A seismic wave from Japan's 2011 magnitude-9 earthquake travelled nearly 2,900 kilometres down to Earth's core, bounced back to the surface 13 minutes later, and shifted the entire country eastward by roughly six millimetres at the same instant - in the first observation of its kind ever recorded

A study led by Sunyoung Park of the University of Chicago, published June 18, 2026, in Science, found that a reflected seismic wave called an ScS wave caused Japan to shift 5-6 millimeters east 15 minutes after the 2011 Tohoku-Oki earthquake. This wave bounced off the Earth’s outer core before returning to the surface to trigger additional fault slip.

Why did Japan shift 5-6 millimeters after the main earthquake?

The displacement happened because of an ScS wave, which is a shear wave that travels from an earthquake’s source straight down into the mantle. According to the University of Chicago, these waves reflect off the core-mantle boundary at a depth of approximately 2,890 kilometers.

Shear waves can’t travel through liquids. Because the Earth’s outer core consists of liquid iron and nickel, the wave bounced back toward the surface like a billiard ball hitting a cushion. The round-trip journey took about 13 minutes.

When the wave hit Japan’s tectonic plate boundaries, it arrived almost simultaneously across the country. Park’s team concludes this energy triggered a small slip on already-stressed megathrust interfaces, releasing energy equivalent to a magnitude-7.5 earthquake.

Did you know? Humans have never directly sampled the mantle or core. The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, only reached 12 kilometers. Everything we know about the Earth’s interior comes from analyzing seismic waves.

How does an ScS wave differ from standard seismic waves?

Seismologists have tracked ScS waves for decades, but they usually don’t cause permanent ground movement. As reported by Scientific American, these waves typically lose most of their energy to attenuation during their long trip through the mantle.

The 2011 Tohoku event was an outlier. The magnitude-9 main shock created an ScS wave with a peak-to-peak amplitude exceeding one centimeter at surface stations. This is far stronger than any previously recorded reflected wave.

The combination of this high amplitude and the fact that Japan sits on heavily stressed plate boundaries allowed the wave to trigger a mechanical effect. In most earthquakes, the wave would simply be a signal in the data; here, it became a physical force.

What makes this a new seismic hazard?

Traditional seismic hazard modeling focuses on the immediate rupture zone and the subsequent aftershock sequence. However, Science News reports that this finding suggests the influence of massive earthquakes extends much further and deeper than previously thought.

Sunyoung Park stated in a University of Chicago announcement that this is a “previously unrecognised source of seismic hazard” due to its unprecedented length and area. The wave traveled 5,800 kilometers round-trip to nudge the surface.

This mechanism implies that a magnitude-9 event could potentially trigger slip events on other plate boundaries across the Pacific Rim. Researchers now plan to examine GPS data from other massive events to see if this pattern repeats. Target events include:

The 1960 Valdivia event in Chile
The 1964 Alaska event
The 2004 Sumatra-Andaman event
The 2010 Chile event

How did researchers rule out other explanations?

The Park team spent years eliminating other possibilities before settling on the ScS-triggering theory. They found that a continued release of energy from the main shock wouldn’t work because that energy would concentrate near the epicentre rather than shifting the whole country uniformly.

According to the researchers, a submarine landslide also failed as an explanation because it couldn’t account for the synchronized timing across all of Japan. Finally, no seismic records matched the signal to any unrecognized aftershock.

The ScS-triggered slip is the only theory that matches the geographic pattern, the timing, and the relationship to the main shock seen in the GPS archives.

Pro Tip: When looking at seismic data, “step-like displacement” refers to a sudden, permanent shift in position rather than a vibration that returns to center. This is why GPS data is more useful than traditional seismometers for detecting these tiny, permanent movements.

Frequently Asked Questions

Was the 5-6mm shift felt by people in Japan?

No. According to the research, the shift was far too small to be felt by humans or to cause any structural damage.

What exactly is an ScS wave?

It’s a shear wave that travels from the earthquake source, reflects off the liquid outer core at 2,890km depth, and returns to the surface as a shear wave.

Why did it take 15 years to find this?

The signal was a tiny anomaly in a vast dataset. It required the specific analysis of GPS archives and the ruling out of all other geological possibilities by Park’s team.

What do you think about the possibility of “remote triggering” of earthquakes? Let us know in the comments, or subscribe to our newsletter for more deep-earth discoveries.