move an entire country, how many other subduction zones have done the same without leaving a clear record?
