Scientists have discovered that seismic waves from Japan’s devastating 2011 earthquake traveled to Earth’s core and back, triggering a second tectonic slip event that shifted the country eastward by up to 6 millimeters.

The newly identified event occurred about 16 minutes after the magnitude 9.0 Tohoku-Oki earthquake struck off Japan’s northeastern coast. Researchers say the waves traveled roughly 3,600 miles (5,800 kilometers) through the planet, reflected off Earth’s liquid outer core, and returned to the surface.

The returning waves appear to have triggered movement along major tectonic plate boundaries around Japan, creating a broad seismic event that had gone unnoticed for years.

The findings help explain a mysterious signal recorded by Japan’s extensive GPS network shortly after the earthquake. Stations across the country registered a sudden eastward shift at nearly the same time, even though no known aftershock had occurred.

Mystery signal explained

The 2011 Tohoku-Oki earthquake ranks among the most powerful ever recorded. Along with the tsunami it generated, the disaster claimed about 20,000 lives and produced one of the most comprehensive sets of earthquake measurements ever collected.

Years later, however, one piece of data continued to puzzle scientists. GPS stations detected a synchronized eastward movement across Japan that did not match any known aftershock activity.

“Most of the time, we would see an offset like this when there’s actual earthquake happening. But here there was no known aftershock at this time, so we were quite curious,” said Sunyoung Park, a geophysicist at the University of Chicago and lead author of the study.

Park and her colleagues ruled out several possible explanations, including an undersea landslide and slow fault movement. Instead, they concluded that seismic energy from the original quake had traveled deep into Earth, struck the outer core, and rebounded back toward the crust.

When the waves returned, they triggered additional slip along plate boundaries surrounding Japan. Researchers believe the intense shaking from the main earthquake may have weakened these faults, making them more susceptible to later movement.

Core bounce effects

The newly recognized event stretched across roughly 1,800 miles (3,000 kilometers), making it the broadest seismic event ever recorded, according to the researchers.

The team estimates it released energy comparable to a magnitude 7.5 earthquake. It also involved multiple major plate boundaries, including the Pacific-Okhotsk boundary and the Philippine Sea-Eurasian boundary.

Scientists have long known that seismic waves can travel through Earth and reflect off deep internal structures. However, this is the first documented case in which those reflected waves appear to have triggered tectonic slip near the surface.

“It’s striking because this is both an unprecedented length and area for a seismic event, and it is a previously unrecognized source of seismic hazard,” said Park.

The discovery could influence how researchers think about large earthquakes and their lingering effects.

“This indicates that large earthquakes can influence the fault even after the main shaking is over,” Park said.

The researchers say the finding highlights how much remains unknown about the behavior of large earthquakes and the complex interactions between seismic waves and tectonic plates.

The study was published in the journal Science.

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