The 2011 earthquake moved Japan about a quarter of an inch farther east than anyone had realized, and the cause went unexplained for more than a decade.
A study published in the journal Science has now identified the mechanism: a seismic wave that traveled nearly 1,800 miles to the Earth’s core, reflected back to the surface, and arrived with enough force to reactivate fault lines beneath the Japanese archipelago.
A Signal That Matched No Known Earthquake
The great Tohoku earthquake, a magnitude 9.0 event, struck the northeastern coast of Japan on March 11, 2011, and together with the tsunami it unleashed, killed roughly 20,000 people.
It was also one of the most thoroughly documented earthquakes in history, recorded by the dense network of GPS stations that blankets the country.
Within that enormous volume of data, an anomaly sat unresolved for years.
About 16 minutes after the main shock, and before any significant aftershocks had arrived, GPS stations across Japan simultaneously registered a displacement toward the east. The movement corresponded to no known aftershock.
A team led by the University of Chicago, in collaboration with the California Institute of Technology and the University of Strasbourg, returned to those records to find an explanation.
Before settling on an explanation, they ruled out several alternatives. A submarine landslide could not account for it — the effect was too geographically widespread. A slow fault slip alone could not explain the synchronized pattern recorded across stations spread so far apart.
The Wave’s Journey to the Core and Back
The explanation that actually fit the data was something else entirely.
Shear waves generated by the main earthquake traveled deep into the Earth until they reached the boundary between the mantle and the outer core, a layer of liquid metal sitting roughly 1,800 miles below the surface. There they reflected and began traveling back up toward the crust.
The full round trip covered approximately 3,600 miles and took about 15 minutes.
When that reflected wave reached the surface again, it encountered fault lines already weakened by the original rupture and pushed them just enough to produce a new slip.
That additional movement permanently shifted sections of Japanese territory up to a quarter of an inch eastward.
It is the first documented instance of this phenomenon triggering a fault slip near the surface.
Seismologists had long known that large seismic waves can travel through the entire planet and reflect off the core. What had never been confirmed until now is that such a reflection could move active tectonic plates thousands of miles from where the earthquake began.
The Largest Seismic Event Ever Recorded
In terms of geographic reach, it is the most widespread seismic episode ever documented.
It affected a corridor roughly 1,900 miles long and released energy comparable to a magnitude 7.5 earthquake.
It is also the first documented case in which a single event ruptured two separate tectonic plate boundaries at once: the boundary between the Pacific and Okhotsk plates, and the one between the Philippine and Eurasian plates.
The reason this signal went unnoticed for so long has to do with the instruments themselves. Seismic sensors are calibrated to detect short, high-frequency signals typical of conventional earthquakes. They are not designed to capture a low-frequency wave returning minutes later amid the noise of a magnitude 9 event and its many aftershocks.
Only by cross-referencing GPS and seismometer data from stations distributed across the country was it possible to isolate the signal.
The finding introduces a variable that seismic risk models have not previously accounted for. A large earthquake can continue reactivating faults long after the main shaking ends, through energy that descends to the planet’s deep interior and returns.
The discovery came from data that had been available for more than a decade, collected by GEONET, one of the densest positioning networks in the world.
That a shift of barely a quarter of an inch went undetected for so long, in the most studied earthquake of recent history, says something about how much remains hidden even in the most complete records we have.
