Japan Shifted 5 Millimetres East 15 Minutes After 2011 Tohoku Quake

Around 15 minutes after the magnitude-9 Tohoku earthquake struck on 11 March 2011, almost the whole of Japan moved 5 millimetres eastward. The displacement resulted from a seismic wave that travelled 5800 kilometres to Earth’s core and back to the surface. GPS stations recorded the eastward step across the whole of Japan.

The movement occurred nearly simultaneously and with similar magnitude across almost all of the country, without any ordinary earthquake at that exact time. The main rupture line of the Tohoku earthquake measured less than one-seventh the 3000-kilometre north-south extent of the observed displacement.

Four adjoining tectonic plates moved in unison during the 5-millimetre displacement. Sunyoung Park at the University of Chicago said the original seismic wave remained powerful enough after reflection from the core to cause the nationwide lurch because the main shock was exceptionally large.

Park said the vigorous shaking from the Tohoku earthquake may have weakened plate boundaries, making them more susceptible to movement when the core-reflected wave arrived.

The Tohoku earthquake generated 40-metre tsunami waves that led to the meltdown of three reactors at the Fukushima Daiichi nuclear plant. aec4190. Robin Lee at the University of Canterbury, New Zealand, said large earthquakes can trigger widespread, delayed fault motion minutes later, and over much larger regions than expected.

Park said after a big earthquake it may be necessary to be aware of potential seismic hazards due to such deep-travelling wave arrivals that can trigger more events, and over very large distances. More research is now needed to understand the implications of this kind of movement for other parts of the world with similar faults, Lee said.