Friday, December 23, 2016

The Earth doesn't just spin : it also shakes and wobbles

Earth does not always spin on an axis running through its poles. Instead, it wobbles irregularly over time, drifting toward North America throughout most of the 20th Century (green arrow).
That direction has changed drastically due to changes in water mass on Earth.
Credit: NASA/JPL-Caltech

From BBC by Nic Fleming

The ground we stand on is not as steady as it seems.
There are a host of factors that cause the entire Earth to judder and topple

The Earth beneath our feet seems reassuringly solid and unchanging most of the time.
But this is an illusion, born of our limited perspective.
Our planet rotates on its axis once every 23 hours, 56 minutes and 4 seconds.
It also orbits the Sun, while our Solar System dashes around the centre of the Milky Way, which is itself hurtling across the Universe towards a region of space called the Great Attractor.
The speeds involved are frankly dizzying.
Even if you ignore all that, the Earth is far from stable.
Beneath us, enormous chunks of rock are constantly grinding past each other to make valleys, pushing together to form mountains, or dragging apart to create rivers and oceans.
The ground under us is forever shifting, stretching and wobbling.
Most of the time, this is nothing to worry about.
However, our growing understanding of these phenomena is driving a better understanding of the inner workings of our planet.
It is also handy for anyone trying to track and land spacecraft.
Here, then, are seven things that make the Earth move for us.

Before about 2000, Earth's spin axis was drifting toward Canada (green arrow, left globe).
JPL scientists calculated the effect of changes in water mass in different regions (center globe) in pulling the direction of drift eastward and speeding the rate (right globe).
Credit: NASA/JPL-Caltech

Under pressure

A desktop globe is a perfect sphere, so it spins smoothly around a fixed axis.
However, the Earth is not spherical, and the mass within it is both unevenly distributed and prone to moving around.
As a result, the axis around which Earth spins, and the north and south rotational poles at each end of the axis, move about.
What's more, because the rotation axis is different to the figure axis around which its mass is balanced, the Earth wobbles as it spins.

This wobble was predicted by scientists as far back as Isaac Newton.
To be more precise, it is made up of a number of distinct wobbles.
The one that has the greatest impact is known as the Chandler Wobble, first observed by American astronomer Seth Chandler Jr in 1891.
It causes movements of the poles of around 26ft (9m) and takes some 14 months to complete a full cycle.
During the 20th Century scientists suggested a wide variety of causes, including changes in continental water storage, atmospheric pressure, earthquakes, and interactions at the boundary of the Earth's core and mantle.
Geophysicist Richard Gross of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California solved the mystery in 2000.
He applied new weather and oceanic models to observations on the Chandler Wobble from 1985-1995.
Gross calculated that two-thirds of the Wobble was caused by fluctuating seabed pressure, and one-third by changes in atmospheric pressure.
"Their relative importance varies with time," says Gross, "but the cause is now widely accepted to be the combination of changes in atmospheric and oceanic pressures."

The relationship between continental water mass and the east-west wobble in Earth's spin axis. Losses of water from Eurasia correspond to eastward swings in the general direction of the spin axis (top), and Eurasian gains push the spin axis westward (bottom).
Credit: NASA/JPL-Caltech 
Water works

The seasons are the second largest influence on the Earth's wobble.
That is because they cause geographical variations in the amount of rain, snow and humidity.
Scientists have been able to pinpoint the poles using the relative positions of the stars since 1899, and using satellites since the 1970s.
But even after removing the impact of the Chandler and seasonal wobbles, the north and south rotational poles still move about with respect to the Earth's crust.
In a study published in April 2016, Surendra Adhikari and Erik Ivins, also at JPL, identified two more important pieces of the Earth wobble jigsaw.
Before the year 2000, the Earth's spin axis was drifting towards Canada, a few inches per year.
But then measurements show the spin axis changed tack, heading instead towards the British Isles. Some scientists suggested this could be the result of the loss of ice caused by the rapid melting of Greenland's and Antarctica's ice sheets.
Adhikari and Ivins set out to test this idea.
They compared GPS measurements of the positions of the poles with data from GRACE, a study that uses satellites to measure changes in mass around the Earth.
They found that the melting of the Greenland and Antarctic ice sheets only explains around two-thirds of the recent shift in the direction of the poles.
The remainder, they concluded, is down to the loss of water held on continents, mostly the Eurasian land mass.
This region has been affected by aquifer depletion and drought.
Then they factored in the position of the areas affected. "From the fundamental physics of rotating objects, we know that movement of the poles is highly sensitive to changes at [around] +/- 45 degree latitudes," says Adhikari. That is exactly where Eurasia had lost water.
The study also identified continental water storage as a plausible explanation for another wobble in the Earth's rotation.
Throughout the 20th Century, researchers were puzzled because the spin axis shifted every six to 14 years, heading 0.5-1.5m east or west of its overall drift. Adhikari and Ivins found that, between 2002 and 2015, dry years in Eurasia corresponded to the eastward swings and wet years corresponded to westward movements.
"We found a perfect match," says Adhikari. "It's the first time anyone successfully identified a one-to-one match between the global-scale inter-annual wet-dry variability and inter-annual polar motion."

This video explains what's happening to Earth

Man-made meandering

While these movements of water and ice are caused by a combination of natural processes and human actions, other changes that impact the Earth's wobbling are all our own doing.
In a 2009 study Felix Landerer, also of the JPL, calculated that, if carbon dioxide levels double between 2000 and 2100, the oceans will warm and expand in such a way that the north pole will shift around 1.5cm per year towards Alaska and Hawai'i over the next century.
Similarly, in a 2007 study Landerer modelled the effects of the ocean warming caused by the same carbon dioxide increase on ocean bottom pressures and circulation.
He found that the changes would shift mass to higher latitudes, and that this would shorten the day by a little over 0.1 milliseconds: 1/10,000th of a second.

Quake shake

It is not just large volumes of water and ice that affect the Earth's rotation if they move around. Shifting rocks have the same effect, if they are big enough.
Earthquakes occur when the tectonic plates that make up the Earth's surface slip past each other suddenly. In theory, that could make a difference.
However, at first the amount of water involved seemed too small to have such an impact.
For example, Gross studied the massive 8.8-magnitude quake that hit the coast of Chile in 2010.
In as as-yet-unpublished study, he calculated that the plate movements shifted Earth's axis of mass balance by around 8cm.
However, this was only a model-based estimate.
Gross and others have since attempted to observe real shifts in the way the Earth is spinning, by following earthquakes in GPS satellite data.
So far this has proved unsuccessful, because it is tricky to remove all the other things that influence how the Earth rotates.
"The models are not perfect and there is residual noise masking the smaller earthquake signals," says Gross.
The movements of mass that take place when tectonic plates slip past each other also affect the length of days.
This is a little bit like an ice skater spinning on one spot: she can speed up by drawing her arms in and thus shifting her mass closer to her body, or slow down by doing the opposite.
For example, Gross calculated that the magnitude-9.1 earthquake that hit Japan in 2011 shortened the length of the day by 1.8 microseconds.

Storm force wobble

When an earthquake happens, it triggers seismic waves that carry its energy through the interior of the Earth.
There are two kinds. "P-waves" repeatedly squeeze and expand the material they pass through, with the vibrations travelling in the same direction as the wave.
Slower "S-waves" wobble rock from side to side, with the vibrations occurring at right angles to their direction of travel.
Intense storms can also create faint seismic waves like those triggered by earthquakes.
These waves are called microseisms.
Until recently, scientists have been unable to determine the sources of S-waves from microseisms.
In a study published in August 2016, Kiwamu Nishida of the University of Tokyo and Ryota Takagi of Tohoku University reported that they had used a network of 202 detectors in southern Japan to track both P- and S-waves.
They traced the waves' origins to a severe North Atlantic storm called a "weather bomb": a storm in which the atmospheric pressure at the centre drops unusually rapidly.
Tracking microseisms in this way will help researchers to better understand the internal structure of the Earth.

Lunar influence

It is not just Earth-bound phenomena that influence our planet's movements.
Recent research suggests that large earthquakes are more likely around full and new moons.
That could be because the Sun, Moon and Earth are aligned, increasing the gravitational force acting on our planet.
In a study published in September 2016, Satoshi Ide of the University of Tokyo and his colleagues analysed the tidal stresses in the two-week periods prior to large earthquakes in the last two decades. Of the largest 12 earthquakes, all of which had a magnitude of 8.2 or higher, nine happened close to full or new moons.
No such relationship was found for smaller quakes.
Ide concluded that the extra gravitational force exerted at these times could increase the forces acting on tectonic plates.
The changes would be small, but if the plates were under stress anyway, the extra force could be enough to turn small rock failures into larger ruptures.
While this may seem plausible, many scientists are sceptical because Ide's study only looked at 12 earthquakes.

Sun shakes

Even more controversial is the idea that vibrations originating deep within the Sun could help explain a number of shaking phenomena on Earth.
When gases move around inside the Sun, they produce two different types of waves.
Those generated by changes in pressure are called p-modes, while those that form when dense material is pulled downwards by gravity are called g-modes.
A p-mode takes a few minutes to complete a full vibrational cycle, while a g-mode takes between tens of minutes and several hours.
This amount of time is the mode's "period".
In 1995, a group led by David Thomson of Queen's University in Kingston, Canada analysed patterns exhibited by the solar wind – a stream of charged particles that flows out from the Sun – between 1992 and 1994.
They reported fluctuations that had the same periods as p-modes and g-modes, suggesting these solar vibrations were somehow influencing the solar wind.
In 2007, Thomson went on to report that unexplained fluctuations in the voltages of undersea communications cables, seismic measurements on Earth and even mobile phone call dropouts also had frequency patterns that matched the waves inside the Sun.
However, other scientists believe Thomson's claims are on shaky ground.
According to simulations, these solar vibrations, especially the g-modes, should be so weak by the time they get to the Sun's surface that they could not affect the solar wind.
Even if that is not the case, the patterns should be destroyed by turbulence in the interplanetary medium long before they get to Earth.
"When we looked at different time periods, the frequencies he had identified were shifting around, when to be g-modes in particular they should remain fairly constant," says Pete Riley of Predictive Science in San Diego, California.
Back in 1996 he published a study questioning Thomson's original results.
"We looked at the same data Dave Thomson looked at and applied the same analysis, and couldn't find any evidence for p-modes or g-modes."
Clearly, Thomson's idea might not pan out.
But there are plenty of other reasons why our planet wobbles and shakes.

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