"That motion causes quite a detectable,
sizable deformation in Australia," said
the study's lead author, Shin-Chan
Han, a professor of engineering at the
University of Newcastle in Australia.
[Spooky! Top 10 Unexplained
Phenomena]
The new findings could help researchers
calculate exactly where the Earth's
center of mass lies, which could provide
a sanity check for ultra-precise GPS
and satellite measurements.
Moving with the seasons
Han and his colleagues wanted to get a
better handle on how Australia moves
with the motion of water in and around
the continent. Though Earth looks like a
sphere, it's actually a little lopsided,
with the distribution of water and
continental crust shifting its center of
mass (CM) slightly, the researchers said.
"I was looking into ground deformation
induced by local climate" in Australia,
Han told Live Science in an email. But as
he began crunching numbers, he
realized that the effect of global water
movement (in the world's oceans)
seemed to be as important as local
water movement (in and around
Australia).
The scientists
looked at
data from
land-based
GPS stations
that tracked
minute
movements of
less than a
millimeter.
From there,
the team
looked at
data from the Gravity Recovery and
Climate Experiment (GRACE), which
calculates the change in the Earth's
gravitational pull from two satellites in
space, and uses that to determine
where water is located on the planet. By
taking the difference between the two,
researchers can calculate where the
center of the mass is at any given time,
and how much the continent was moving
as a result of global effects.
The continent moved northwest by a
millimeter during Australia's summer
(winter in the Northern Hemisphere),
with its northwestern edge lifting up.
Meanwhile, the opposite corner of the
continent goes up by 2 to 3 millimeters
during this same time. Six months later,
during Australia's winter, the trends
reverse, the researchers reported in the
Journal of Geophysical Research: Solid
Earth.
Mass migration
Han found that much of Australia's
motion was caused by the movement of
Earth's water in locations far from the
continent. For instance, in the Northern
Hemisphere's winter, the vast
accumulation of ice and snow up north
shifts Earth's center of gravity north.
Once all that water melts, it evaporates
into the atmosphere and the center of
mass migrates closer to the South
Pacific. Australia sits smack in between
these two areas, Han said.
"Australia is in between two peaks and
thus suffers from large extensional
stress," meaning it is always chasing the
center of mass, in a sense, Han added.
Still, "all continents will be affected by
this global pattern of mass migration
(CM motion) to different extents," Han
said.
The new findings also suggest
Australia's GPS instruments could be off
by as much as a millimeter. While that
may not seem like much, it could have
impacts in tracking sea- level rise . For
instance, deep ocean heights are
measured by satellites, while coastal
areas are tracked by tide gauges on the
ground. Having a better understanding
of how the Earth's center of mass
migrates may improve how those
measurements are reconciled, Han said.
"This is systematic distortion," Han
said. "When dealing with [millimeter]-
level precision positioning, this should be
understood and removed."
