The massive Tohoku earthquake sent a seismic wave that changed all of Japan

In March 2011, the Tohoku earthquake measuring 9 on the Richter scale struck off the northeastern coast of Honshu Island, Japan. The violent ground shaking lasted for about six minutes and generated a massive tsunami, ultimately killing more than 18,000 people.

But 15 minutes after the main shock, something strange happened. Researchers led by Sunyoung Park, an assistant professor in the Department of Geophysical Sciences at the University of Chicago, were examining GPS records of the Tohoku earthquake when they noticed an eastward ground shift of up to 6 mm across Japan.

“We were very puzzled because these displacement signals are usually seen when there is an earthquake, but there has been no known earthquake that matches this timing,” Park told Gizmodo. What was also unusual, she said, was that the shift occurred uniformly across the entire country at once, and occurred at multiple plate boundaries.

In a study published today in the journal Science, Park and her colleagues analyze this shift and offer a possible explanation for it. Their findings suggest that the Tohoku earthquake generated a massive seismic wave that traveled deep into the Earth, bounced off the Earth’s core, and returned to the surface 13 minutes after the main shock, causing the entire country to shift east toward the Japan Trench. To their knowledge, this is the first known observation of a seismic event caused by this type of wave.

Spying on seismic shift

When earthquakes strike, they generate seismic waves that radiate outward in all directions from the epicenter. Earth shaking originates primarily from surface waves, but earthquakes also produce ScS waves that descend below the surface, reflect off the Earth’s core, and then rise back to the surface.

As ScS waves travel deeper into the Earth’s interior, they lose energy. Park explained that this could explain why researchers have never seen this type of signal before, as these waves usually fade out before bouncing back to the surface. But the Tohoku earthquake was so powerful that the ScS wave survived a 3,600-mile (5,800 km) round trip down to the Earth’s core and back again. When it returned, it likely still retained enough energy to reactivate the tectonic plate boundary and cause an eastward shift.

Park and her colleagues also looked at whether this shift was directly caused by the main shock itself. For example, an earthquake could have continued to release energy for longer than previously thought, or it could have triggered an underwater landslide that triggered a shift. However, Park said these possibilities can only explain eastward movement near the main shock zone, and not across the entirety of Japan. Moreover, a ScS wave is clearly detected at stations all over Japan, followed by an eastward displacement.

A new earthquake danger appears

The findings change our understanding of the risks of large earthquakes. Long after the tremors stop, ScS waves can return to the surface and trigger another seismic event, which could lead to more regular aftershocks or additional ruptures at other locations, Park explained.

“This is something we didn’t know about before,” she said.

Measuring the impact of this ScS wave across Japan already suggests that these waves could have a far-reaching impact, but the study was limited by a lack of marine GPS data. Park said the eastward shift likely extended beyond Japan. She and her colleagues are also curious to know whether this type of event has occurred in other subduction zones, such as the San Andreas Fault in the western United States, but the availability of data may limit studies there as well.

It is impossible to know when the next big earthquake will strike, but gaining a deeper understanding of its processes and risks can improve preparedness. Fifteen years later, seismologists are still learning from the Tohoku earthquake. Their ideas could help prevent a similar tragedy in Japan and other seismically active parts of the world.

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