The March 11, magnitude 9.0 earthquake in Japan may have
shortened the length of each Earth day and shifted its axis. But don’t worry—you
won’t notice the difference.
Using a United States Geological Survey estimate for how the fault
responsible for the earthquake slipped, research scientist Richard Gross of
NASA’s Jet Propulsion Laboratory, Pasadena, Calif., applied a complex model to
perform a preliminary theoretical calculation of how the Japan earthquake-the
fifth largest since 1900-affected Earth’s rotation. His calculations indicate
that by changing the distribution of Earth’s mass, the Japanese earthquake
should have caused Earth to rotate a bit faster, shortening the length of the
day by about 1.8 microseconds.
The calculations also show the Japan quake should have shifted the
position of Earth’s figure axis by about 17 cm (6.5 in), towards 133 degrees
east longitude. Earth’s figure axis should not be confused with its north-south
axis; they are offset by about 10 m (about 33 ft). This shift in Earth’s figure
axis will cause Earth to wobble a bit differently as it rotates, but it will
not cause a shift of Earth’s axis in space—only external forces such as the
gravitational attraction of the sun, moon, and planets can do that.
Both calculations will likely change as data on the quake are
further refined.
In comparison, following last year’s magnitude 8.8 earthquake in
Chile, Gross estimated the Chile quake should have shortened the length of day
by about 1.26 microseconds and shifted Earth’s figure axis by about 8 cm (3 in).
A similar calculation performed after the 2004 magnitude 9.1 Sumatran earthquake
revealed it should have shortened the length of day by 6.8 microseconds and
shifted Earth’s figure axis by about 7 cm, or 2.76 in. How an individual
earthquake affects Earth’s rotation depends on its size, location, and the details
of how the fault slipped.
Gross said that, in theory, anything that redistributes Earth’s
mass will change Earth’s rotation.
“Earth’s rotation changes all the time as a result of not
only earthquakes, but also the much larger effects of changes in atmospheric
winds and oceanic currents,” he said. “Over the course of a year, the
length of the day increases and decreases by about a millisecond, or about 550
times larger than the change caused by the Japanese earthquake. The position of
Earth’s figure axis also changes all the time, by about 1 meter (3.3 ft) over
the course of a year, or about six times more than the change that should have
been caused by the Japan
quake.”
Gross said that while we can measure the effects of the
atmosphere and ocean on Earth’s rotation, the effects of earthquakes, at least
up until now, have been too small to measure. The computed change in the length
of day caused by earthquakes is much smaller than the accuracy with which
scientists can currently measure changes in the length of the day. However,
since the position of the figure axis can be measured to an accuracy of about 5
centimeters (2 in), the estimated 17-cm shift in the figure axis from the Japan quake may
actually be large enough to observe if scientists can adequately remove the
larger effects of the atmosphere and ocean from the Earth rotation
measurements. He and other scientists will be investigating this as more data
become available.
Gross said the changes in Earth’s rotation and figure axis caused
by earthquakes should not have any impacts on our daily lives. “These
changes in Earth’s rotation are perfectly natural and happen all the
time,” he said. “People shouldn’t worry about them.”
