A seismic event bounced off Earth’s core and shifted an island country

Key Points:

• Following the massive 9.0 magnitude earthquake in Japan on March 11, 2011, GPS data revealed a small but permanent eastward shift of nearly the entire country, measuring 5 to 6 millimeters, occurring about 15 minutes after the quake began.
• University of Chicago geophysicist Sunyoung Park and her team identified this movement as a previously undocumented seismic phenomenon caused by seismic waves traveling down to Earth's core and rebounding to the crust, displacing four major tectonic plates across approximately 1,800 miles.
• Unlike typical localized ground shifts from earthquakes, this broad, uniform movement released energy equivalent to a magnitude 7.5 earthquake but was distributed over a large area, making it less damaging and difficult to distinguish from the main quake and aftershocks.
• The discovery suggests a new type of seismic hazard that could potentially be anticipated due to the predictable 15-minute travel time of seismic waves to the core and back, providing new insights into fault system behavior beyond traditional aftershock activity.
• Experts highlight the significance of this finding, noting that such deep-core seismic wave effects may occur globally but remain undocumented in less instrumented regions, and emphasize the need for further study to better understand fault dynamics and earthquake impacts.