"We were quite puzzled because usually these offset signals are seen when there’s an earthquake, but there was no known earthquake corresponding to this timing," Sunyoung Park recalled, pointing to a tiny, synchronized jump in GPS traces recorded across Japan after the March 2011 shaking.
Researchers led by Sunyoung Park linked that puzzling signal to a seismic wave that traveled to Earth’s core and back: the study, published in Science, says the ScS seismic wave returned to the surface about 13 minutes after the main shock and corresponded with an eastward ground shift of up to 6 millimeters across all of Japan.
Japan-wide GPS Measurements
Sunyoung Park and her team at the University of Chicago examined GPS records and found a step-like, eastward displacement occurring about 15 minutes after the main shock of the magnitude 9 Tohoku earthquake in March 2011. GPS stations across Japan registered the shift uniformly, the study reports, with the movement observed at multiple plate boundaries throughout the entirety of Japan.
The Tohoku earthquake ScS Return
The Tohoku earthquake produced violent ground shaking that lasted for about six minutes and triggered a massive tsunami; more than 18,000 people died. The study traces a separate wave type—the ScS—that plunged below the surface, reflected off Earth’s core, and returned to the surface on a roughly 3,600-mile (5,800-kilometer) round trip. "That’s something we didn’t actually know about before," Sunyoung Park said about the returned ScS wave and its surface effects.
Sunyoung Park and Alternatives
Researchers considered whether the eastward shift might be explained directly by the main shock continuing to release energy or by a submarine landslide, but they said those mechanisms could only account for movement near the rupture zone, not a uniform shift across Japan. The study notes the ScS wave was detected at stations across Japan before the eastward displacement, and the authors propose the returned ScS wave retained enough energy to reactivate the tectonic plate boundary and push the country eastward toward the Japan trench.
The finding is framed as the first known observation of a seismic event triggered by this type of ScS wave, and it points to how very large earthquakes can send deep-reaching waves that later produce surface motion far from the original rupture. Seismic waves generally lose energy as they travel into Earth’s interior; by comparison, the study says the Tohoku event was powerful enough for its ScS wave to survive the long round trip and still perturb plate-boundary stresses when it returned.
Because the study relied on onshore GPS records, the researchers note the analysis was limited by a lack of offshore GPS data; that shortfall leaves an immediate, practical question for anyone tracking regional hazards: how far beyond Japan did the eastward shift extend given the absence of offshore measurements?
