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There are news reports saying that the Sendai earthquake a couple days ago has shifted the earth on its axis and moved the island 8 feet.

Last year, there were similar reports with the earthquake in Chile.

Does this affect GPS devices at all? Is an inaccuracy introduced? How does GPS still work if the earth has shifted on its axis?

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The GPS satellites are recalibrated (ephemeris data update) from ground-based stations every couple hours. If the whole earth shifted uniformly, would GPS automatically get updated to the new position within a few hours? –  freiheit Mar 13 '11 at 17:59
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@freiheit The whole earth indeed is constantly "shifting uniformly" as it rotates about its axis. In order to change that measurably, you would need to bring a truly massive gravitational body close to the earth for some time. Remember, too, Newton's laws: the action of the earthquake generated a reaction within the earth's crust; the net motion was zero. –  whuber Mar 13 '11 at 18:04
    
@whuber: yes, I'm aware of the uniform shifts of the surface due to rotation, as well as fairly uniform shifts due to wobbling of that axis on several periods. The news reports I saw were indicating some kind of new shift, both of Japan and the whole surface (smaller magnitude). –  freiheit Mar 14 '11 at 18:00
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@freiheit There's an article on this in today's NY Times (nytimes.com/2011/03/14/world/asia/14seismic.html?hp ) Buried at the end is this important statement: "Such changes are not unusual, and even without earthquakes, changes in ocean currents and atmospheric conditions usually have even greater effects [on mass shifts within the earth]. “The Earth is always wobbling, and the length of the day is always changing,” Dr. Gross said." –  whuber Mar 14 '11 at 18:07
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5 Answers

Geologically speaking, the Earth's surface is always on the move due to plate tectonics. On average, your location is moving anywhere from 0 to 10 cm per year (relative to other positions on Earth).

You can see global changes in GPS positions throughout the world here.

Snapshot of the GPS positions

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+1 Nice illustration. –  whuber Mar 13 '11 at 20:18
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I think this is a different effect than the one mentioned in the question. While there was a plate techtonic element to the movement of Japan due to the earthquake. The questioner is asking about the change to the world due to mass movement. –  iant Mar 14 '11 at 13:54
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Here's a great article from MSNBC on how the earthquake affects GPS data:

http://photoblog.msnbc.msn.com/_news/2011/03/12/6256280-how-the-quake-shifted-japan?chromedomain=cosmiclog&GT1=43001

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 [Ken Hudnut, a geophysicist at the U.S. Geological Survey's Earthquake Hazards Program] 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."

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Maybe it is just me, but this is a change in what is at the location, it is not a flaw of GPS itself. The GPS still knows where you are, it just happens the channel moved 3 feet to the left. –  Kortuk Mar 28 '11 at 15:59
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If the group moves to the left 8 feet and you move with it, your GPS will now say you are to the left 8 feet(if it had perfect accuracy).

If you tried to load a map of where you were the map would be off by 8 feet until they updated it. GPS is based on satellites, so to mess it up you would have to shift the world, but that has its own set of issues involving orbits.

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Right, but the news reports are that the world has been moved. –  Marcin Mar 13 '11 at 13:05
    
@Marcin, which it has not. I mean the earth shifting in space by X meters while the rest of the universe is held still. The GPS system knows where you are as long as their is not a space level event. Your maps based on GPS need an update. –  Kortuk Mar 13 '11 at 13:45
    
@Marcin, to be more clear, as long as the satellites do not move they can tell where you are in reference to the satellites, if a plate moves you will map incorrectly to where the road is, but they will know where you all. –  Kortuk Mar 28 '11 at 15:58
    
maps will be changed, but your position on the globe does not. GPS positioning is a function of time and space and does not rely on terrestrial maps. –  monkut Apr 12 '12 at 2:17
    
@monkut, that is what I was attempting to explain. The fact that the earth shifted 8 feet will be correctly noted, but until they update your map they will have you offset from a real location by 8 feet. now will a plate every shift a whole area by 8 feet, no idea, i have no idea what goes on with an earthquake. But I know what gps is doing and my point is that it would still know your location, the maps would just be incorrect and have you 8 feet off the road you are on in this hypothetical case. –  Kortuk Apr 12 '12 at 12:13
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I expect the change due to the earthquake to be on an order of magnitude as the Chandler Wobble and I assume that GPS can handle that as it happens constantly.

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not sure if the 03122011 post-tsunami Geo-Eye images on Google Earth / Maps are just not geo-referenced or what, but there is about a 40 foot discrepancy on the old and new aerial images, doubtful the land moved that much, but who knows...

it's amazing how fast they have put these images together so quickly... a word of warning, the after aerial images are pretty stunning, lots of houses and entire towns completely swept away... link is here: Satellite imagery of Japan Earthquake

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before / after Aerials of Japanese tsunami. If you scroll over the photos it gives you the before and after perspective. abc.net.au/news/events/japan-quake-2011/beforeafter.htm –  sirgeo Mar 13 '11 at 21:01
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