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Japan Shifted 5 Millimeters East 15 Minutes After 2011 Tohoku Quake
A study published in Science shows a core-reflected seismic wave from the magnitude-9 Tohoku earthquake moved nearly all of Japan 5 millimeters eastward over 3000 kilometers. Four tectonic plates shifted in unison after the wave weakened plate boundaries. The finding points to previously unrecognized hazards from deep-traveling waves.
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A magnitude-9 earthquake off Japan's coast on 11 March 2011 produced a seismic wave that reached Earth's core and returned to shift almost the entire country 5 millimeters eastward 15 minutes later. The movement spanned 3000 kilometers, nearly seven times the length of the quake's main rupture, and occurred simultaneously across GPS stations throughout Japan without an ordinary earthquake at that moment.
Four adjoining tectonic plates moved together after the initial shaking weakened their boundaries.
Sunyoung Park at the University of Chicago said the original wave remained strong enough on its return to trigger the nationwide lurch. “We see a small 5-millimetre eastward step that happens nearly simultaneously and with similar size across almost all of Japan, without any ordinary earthquake at that exact time,” she stated.
Park added that the vigorous shaking from the Tohoku event likely made the plate boundaries more susceptible to motion when the reflected wave arrived.
The Tohoku quake also produced localized land shifts of many meters and 40-meter tsunami waves that caused three reactors at the Fukushima Daiichi plant to melt down. Robin Lee at the University of Canterbury said the event demonstrates that large earthquakes can trigger widespread, delayed fault motion minutes later and over much larger regions than expected.
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