The seismic wave generated by Japan’s devastating 2011 magnitude 9.0 Tohoku-oki earthquake traveled a distance of about 2,900 kilometers to the edge of the Earth’s core, reflected back about 13 minutes later and shifted the entire country to the east by about six millimeters, according to a study published in the journal Science. The movement was so small that no one could notice it, but Japan’s highly sensitive GPS network picked up the nationwide change almost simultaneously. Scientists were unable to explain this unusual signal for 15 years because it did not match the main earthquake, any aftershocks, or any other known geological phenomenon. New research identifies a returning seismic wave as the cause, the first documented observation of such a phenomenon.
How a wave traveled nearly 2,900 km below the Earth’s surface
The study was led by Sunyoung Park, assistant professor of geophysical sciences 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 an SCS wave, a type of seismic shear wave produced during powerful earthquakes. Unlike the surface waves that are responsible for most shaking people experience, ScS waves travel deep through the Earth’s solid mantle. After the 2011 Tohoku-oki earthquake, one such wave descended about 2,890 kilometers until it reached the core-mantle boundary, the boundary separating Earth’s rocky mantle from its liquid outer core.Shear waves cannot pass through fluids, so when the ScS wave reached the molten iron and nickel of the outer core, it could not continue its journey. Instead, it was reflected off the core-mantle boundary and bounced back toward the surface like an echo bouncing off a wall. The entire journey covered a distance of approximately 5,800 kilometres, making it one of the deepest seismic journeys ever conducted involving measurable effects on the Earth’s surface.Researchers say the extraordinary intensity of the Tohoku-Oki earthquake generated an unusually powerful SCS wave, which provided enough energy to survive a long journey into the Earth’s interior. By the time it returned to Japan, the wave was still strong enough to impact faults that had already been pushed close to their breaking point by the main earthquake.
Why did the wave return after 13 minutes?
Scientists calculated that the trip to the core-mantle boundary and back would take about 13 minutes, matching unexplained GPS signals recorded across Japan.When the reflected wave arrived, it reached most of the country at almost the same time. Researchers believe this triggered small slippages at tectonic plate boundaries, which were already under extreme stress from the main earthquake. Those small movements permanently shifted Japan to the east by about five to six millimeters.Although the displacement was barely measurable, the combined fault slips are estimated to have released energy equivalent to a magnitude 7.5 earthquake.
Why has this never been seen before?
Seismologists have studied SCS waves for decades and routinely use them to probe Earth’s deep interior. What was never seen was someone returning with enough energy to leave permanent marks on the Earth’s surface.According to researchers, the Tohoku-Oki earthquake was extraordinary because of its immense intensity. The reflected SCS wave recorded across Japan had a peak-to-peak amplitude of more than a centimeter, making it far stronger than waves generated by most earthquakes. Combined with already stressed faults, that unusual energy appears to be enough to trigger additional ground movement.
The GPS signal that puzzled scientists for 15 years
Japan operates one of the densest GPS monitoring networks in the world, with more than 1,300 continuously operating stations capable of detecting even the smallest ground movement.About 15 minutes after the earthquake struck, instruments recorded an almost simultaneous eastward shift across the country. This pattern did not match any major rupture, any known aftershock, or submarine landslide, leaving scientists without a solid explanation for more than a decade.The new analysis finally links that long-standing anomaly to a returning SCS wave.
How researchers ruled out other possibilities
Before reaching their conclusion, the team evaluated several competing explanations.If the main rupture had continued to release energy, the movement should have been strongest near the epicenter of the earthquake rather than appearing almost evenly throughout Japan. Similarly, neither an unrecorded shock nor an underwater landslide can explain both the timing and nationwide patterns observed by the GPS network.According to the researchers, the reflected SCS wave is the only mechanism that accounts for all the observations.
Implications of the discovery for seismology
The findings suggest that the effects of Earth’s largest earthquakes may extend much deeper and farther than previously thought.Until now, seismic hazards have been mainly associated with main ruptures, aftershocks and tsunamis. The study indicates that seismic waves traveling thousands of kilometers from the Earth’s interior may also be capable of triggering additional fault movement after being reflected off the boundary above the outer core.Researchers now plan to re-examine data from other giant earthquakes, including the 2004 Sumatra-Andaman earthquake, the 1960 Valdivia earthquake in Chile, the 1964 Alaska earthquake, and the 2010 Maule earthquake, to determine whether the same mechanism has occurred elsewhere.
What does this discovery reveal about Earth’s hidden interior?
The study also sheds light on how scientists investigate parts of the planet that are physically inaccessible. The deepest hole ever drilled, Russia’s Kola Superdeep Borehole, reaches only 12 kilometers, while Earth’s mantle extends about 2,890 kilometers before meeting the liquid outer core.Since humans cannot directly explore these depths, researchers rely on seismic waves generated 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 impact, the Tohoku-oki earthquake has provided an unprecedented glimpse into the dynamic relationship between the Earth’s deep interior and its crust.