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How the quake shifted Japan


Images from NASA's Terra satellite show the coastline of Japan's Honshu island in the area around Sendai before and after Friday's earthquake. The left image is from Feb. 26, and the right image is from today. The images are color-coded to reflect surface composition rather than what the eye would see. The "Flood" label helps you gauge the extent of the flooding caused by the tsunami that followed the quake.

This week's earthquake caused the main island of Japan to shift as much as 13 feet to the east, seismologists say. That may sound like a shocker, but it's just one of the natural changes that come along with an 8.9-magnitude temblor — like the 1.8-microsecond speed-up of Earth's daily rotation and the 6.5-inch shift in Earth's axis.

The eastward shift was documented by Japan's Geonet network of GPS monitoring stations, based in Tsukuba, said Ken Hudnut, a geophysicist at the U.S. Geological Survey's Earthquake Hazards Program in Pasadena, Calif. Similar shifts took place during last year's 8.8 earthquake off the Chilean coast, as well as the 9.1 earthquake near Sumatra that caused a disastrous tsunami in 2004.

"It's the same phenomenon in all three cases," Hudnut said. The movement is linked to the release of the strain that builds up when one tectonic plate grinds against another in a subduction zone. 

"What's going on is that the plate going down drags along with it the upper plate as strain is stored in between earthquakes," he explained. "When the earthquake occurs, the upper plate lurches eastward over the subducting plate. The oceanic plate that's going down is relatively rigid, but the upper plate is like a wedge of material that's more elastic. So picture that upper wedge as being almost like an accordion that's being compressed between the times of earthquakes. It's like a spring. You're loading up the spring between earthquakes — in other words, you're compressing the eastern edge of the spring toward the main island of Japan. The earthquake allows that material to spring out toward the east."

Japan's network of 1,200 GPS monitoring stations, operated by the Geographical Survey Institute, shows a maximum springing-out effect of 13 feet (4 meters), with an average displacement of about 8 feet (2.5 meters) along a stretch measuring more than 300 miles (500 kilometers).

Everything that links GPS readings to maps, ranging from driving directions to property records, will have to be changed as a result of the shift, Hudnut told me. "Their national network for property boundary definitions has been warped," he said in an e-mail. "For ships, the nautical charts will need revision due to changed water depths, too (of about 3 feet). Much of the coastline dropped by a few feet, too, we gather."

We're starting to get pictures from space that document how the coastline has changed due to the earthquake and the tsunami. The NASA photos at the top of this posting show the coastline around the city of Sendai, which was one of the hardest-hit areas. The left photo was taken on Feb. 26 by the Moderate Resolution Imaging Spectrometer, or MODIS, aboard NASA's Terra satellite. The right photo was taken today by the same instrument. You can see that wide stretches of the coast are still flooded, in part because barriers erected at the coastline's edge are now keeping water in rather than keeping it out.

Other satellite pictures, distributed by Google, provide a closer-in view of the devastation caused by the tsunami. In each of the before-and-after sets below, the left picture was taken before the earthquake and the right picture was taken afterward. We've put together an eye-opening slideshow of before-and-after imagery that gives you control of the slider. And you can check out Google's blog posting and this Picasa Web album for still more.

Google / GeoEye / DigitalGlobe

Satellite photos provide before-and-after views of Kamaishi, a coastal city north of Sendai in Japan.


These photos show before-and-after views of Japanese communities, with the left views taken before the earthquake and the right view taken today. The upper set shows Yuriagi and the lower set shows Yagawahama, both in Japan's hard-hit Miyagi prefecture.

Update for 7:25 p.m. ET March 13: I've updated the figures for the change in the day's length and the position of Earth's axis to reflect fresh figures from Richard Gross, a geophysicist at NASA's Jet Propulsion Laboratory.

More post-earthquake imagery:

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