Researcher: Gravity of moon, sun likely unleashed Tohoku quake

December 18, 2012

By TAKASHI SUGIMOTO/ Staff Writer

The gravity of the moon and the sun likely triggered the Great East Japan Earthquake last year, according to a Japanese researcher.

Sachiko Tanaka, of the National Research Institute for Earth Science and Disaster Prevention, studied the correlation between gravitational pull and earthquakes off the coast of the Tohoku region over a 36-year period.

She said she found that the quakes before the disastrous temblor on March 11, 2011, tended to strike when the gravitational effects were strong.

The Earth goes through two cycles of expansion and contraction by about 20 centimeters a day due to the gravitational pull of the moon and the sun. Although the force from the gravity is only around one-thousandth of the typical stress that has built up in a tectonic plate, it could be enough to set off an earthquake, she said.

“As stress that can cause a huge earthquake has built up in a plate, a small force could be a trigger,” Tanaka said.

She said studies of the gravitational effects and the frequency of earthquakes could provide signs that a huge quake is approaching.

Specifically, Tanaka analyzed about 500 temblors with a magnitude of 5.0 or greater between 1976 and 2011 in a 500-kilometer-by-200-km area of the fault that caused the magnitude-9.0 earthquake on March 11 last year.

She detected no correlation between the gravity and earthquake occurrences in the 25 years from 1976.

But for the 3,000 days before the March 11 earthquake, she found that quakes tended to occur when relatively strong gravity was applied to the fault in the direction that it slides.

Similar patterns were found before the huge earthquakes off Sumatra in 2004 and 2007, she said.

By TAKASHI SUGIMOTO/ Staff Writer
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A sightseeing boat sits amid the wreckage in Miyako, Iwate Prefecture, following the Great East Japan Earthquake and tsunami in March 2011. (Asahi Shimbun file photo)

A sightseeing boat sits amid the wreckage in Miyako, Iwate Prefecture, following the Great East Japan Earthquake and tsunami in March 2011. (Asahi Shimbun file photo)

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  • A sightseeing boat sits amid the wreckage in Miyako, Iwate Prefecture, following the Great East Japan Earthquake and tsunami in March 2011. (Asahi Shimbun file photo)
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