How a seismic wave made the whole of Japan move

A magnitude of 7.2 jolted Japan's northern coast today (June 25), injuring at least 10 people, according to reports. The quake was also felt in Tokyo. No tsunami warning has been issued.

Speaking to reporters, Prime Minister Sanae Takaichi said there was “no tsunami concern” and urged residents in the affected areas "to remain vigilant for the possibility of another earthquake of similar magnitude."

Whenever an earthquake hits Japan, it reminds me of the nation's 2011 quake- the largest ever recorded in Japan—which struck off the coast of the Tōhoku-Oki region.

A tremor so strong that it caused the entire country to “slip,” in what is described in new research as "as much as five or six millimetres." However, what led to this?

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We take a look.

Did seismic waves lead Japan to "slip"? 

In a new study published on June 18 in Science, the seismic waves are likely considered the key triggering mechanism that bounced off Earth’s core in the wake of Japan's 9.0-magnitude quake in 2011.

Sunyoung Park, the lead author of the study and an assistant professor of geophysics at the University of Chicago, called it "a never-before-seen event." This event may present a previously unrecognised hazard linked to earthquakes, Park notes.

Park and her colleagues analysed a vast Global Navigation Satellite System (GNSS) dataset to monitor slight shifts at locations across Japan in the minutes after the 2011 Tohoku-Oki quake. What they witnessed was unexpected and baffling, according to Park.

Seismic waves that travel through Earth’s mantle reflect off its iron core and return to the surface. Image courtesy: X

“The co-authors and I were all kind of initially puzzled by the observation,” the lead author noted, referring to Japan's movement. Adding, “Because this was such an unusual thing, we took a lot of time going through different possibilities," as quoted by SCIAM.

The researchers were looking into a processing error in the GNSS data; however, they ruled out that possibility and concluded that the signals were “ScS waves”—seismic waves.

Seismic waves that travel through Earth’s mantle, reflect off its iron core, and return to the surface cause Japan's position to shift, the researcher found.

According to the author, the size of the movement was not the most surprising part, as strong tremors cause the ground to move by much more than 5–6 millimetres. But researchers discovered ScS waves caused a subtle movement of the entire Japanese archipelago.

Park notes that researchers had never before documented an entire nation being "nudged" in this way by these seismic waves.

The lead author further explains how seismic waves moved the whole of Japan: "In this instance, the seismic waves took roughly 15 minutes to journey to Earth’s core and back. The resulting 'slip' appears to have unfolded gradually over about 100 to 200 seconds, so people in Japan probably wouldn’t have felt it."

Though it remains unclear whether this would be the case for future seismic-triggered slips in Japan or elsewhere, according to him.

How experts are reacting to new research

The research is called "extraordinary observation" by many.

Earthquake geologist Wendy Bohon said, "Dynamic earthquake triggering,” or “when seismic waves from an earthquake ‘nudge’ a fault already close to breaking into having an earthquake, is well documented."

"However, this paper outlines a previously unrecognised 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 are extraordinary," Bohon added, as quoted by SCIAM.

Amanda Thomas, a geophysicist at the University of California, Davis, who was not part of the latest research, said, "If their interpretation of the data is correct, the research is 'very significant," CNN reported.

“The study’s broader implication is that large earthquakes may continue influencing fault systems in unexpected ways for many minutes after the main rupture, not just through aftershocks but through the passage of later-arriving seismic waves,” she said.

“We still don’t fully understand how faults work, and this sort of observation gives us another piece of the puzzle," the report noted.

Park even notes that new findings can help people better prepare for possible dangers hidden in the aftermath of earthquakes.

"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. This new type of seismic hazard is one which “we might want to think about," Park added, according to SCIAM.

With inputs from agencies