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Japan’s catastrophic Tohoku-Oki earthquake in 2011 was so strong that it caused the entire country to “slip,” in some areas by as much as five or six millimeters, according to new research.

This “extraordinary observation,” as it is described in a new study published today in Science, was likely triggered by seismic waves bouncing off Earth’s core in the wake of the magnitude 9.0 quake. This never-before-seen event could present a previously unknown hazard associated with earthquakes, says Sunyoung Park, the study’s lead author and an assistant professor of geophysics at the University of Chicago.

Park and her team relied on an extensive Global Navigation Satellite System (GNSS) dataset to document subtle movement at sites across Japan in the minutes after the 2011 Tohoku-Oki quake. What they saw baffled them, Park says.

“The co-authors and I, we were all kind of initially puzzled by the observation,” Park says, referring to the movement of Japan. “Because this was such an unusual thing, we took a lot of time going through different possibilities.”

After ruling out other possible explanations for what they were seeing, such as a processing error in the GNSS data, the researchers concluded that “ScS waves”—seismic waves that travel through Earth’s mantle, ping off the planet’s iron core and return to the surface—had made Japan shift position.

Five or six millimeters—about the length of an average adult’s pinky toenail—might not sound like a lot. And it’s not uncommon for land to shift much more than that during earthquakes, causing “offsets,” where you might see, say, a disconnected road. But those movements are typically localized to areas near the center of the quake, Park says. Until now, researchers had never documented land movement at this scale—an entire country “nudged” by ScS waves, Park says.

“Dynamic earthquake triggering,” or “when seismic waves from an earthquake ‘nudge’ a fault already close to breaking into having an earthquake,” is well documented, says earthquake geologist Wendy Bohon. “However, this paper outlines a previously unrecognized source for this type of occurrence”: ScS waves. “The authors’ observations of triggered slip from this source across an area six to seven times greater than the area that broke during the main shock is extraordinary.”

In this case, it took about 15 minutes or so for the waves generated by the quake to travel to Earth’s core and back. The “slip” appears to have happened gradually, possibly over the course of around 100 or 200 seconds, so people in Japan probably wouldn’t have felt it, Park says. But it’s unclear if that would be the case for future ScS-triggered slips in Japan or elsewhere.

More research is needed to better understand why exactly this earthquake made Japan slip and whether future events such as this might be more damaging. The Tohoku-Oki quake was one of the world’s largest and most devastating: the initial shock and following tsunami killed more than 18,000 people and caused an estimated $220 billion in damage (in 2011 dollars), according to the U.S. National Oceanic and Atmospheric Administration.

The new findings could help people around the world better prepare for possible dangers hidden in the aftermath of quakes, Park says.

“I think we should be aware of the fact that there could be this potential triggering of an event many minutes after [an earthquake’s] main shaking has passed,” she says.

This “new type of seismic hazard” is one which “we might want to think about,” she adds.

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