A seismic wave from the devastating 2011 Tohoku-oki earthquake, measuring 9.0 on the Richter scale, traveled about 2,900 kilometers to the Earth’s core, reflected about 13 minutes later, and moved the entire country eastward by about six millimetres, 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 match the main earthquake, any aftershock or other known geological event. The new research identifies a returning seismic wave as the cause, marking the first documented observation of such a phenomenon.
How a wave traveled nearly 2,900 kilometers below the Earth’s surface
The study was led by Sunyoung Park, assistant professor of geophysical sciences at the University of Chicago, along with Hiro Kanamori of the California Institute of Technology and Luis Rivera of the University of Strasbourg.The researchers found that the unexplained GPS signal was caused by a ScS wave, a type of seismic wave produced during strong earthquakes. Unlike surface waves, which are responsible for much of the vibration people experience, ScS waves travel deep through the Earth’s solid mantle.
After the 2011 Tohoku-oki earthquake, one of these waves descended about 2,890 kilometers to reach the mantle boundary, the boundary that separates the Earth’s rocky mantle from its liquid outer core.Shear waves cannot pass through liquids, so when a ScS wave reaches the molten iron and nickel in the outer core, it cannot continue its journey. Instead, it reflected off the core boundary of the mantle and returned toward the surface, much like an echo bouncing off a wall.
The entire round trip covered approximately 5,800 kilometres, making it one of the deepest seismic journeys ever associated with a measurable impact on the Earth’s surface.The exceptional strength of the Tohoku-oki earthquake produced an unusually strong ScS wave, allowing enough energy to survive the long journey through the Earth’s interior, researchers say. By the time it returned to Japan, the wave was still strong enough to impact faults that the main quake had already pushed to the breaking point.
Why did the wave come back after 13 minutes?
Scientists calculated that the trip to the core-mantle boundary and back would take about 13 minutes, which is consistent with the unexplained GPS signal recorded throughout Japan.When the reflected wave arrived, it reached most of the country at about the same time. Researchers believe it caused small slides along the boundaries of tectonic plates that were already under enormous pressure from the main quake. These small movements moved Japan permanently eastward by about five to six millimeters.Although the displacement was barely measurable, the combined fault slips were expected to release energy similar to a 7.5 magnitude earthquake.
Why wasn’t this noticed before?
Seismologists have studied ScS waves for decades and routinely use them to explore the Earth’s deep interior. What was never noticed was his return with enough energy to leave a permanent mark on the Earth’s surface.According to researchers, the Tohoku-oki earthquake was exceptional because of its enormous power.
The reflected ScS wave recorded across Japan had a peak-to-peak amplitude exceeding one centimeter, making it much stronger than that generated by most earthquakes. Combined with already stressed faults, it appears that this unusual energy was enough to stimulate additional ground motion.
The GPS signal that has 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 movements.About 15 minutes after the quake, instruments recorded an almost simultaneous eastward shift across the country. This pattern did not match the main rupture, any known aftershock, or undersea landslide, leaving scientists without a convincing explanation for more than a decade.The new analysis finally links this long-term anomaly to the returning ScS wave.
How researchers ruled out other possibilities
Before arriving at this conclusion, the team evaluated several competing explanations.Had the main rupture continued to release energy, the movement should have been stronger near the epicenter rather than appearing almost uniformly across Japan. Likewise, no unrecorded aftershock or underwater landslide could explain the timing and national pattern observed by the GPS network.According to the researchers, the reflected ScS wave is the only mechanism that explains all the observations.
Implications of the discovery for seismology
The results suggest that the effects of the largest earthquakes to hit Earth may extend much deeper and farther than previously thought.Until now, seismic hazards have been largely associated with major ruptures, aftershocks, and tsunamis. The study indicates that seismic waves traveling thousands of kilometers through the Earth’s interior may also be able to cause additional fault motion after reflection from the boundary above the outer core.The 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 occurred elsewhere.
What this discovery reveals about the hidden interior of the Earth
The study also highlights how scientists are investigating parts of the planet that remain physically inaccessible. The deepest hole ever drilled, the Kola Deep Well in Russia, is only about 12 kilometers deep, while the Earth’s mantle extends approximately 2,890 kilometers before meeting the liquid outer core.Since humans cannot explore these depths directly, 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, then returned to the surface and created a measurable impact, the Tohoku-oki earthquake provided an unprecedented glimpse into the dynamic relationship between the Earth’s deep interior and its crust.