Japan shifted eastward by core-reflected waves after Tohoku earthquake

About 15 minutes after the magnitude-9 Tohoku earthquake on March 11, 2011, nearly all of Japan shifted eastward by approximately half a centimeter. This slight move resulted from a powerful seismic wave that traveled 5,800 kilometers to the Earth’s core and then reflected back to the surface.

In light of the extensive damage caused by the earthquake, including localized land shifts of several meters and 40-meter tsunami waves that led to three reactors melting down at Fukushima Daiichi, such a small 5-millimeter movement may seem trivial.

However, this shift covered a distance of 3,000 kilometers, nearly seven times the length of the main rupture line of the quake and exceeding any recorded slip.

The timing and pattern of this movement are also unusual, notes Sunyoung Park from the University of Chicago. “We observe a small 5-millimeter eastward step occurring almost simultaneously and uniformly across nearly all of Japan, without any typical earthquake at that exact moment.” The shift wasn’t just north-south; it spanned the entire country and extended into the ocean.

“This isn’t merely a narrow ‘edge’ shift,” Park explains. “The eastward movement extends across all of Japan where GPS data exist. With more seafloor instruments, we could better determine how far offshore this motion reaches, but on land, the shift is observable nearly everywhere in Japan.”

Through thorough analysis of GPS and seismic data collected during the catastrophe, Park and her team have explored how such a massive shift was triggered and why it occurred 15 minutes after the main quake.

The initial Tohoku quake was so powerful that the seismic wave, though weakened, still retained enough energy upon returning to the surface to cause the entire country to shift as four tectonic plates moved collectively.

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“We believe the intense shaking from the initial earthquake may have weakened the plate boundaries, making them more prone to movement once the core-reflected wave arrived,” Park states.

This event reveals previously unrecognized mechanisms of earthquake-related damage, emphasizing the need to be aware of potential hazards posed by deep-traveling waves that can trigger additional seismic activity over large distances.

Further research is necessary to understand how such movements might affect other regions with similar faults, notes Robin Lee from the University of Canterbury, New Zealand. “This shows that large earthquakes can induce widespread, delayed fault motions over much larger areas than anticipated,” Lee remarks.

Typically, earthquakes generate waves that travel deep within Earth and reflect off the core, but these waves usually diminish significantly by the time they reach the core and return to the surface.