A seismic wave from the 2011 magnitude 9 earthquake in Japan traveled nearly 2,900 kilometers into the Earth’s core, then rebounded to the surface 13 minutes later, moving the entire country eastward by about six millimeters at the same instant – in the first observation of its kind ever recorded.
For 15 years after the 2011 Tohoku-oki earthquake — among the most precise natural disasters in history — a small anomaly in GPS data remained unresolved in the archives. About 15 minutes after the 9-magnitude main shock struck off the coast of northeastern Honshu, GPS stations distributed across Japan recorded a small but unambiguous step-like displacement to the east, of about five to six millimetres, occurring almost simultaneously across the country. The shift was too small for anyone to feel, too small to cause damage, and too late to be attributed to the main rupture itself. It did not correspond to any known aftershock. It was reported in later Tohoku data sets, but was, for more than a decade, an unexplained signal lurking in the corner of a massive data set that researchers had yet to figure out how to explain. On June 18, 2026, a team led by Sunyoung Park, assistant professor of geophysical sciences at the University of Chicago, published research in sciences The suggestion they are arguing is the correct interpretation. The researchers concluded that this shift was caused by a seismic wave that traveled from the epicenter straight down through the Earth’s mantle, bounced off the outer core, returned to the surface, and reached the boundary of Japan’s tectonic plates with enough energy left to push the entire country a few millimeters eastward.
according to University of Chicago announcement of resultsThe specific wave responsible is what seismologists call a ScS wave — a shear wave that travels downward from the earthquake’s source, reflects off the underlying mantle boundary at a depth of about 2,890 kilometers, and returns to the surface as a shear wave. Reflection occurs because shear waves cannot propagate through liquids, and the Earth’s outer core consists of liquid iron and nickel. The wave bounces off the boundary in the same way that a billiard ball bounces off a cushion. The round-trip travel time of the wave originating at the depth of the Tohoku rupture is about 13 minutes. When the wave returned to Japan, it reached simultaneously across the country, and was strong enough, in Park’s team’s interpretation, to cause a small additional slip on the already stressed massive thrust fronts along Japan’s tectonic plate boundaries. The slide event itself, although displaced by only a few millimeters, released energy roughly equivalent to a 7.5-magnitude earthquake.
Why this has never been seen before
Seismologists have long known that large earthquakes generate seismic waves that travel through the planet’s interior and reflect various internal boundaries. ScS waves, in particular, have been cataloged and studied for decades as a routine feature of the seismic record. What has not been observed before is the ScS wave returning to the surface with enough energy to cause permanent, measurable ground displacement. As reported Scientific American’s coverage of Park’s studyThe main reason is that ScS waves typically dissipate greatly during their long round-trip through the mantle – the deeper they travel, the more energy they lose to attenuation, and by the time they return to the surface, they are usually too weak to produce any detectable mechanical effect. The Tohoku event was different. The 9 magnitude main shock generated a ScS wave with peak-to-peak amplitude exceeding one centimeter at surface stations in Japan – a reflected wave much stronger than any previously recorded. The combination of unusually high amplitude and simultaneous arrival across a country that already lies on a highly compressed plate boundary produced, in Park’s interpretation, the conditions necessary for the wave to trigger small slip events along that boundary.
Park’s team spent a long time ruling out alternative explanations before settling on the ScS-triggering explanation. The possibility that the main shock would continue to release energy for longer than thought could not explain a uniform shift across the entire country; The energy from the main rupture was assumed to produce displacement concentrated near the epicenter and fall off with distance. The possibility of a subsea landslide triggered by the mainshock cannot explain the synchronous timing or nationwide scale. The possibility of an unrecognized aftershock does not match any seismic record. Each alternative explanation can explain some, but not all, features of the signal. In contrast, the ScS slip interpretation predicts exactly the geographic pattern, synchronous timing, and relationship to the mainshock that GPS data actually show.
What does this mean for seismic risk?
per Science news coverage of the implicationsPark’s discovery has fundamental implications for how seismologists model the propagation of impacts from large earthquakes. The conventional picture is that the risk of a major earthquake is concentrated in the immediate vicinity of the rupture and in the aftershock sequence that follows it. The new findings suggest that the impact of major earthquakes may extend much further, along much deeper geological paths, than this picture captures. A wave that travels 5,800 kilometers back and forth through the Earth’s interior, bounces off the Earth’s core, returns to the surface, and triggers an additional fault slip on the far side of the planet, acts at a different scale and through a different mechanism than the pattern of surface wave aftershocks that has dominated seismic hazard models for the past century.
Park’s formulation of the finding, quoted in the University of Chicago’s announcement, is straightforward: “It is astonishing because this is an unprecedented length and extent of a seismic event, a previously unrecognized source of seismic hazard.” The practical implication is that the Tohoku event may have triggered small strike-slip events not only around Japan but likely across other plate boundaries around the Pacific Rim that were exposed to the same ScS returning wave. Whether this type of excitation turns out to be characteristic of all sufficiently large earthquakes, or whether it is specific to the unusual configuration of the Tohoku event, will require examining GPS data from other recent magnitude 9 earthquakes—the 2004 Sumatra-Andaman event, the 1960 Valdivia event in Chile, the 1964 Alaska event, and the 2010 Chile event—to see if signals can be detected. Comparable in available data.
The deeper picture
As shown in Summary of the paper by Park, Kanamori and Rivera in sciencesThis finding fits into a longer scientific tradition of using large earthquakes as natural experiments on the structure of the Earth’s interior. Seismic waves from major events are, in practice, the only tool humans have to directly explore the planet’s structure below the relatively shallow depths to which they can be drilled. The deepest hole ever drilled – the Kola Deep Well on Russia’s Kola Peninsula – reached about 12 kilometers before heat and pressure exceeded the capacity of the drilling equipment. The Earth’s mantle starts at approximately 35 km deep and continues up to 2890 km. The outer core extends from there 5,150 kilometers. The inner core lies below that, reaching the center of the planet at an altitude of 6,371 kilometers. None of these layers were directly sampled. Everything humans know about the planet’s structure beneath the upper crust has been inferred from seismic waves—their travel times, reflections, refractions, and speed changes as they pass through different layers—generated by earthquakes and recorded at the surface.
Subsequently, the 2011 Tohoku event generated the largest and most accurate seismic data set of any earthquake in human history. The five-to-six millimeter shift across the country toward the east that no one initially knew how to explain has now been identified, 15 years later, as the surface imprint of a wave that traveled deep enough to touch the boundary of the molten outer core and then return. It is, in an objective sense, the first time this species has directly witnessed the planet’s internal seismicity reaching up to push the surface. The signal is small. The effects are not.
