how the japan earthquake shortened earth's day

by Beth LebwohlMarch 15, 2011

from FastCompany Website

The 8.9-magnitude earthquake inJapan shortened Earth's dayby 1.8 millionths of a second.

While this might sound striking,perhaps even scary, don't panic:

Earth is shifting slightly all thetime,

owing mostly to atmospheric andocean currents.

The 8.9-magnitude earthquake in Japan on March 11 shortened Earth's day by 1.8 millionths of a second,according to NASA scientists. Earth still tilts on its axis by 23.5 degrees, as before. But, since the March 11 earthquake, Earth is spinningfaster than before, and our day is ever so slightly shorter from sunrise to sunset. This change is so small that sophisticated instruments cannot detect it. Instead, a team of scientists atNASA's Jet Propulsion Laboratory calculated the change in Earth's rate of spin, using computers andearthquake data. EarthSky's Beth Lebwohl spoke to Richard S. Gross, the team's leader and a senior scientist at NASA'sJet Propulsion Laboratory. His group used seismic data showing the amount of slippage in the fault linenecessary to create the Japan earthquake, in order to calculate the shift in mass inside Earth and

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subsequent change in Earth's rate of spin. Imagine a spinning figure skater. As she moves her arms closer to her body, she spins faster. The Earth issimilar to that. If the mass of the Earth moves closer to its rotation axis, the planet will spin faster. Overall, Earth's axis of rotation is tilted at an angle of 23.5 degrees relative to the orbital plane at whichEarth travels around the sun. This tilt causes our four seasons, and this tilt has not changed. What has changed is the orientation of thesolid Earth with respect to our planet's tilt. In other words, the earthquake rearranged Earth's mass,bringing more mass a bit closer to the Earth's rotation axis, causing the Earth to rotate slightly faster andthe length of the day to shorten. Says Gross:

This change doesn't effect the (degree) of tilt of the axis of Earth in space, or the orbit of theEarth around the sun. The only way Earth's tilt or orbit can be affected is if some externalforce - like an asteroid - hits the Earth. These are internal processes - earthquakes or winds or currents. They can only change howthe Earth's mass is balanced. The Earth is a big massive rotating body. Anything that isreasonable to happen is going to cause only a very small change. Earth really is a very stablesystem.

Gross told EarthSky that while a shift in Earth's axis and rotational speed might sound striking, Earth isshifting slightly all the time, owing mostly to atmospheric and ocean currents.

It's a perfectly natural motion of the Earth, and the biggest cause of this motion are changesin the atmospheric winds, and changes in the ocean currents. The winds and the currentscarry a lot of energy with them, and that energy can be exchanged with the solid Earth tocause Earth's rotation to change.

If you were to stop the winds completely, Gross said, you would change the length of Earth's day by about4 milliseconds. He added that other earthquakes have also shifted Earth's axis, for example, the February 2010earthquake in Chile, and an even stronger earthquake in Chile in 1960.

The largest earthquake that has happened in Earth's recorded history was the 1960earthquake in Chile. I did the same calculations for that earthquake (as for the 2011 Japan earthquake and 2010Chile earthquake), and, according to my calculations, the 1960 earthquake should haveshortened the length of the day by 8 microseconds.

He explained that his figures on the shifting caused by Japan's earthquake are preliminary. His team is still working on the calculations.

We're looking at the observations right now to see if these predicted effects are actuallyobservable, but it will take some time for us to reduce the data - that is, to reduce the effectsof the atmosphere and oceans - to see the much smaller effects caused by the earthquake. But if we're successful, this will be the first time that we'll see a verified effect of theearthquake on the Earth's rotation.

The magnitude-8.9 earthquake that struck northern Japan on Friday not only changed the balance of theplanet and altered Earth's spin. It also moved the coastline of the island nation of Japan. Global positioning stations closest to theearthquake's epicenter jumped eastward by up to 13 feet. In the meantime, our day has gotten shorter by1.8 millionths of a second.


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Chile Earthquake Might Have...

...but How?by Emily Howard

March 15, 2010from EarthSky Website

The massive8.8-magnitude

earthquake thatstruck Chile in

February 2010 mighthave shortened

Earths day by 1.26microseconds,according to

calculations by aNASA scientist.

An EarthSky Facebook friend asked us how. And the answer is for the day to get shorter, Earths rotation has to speed up. EarthSky spoke to RichardGross, of NASAs Jet Propulsion Laboratory. He is the scientist who performed the calculations indicating the minuscule shift in Earths day length.

The Earths rotation changes all the time. It speeds up and it slows down and it wobbles as itrotates. These changes are caused by anything that moves mass around on the Earth.

He said the Chilean earthquake re-arranged what he called the mass balance of Earth. He explained:

This is just like an ice skater, a spinning ice skater who will spin faster as she brings her armscloser to her body. The net result of the mass motion that was caused by the earthquake everywhere within theearth was to rearrange the Earths mass in such a manner that it brought it a bit closer to theEarths rotation axis causing the Earth to rotate a bit faster and the length of the day to be abit shorter.

Dr. Gross said there wont be any practical consequences from this shortening of our day. In fact, winds and oceans currents have a bigger effect on the length of the day than this February 2010earthquake in Chile. In the end, the change is too small to detect even with the most sophisticatedinstruments. Dr. Gross added that the way an earthquake effects the Earths rotation differs based on the latitude whereit occurs.


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In order to change the Earths rotation you have to move mass vertically, up and down, and itturns out that if the earthquake is located on the equator, that vertical mass motion is mosteffective in changing the length of the day, and if the earthquake is located at mid-latitudes itsmost effective at changing the position of the figure axis. The Chilean earthquake was located at mid-latitudes and so it was quite effective in changingthe figure axis of the Earth.

The figure axis, Gross went on to explain, is the axis about which the Earths mass is balanced. This isdifferent from Earths rotational axis. This difference of axes causes the Earth to literally wobble as itrotates.

So the Earth is wobbling slightly differently than it was before the earthquake.

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how the japan earthquake shortened earth's day

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