A seismic wave that reached Earth's core and bounced back caused a nationwide eastward shift in Japan 15 minutes after the 2011 Tohoku earthquake.
Around 15 minutes after the magnitude-9 Tohoku earthquake struck on 11 March 2011, almost the whole of Japan moved 5 millimetres to the east. The movement resulted from a powerful wave that travelled 5800 kilometres to the planet’s core before returning to the surface.
The shift spanned 3000 kilometres and affected the entire country where GPS stations recorded it. Sunyoung Park at the University of Chicago noted that the step occurred nearly simultaneously across the region without any ordinary earthquake at that time.
Researchers analysed GPS and seismic data to determine that the main shock weakened plate boundaries. This allowed four adjoining tectonic plates to move together when the reflected wave arrived.
Park stated that the vigorous shaking from the original earthquake likely made the boundaries more susceptible to motion. Robin Lee at the University of Canterbury said the event shows large earthquakes can trigger widespread delayed fault motion over much larger regions than expected.
