by Beth Lebwohl
March 15, 2011
from FastCompany Website
 

The 8.9-magnitude earthquake in Japan shortened Earth's day

by 1.8 millionths of a second.

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

Earth is shifting slightly all the time,

owing mostly to atmospheric and ocean 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 spinning faster 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 at NASA's Jet Propulsion Laboratory calculated the change in Earth's rate of spin, using computers and earthquake data.

EarthSky's Beth Lebwohl spoke to Richard S. Gross, the team's leader and a senior scientist at NASA's Jet Propulsion Laboratory. His group used seismic data showing the amount of slippage in the fault line necessary to create the Japan earthquake, in order to calculate the shift in mass inside Earth and 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 is similar 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 which Earth travels around the sun.

This tilt causes our four seasons, and this tilt has not changed. What has changed is the orientation of the solid 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 and the 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 the Earth around the sun. The only way Earth's tilt or orbit can be affected is if some external force - like an asteroid - hits the Earth.

 

These are internal processes - earthquakes or winds or currents. They can only change how the Earth's mass is balanced. The Earth is a big massive rotating body. Anything that is reasonable to happen is going to cause only a very small change. Earth really is a very stable system.

Gross told EarthSky that while a shift in Earth's axis and rotational speed might sound striking, Earth is shifting 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 changes in the atmospheric winds, and changes in the ocean currents. The winds and the currents carry a lot of energy with them, and that energy can be exchanged with the solid Earth to cause 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 about 4 milliseconds.

He added that other earthquakes have also shifted Earth's axis, for example, the February 2010 earthquake in Chile, and an even stronger earthquake in Chile in 1960.

The largest earthquake that has happened in Earth's recorded history was the 1960 earthquake in Chile.

 

I did the same calculations for that earthquake (as for the 2011 Japan earthquake and 2010 Chile earthquake), and, according to my calculations, the 1960 earthquake should have shortened 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 actually observable, but it will take some time for us to reduce the data - that is, to reduce the effects of 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 the earthquake on the Earth's rotation.

The magnitude-8.9 earthquake that struck northern Japan on Friday not only changed the balance of the planet and altered Earth's spin.

It also moved the coastline of the island nation of Japan. Global positioning stations closest to the earthquake's epicenter jumped eastward by up to 13 feet. In the meantime, our day has gotten shorter by 1.8 millionths of a second.

Chile Earthquake Might Have...

Shortened Earth’s Day

...but How?
by Emily Howard
March 15, 2010
from EarthSky Website

The massive 8.8-magnitude earthquake that struck Chile in February 2010 might have shortened Earth’s day by 1.26 microseconds, according to calculations by a NASA scientist.

An EarthSky Facebook friend asked us how.

And the answer is for the day to get shorter, Earth’s rotation has to speed up. EarthSky spoke to Richard Gross, of NASA’s Jet Propulsion Laboratory.

He is the scientist who performed the calculations indicating the minuscule shift in Earth’s day length.

The Earth’s rotation changes all the time. It speeds up and it slows down and it wobbles as it rotates. 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 arms closer to her body.

 

The net result of the mass motion that was caused by the earthquake everywhere within the earth was to rearrange the Earth’s mass in such a manner that it brought it a bit closer to the Earth’s rotation axis causing the Earth to rotate a bit faster and the length of the day to be a bit shorter.

Dr. Gross said there won’t 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 2010 earthquake in Chile. In the end, the change is too small to detect even with the most sophisticated instruments.

Dr. Gross added that the way an earthquake effects the Earth’s rotation differs based on the latitude where it occurs.

In order to change the Earth’s rotation you have to move mass vertically, up and down, and it turns out that if the earthquake is located on the equator, that vertical mass motion is most effective in changing the length of the day, and if the earthquake is located at mid-latitudes it’s most effective at changing the position of the figure axis.

 

The Chilean earthquake was located at mid-latitudes and so it was quite effective in changing the figure axis of the Earth.

The figure axis, Gross went on to explain, is the axis about which the Earth’s mass is balanced. This is different from Earth’s rotational axis. This difference of axes causes the Earth to literally wobble as it rotates.

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