The seismic wave caused by Japan’s devastating 2011 magnitude 9.0 Tohoku-Oki earthquake traveled nearly 2,900 kilometers to the edge of the Earth’s core, reflected back about 13 minutes later and shifted the entire country east by about six millimeters, according to a study published in the journal Science. The movement was too small to be noticed, but Japan’s highly sensitive GPS network detected the nationwide shift almost simultaneously. Scientists were unable to explain the unusual signal for 15 years because it did not correspond to the main earthquake, any aftershock or other known geological event. The new study identifies the cause in a returning seismic wave, marking the first documented observation of such a phenomenon.
How the wave traveled almost 2900 km below the Earth’s surface
The study was led by Soonyoung Park, assistant professor of geosciences at the University of Chicago, along with Hiroo Kanamori of the California Institute of Technology and Luis Rivera of the University of Strasbourg.The researchers discovered that the unexplained GPS signal was caused by a ScS wave, a type of seismic shear wave that occurs during powerful earthquakes. Unlike surface waves, which are responsible for most of the shaking people experience, ScS waves travel deep through the Earth’s solid mantle. After the 2011 Tohoku-Oki earthquake, one such wave sank nearly 2,890 kilometers until it reached the core-mantle boundary, the boundary separating Earth’s rocky mantle from its liquid outer core.Transverse waves cannot travel through liquids, so when the ScS wave reached the molten iron and nickel of the outer core, it was unable to continue its journey. Instead, it bounced off the core-mantle boundary and headed back toward the surface, like an echo bouncing off a wall. The full round trip was nearly 5,800 kilometers, making it one of the deepest seismic journeys ever associated with a measurable impact on the Earth’s surface.The core-mantle boundary experiences extreme conditions, with temperatures estimated to exceed 3,500°C and pressures reaching over 130 gigapascals, making it one of the most inaccessible and scientifically important regions within the Earth.Researchers say the exceptional magnitude of the Tohoku-Oki earthquake produced an unusually powerful ScS wave, capable of surviving a long journey through the Earth’s interior. By the time the wave returned to Japan, the wave was still strong enough to affect faults that were already close to the point of failure due to the main earthquake.
Why did the wave return after 13 minutes?
Scientists estimate that the journey to the core-mantle boundary and back will take about 13 minutes, which matches the unexplained GPS signal recorded throughout Japan.When the reflected wave arrived, it reached most of the country almost simultaneously. Researchers believe this caused tiny shifts along the boundaries of tectonic plates that were already under enormous stress from the main earthquake. These small movements permanently shifted Japan eastward by about five to six millimeters.Although the displacement could barely be measured, the combined fault movements were estimated to release energy comparable to a magnitude 7.5 earthquake. Unlike the main earthquake, these triggered movements caused little additional shaking because the movement occurred slowly along numerous small fault segments rather than through one large rupture, allowing the ground to shift without triggering another major seismic event. 
Why has this never been seen before?
Seismologists have studied ScS waves for decades and regularly use them to probe the Earth’s deep interior. What has never been observed is to return with enough energy to leave an indelible mark on the surface of the Earth.According to researchers, the Tohoku-Oki earthquake was exceptional due to its enormous strength. The reflected ScS wave recorded across Japan had a peak-to-peak amplitude greater than one centimeter, making it much stronger than the waves produced by most earthquakes. Combined with already stressed faults, this unusual energy was apparently enough to cause additional ground movement.
The GPS signal that has puzzled scientists for 15 years
Japan has one of the world’s densest GPS monitoring networks, with more than 1,300 constantly operating stations capable of detecting even the slightest ground movements.About 15 minutes after the earthquake, instruments recorded an almost simultaneous eastward shift across the country. The pattern did not match the main rupture or any known aftershock or underwater landslide, leaving scientists without a convincing explanation for more than a decade.New analysis finally links this long-standing anomaly to the returning ScS wave.
How the researchers ruled out other possibilities
The team evaluated several competing explanations before reaching this conclusion.If the main rupture had continued to release energy, the movement should have been strongest near the epicenter rather than appearing almost uniformly across Japan. Likewise, neither an undocumented aftershock nor an underwater landslide could explain the timing or the nationwide pattern observed by the GPS network.According to the researchers, the reflected ScS wave is the only mechanism that explains all observations.
Significance of the discovery for earthquake science
The results show that the effects of the largest earthquakes on Earth can extend much deeper and farther than previously thought.Until now, seismic hazards have largely been associated with the main fault, aftershocks and tsunamis. The study shows that seismic waves traveling thousands of kilometers through the Earth’s interior can also cause additional movement of faults after being reflected from the boundary above the outer core.The researchers now plan to review data from other giant earthquakes, including the 2004 Sumatra-Andaman earthquake, the 1960 Valdivia earthquake in Chile, the 1964 Alaska earthquake and the 2010 Maula earthquake, to determine whether the same mechanism occurred elsewhere.
What does the find say about the hidden depths of the Earth?
The study also shows how scientists are exploring parts of the planet that remain physically inaccessible. The deepest well ever drilled, the Kola Superdeep Well in Russia, reached a depth of only about 12 kilometers, while the Earth’s mantle extends some 2,890 kilometers before meeting its liquid outer core.Because humans cannot directly probe these depths, researchers rely on seismic waves created by powerful earthquakes to understand the planet’s internal structure. By showing that a seismic wave traveled to the edge of the Earth’s core, returned to the surface, and produced a measurable effect, the Tohoku-Oki earthquake provided an unprecedented glimpse into the dynamic connection between the Earth’s deep interior and its crust.