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## Richard Gross: Japan earthquake shortened Earth’s day 1.4 millionths of a second

The 9.0-magnitude earthquake in Japan on March 11, 2011 caused Earth to spin faster, making our day 1.4 millionths of a second shorter.

After the 9.0 magnitude earthquake in Japan on March 11, 2011, many reported that Earth’s day had been shortened by 1.8 millionths of a second, according to NASA scientists. In other words, since the earthquake, Earth spins faster than before, and our day is ever so slightly shorter from sunrise to sunset. The scientists 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. In early March 2012, EarthSky asked Richard Gross of NASA’s Jet Propulsion Laboratory, who helped make this calculation in 2011, if the original number – 1.8 milliseconds – still held.

A year ago, you said would refine your calculation of the shortening of Earth’s day due to the March 11, 2011 Japan. How does it look today?

The 1.8 microsecond shortening of the length of the day was based upon a preliminary USGS model of the earthquake fault. Using a more recent, updated model from the USGS, I now calculate that the length of the day should have become shorter by about 1.4 microseconds as a result of the 2011 Japan earthquake.

Using the updated fault model from the USGS for the 2011 Japan earthquake, I also calculate that the position of the point where the figure axis pierces the Earth’s crust near the North pole should have changed its position by about 13.7 centimeters (5.4 inches) towards 132 degrees East longitude.

So since that earthquake, our planet is spinning faster?

Yes. This is just like a spinning ice 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.

Let me see if I understand this. The tilt of Earth 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. Right?

Yes. 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.

You said a year ago you would be looking for signs of this change in observational data. Did you find any?

I have searched the Earth rotation observations for these theoretical changes caused by the earthquake but without success. Unfortunately, these rather small changes caused by the earthquake are masked by much larger changes caused by the atmosphere and oceans.

Yes, let’s talk about those. We understand Earth shifts its rate of spin all the time due to winds 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.

Have other earthquakes have also shifted Earth’s axis and changed our planet’s spin rate?

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.

Chile earthquake might have shortened Earth’s day, but how?

Bottom line: The magnitude-9.0 earthquake that struck northern Japan on March 11, 2011 slightly changed the balance of the planet and altered Earth’s spin by 1.4 milliseconds. 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.

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