TOHOKU REGION — Researchers led by University of Chicago seismologist Sunyoung Park published findings in the journal Science on June 18, detailing ground displacement following the 2011 Tohoku earthquake. A seismic wave reflecting off Earth's core caused the Japanese archipelago to shift a few millimeters eastward.

This event marks the first documented instance of a core-reflected seismic wave triggering fault slippage. Park and colleagues analyzed archival seismic and GPS data from the March 11, 2011, magnitude 9.0 Tohoku earthquake. The core-reflected wave was detected approximately 15 minutes after the mainshock.

Hundreds of GPS sensors across Japan recorded ground displacement that coincided with the arrival of the reflected wave. Park said, "We see this permanent offset." The ground shift extended from Hokkaido in the north to Kyushu in the south, and researchers determined that two separate plate boundaries totaling at least 3,000 kilometers experienced fault slippage.

Core-reflected S waves travel through 2,900 kilometers of Earth's mantle, reaching the core's boundary before returning toward the surface. The fault slippage occurred over approximately three minutes. Caltech seismologist Zachary Ross said, "That implies that there's some amount of fault slip."

The rupture length associated with the core-reflected wave event exceeded twice the rupture length of the 2004 Sumatra earthquake. The seismic energy was distributed across a large geographic area, making the resulting ground movement imperceptible to human sensation. Purdue University geophysicist Andrea Donnellan said, "I think it's very plausible."

"That's a type of seismic hazard that we didn't think about before." Park said. Researchers stated that future core-reflected wave events could produce more detectable ground displacement.