Japan slipped – A new Science study says the magnitude 9.0 Tohoku-Oki earthquake in 2011 didn’t just shake the ground—it also nudged Japan as a whole. Using GNSS data, researchers traced subtle national movement to seismic waves that bounced off Earth’s iron core, raising a n
Minutes after Japan’s 2011 Tohoku-Oki earthquake, the ground didn’t simply settle. In a few places, the country “slip[ped]” by as much as five or six millimeters—an amount too small for most people to feel, yet large enough to leave researchers stunned.
The finding comes from a study published today in Science. The authors describe the observation as “extraordinary,” because it points to an event that researchers had not documented before: Japan shifting position across the country, apparently triggered by seismic waves bouncing off Earth’s core.
Sunyoung Park. the study’s lead author and an assistant professor of geophysics at the University of Chicago. said the team spent time questioning what they were seeing. “The co-authors and I. we were all kind of initially puzzled by the observation. ” Park said. explaining that because the movement was so unusual. they “took a lot of time going through different possibilities.”.
Park and her team leaned on an extensive Global Navigation Satellite System (GNSS) dataset. tracking subtle motion at sites across Japan in the minutes after the magnitude 9.0 Tohoku-Oki quake. After ruling out other explanations—including a processing error in the GNSS data—the researchers concluded the movement came from “ScS waves.” These are seismic waves that travel through Earth’s mantle. ping off the planet’s iron core. and return to the surface.
Five or six millimeters—roughly the length of an average adult’s pinky toenail—may sound insignificant. Park said it’s not unusual during earthquakes for land to shift by more than that. creating visible “offsets” such as a disconnected road. But those shifts, she said, are usually localized to areas near the center of the quake. What researchers hadn’t seen before was movement on a scale of an entire country. with Japan “nudged” by ScS waves.
Wendy Bohon, an earthquake geologist, said the mechanism matters for what it changes and where it comes from. “Dynamic earthquake triggering. ” she explained. is well documented—when seismic waves from an earthquake “nudge” a fault already close to breaking. But in her view. the paper points to a previously unrecognized source for that kind of triggering: “ScS waves.” She added that 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.”.
The timing also stands out. 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. Park said the “slip” appears to have happened gradually—possibly over about 100 or 200 seconds—so people in Japan likely wouldn’t have felt it. But she also said it’s unclear whether future ScS-triggered slips in Japan or elsewhere would produce the same lack of sensation.
The study leaves key questions open. Park said more research is needed to understand why the Tohoku-Oki earthquake made Japan slip and whether future events like this might be more damaging.
The original disaster was already among the world’s deadliest. 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.
Against that backdrop, the new research shifts attention to what can happen after the main shaking has passed. Park said the findings could help people worldwide prepare for dangers hidden in the aftermath of quakes. “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 said. She added that “this new type of seismic hazard” is something “we might want to think about.”.
Japan earthquake Tohoku-Oki 2011 ScS waves GNSS seismic hazard Earth’s core dynamic earthquake triggering