Study Finds 2011 Japan Earthquake Triggered Rare Seismic Rebound From Earth's Core

When the magnitude-9.0 earthquake struck Japan on March 11, 2011, the ground underwent a subtle but permanent shift.

Scientists say seismic waves from Japan's 2011 earthquake rebounded from Earth's core, causing a subtle but permanent shift across the country.

About 15 minutes after the quake began at 2:46 p.m. local time, nearly the entire country moved eastward, according to GPS measurements, Katie Hunt reported for CNN.

The movement measured just 5 to 6 millimeters (0.20 to 0.24 inches), but it was permanent. At the time, the signal was largely overlooked or dismissed as a data anomaly.

University of Chicago geophysicist Sunyoung Park believed the measurements reflected a genuine geological event. According to a new study, the movement represents an "extraordinary" and previously undocumented seismic phenomenon.

"What was unusual about this movement is basically the whole of Japan was moving nearly uniformly at the same time," Park, who led the study, said.

She noted that the movement affected mainland Japan—from Hokkaido to Kyushu, spanning roughly 3,000 kilometers (1,800 miles)—and did not coincide with the initial earthquake or any significant aftershocks.

After years of analyzing GPS and seismic data, Park and her colleagues concluded that seismic waves from the earthquake traveled to Earth's core before rebounding back to the crust, displacing four major tectonic plates.

Although scientists have long known that seismic waves can travel to Earth's liquid outer core and reflect back, they generally believed the energy dissipated before reaching the surface.

"That type of deep-diving wave triggering some kind of event is new, and this event is very unusual because it's so broad," Park said.