Earthquake Reflection Waves Pushed Japan Eastward, Study Finds

The massive 9.0 magnitude earthquake that struck Japan on March 11, 2011, not only triggered devastating tsunamis and a nuclear disaster but also caused the entire Japanese archipelago to shift eastward by up to 6 millimeters, according to new research. The study, published in the prestigious journal Science, attributes this widespread crustal movement to seismic reflection waves.

A team led by Professor Sunyoung Park of the University of Chicago analyzed GPS data and seismic waves recorded during the 2011 Great East Japan Earthquake. Their findings indicate that approximately 15 minutes after the main earthquake, seismic waves known as ScS waves traveled down to the Earth's core, reflected, and then struck the surface again. This secondary impact caused the Japanese islands, stretching about 3,000 kilometers from north to south, to move eastward.

While it's known that seismic waves can reflect off the Earth's core, this is the first time scientists have demonstrated that these reflected waves can induce movement in the crustal plates. Professor Park described this as a "previously unknown risk factor" that could potentially trigger additional earthquakes.

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Reflected seismic waves can cause another earthquake.

The eastward shift was observed across the entire archipelago, from Hokkaido in the north to Kyushu in the south, with the movement detected simultaneously at various observation points. The area closest to the epicenter experienced the largest shift of 5.5 mm, while the northernmost and southernmost regions moved by 2 to 3 mm. This synchronized movement across such a vast area is unusual compared to typical earthquakes, which often involve localized shifts with time differences.

Japan's unique geological location, situated at the convergence of four tectonic plates, likely played a role in this phenomenon. The researchers concluded that the intense vibrations from the main earthquake had already loosened the boundaries between these plates. Consequently, the weakened reflected seismic waves, upon returning from the Earth's core after a 5,800 km round trip, were able to trigger a chain reaction, causing the plates to slide eastward.

This groundbreaking study, which involved comparing seismic data with potential causes like undersea landslides, provides the first evidence that large earthquakes can trigger seismic activity over a much wider area than previously understood, potentially minutes after the initial event. Robin E. L. Lee, a professor at the University of Canterbury in New Zealand, commented to New Scientist that the research shows "megathrust earthquakes can trigger fault motion over a much wider area than expected, minutes after the main shock."

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This study shows that megathrust earthquakes can trigger fault motion over a much wider area than expected, minutes after the main shock.