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.
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.
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.
He explained that his figures on the shifting caused by Japan's earthquake are preliminary.
His team is still working on the calculations.
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.
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.
He said the Chilean earthquake re-arranged what he called the mass balance of Earth.
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.
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.