A Japan seismic shift after the 2011 magnitude 9.0 earthquake came from seismic waves that travelled to Earth’s core and bounced back, researchers said.
The movement pushed nearly all of mainland Japan eastward by 5-6 millimetres. It occurred about 15 minutes after the quake began at 2:46 p.m. local time.
University of Chicago geophysicist Sunyoung Park led the study. Her team analysed GPS and seismic data from the March 11, 2011, event.
Park said the unusual finding was the scale of the movement. Almost all of Japan moved nearly uniformly at the same time.
The shift covered mainland Japan from Hokkaido to Kyushu. That area stretches about 3,000 kilometres.
Researchers said the returning wave moved four major tectonic plates. They identified the Pacific, Okhotsk, Philippine Sea and Eurasian plates.
The movement was small but permanent, according to the study. Researchers said it released energy close to a magnitude 7.5 earthquake. Scientists already knew that waves from large earthquakes can travel through the Earth. They can also bounce off the liquid-metal outer core.
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Park said the new finding showed something different. A deep-diving wave can return to the crust and trigger broad ground movement. The 15-minute round-trip from the crust to the core could help scientists anticipate similar events.
Goran Ekstrom, a Columbia University geophysicist, did not work on the study. He said the 2011 quake also moved Honshu about 20 centimetres east during the main rupture.
Vedran Lekić, a University of Maryland professor, said Japan’s dense monitoring network helped researchers document the event.
Amanda Thomas, a University of California, Davis geophysicist, also did not work on the study. She said large earthquakes may affect fault systems long after the main rupture.
The Japanese seismic shift may still be hard to link to visible damage. Park said the energy spread over a very broad area, unlike a typical concentrated earthquake.
