The ground on which we stand is not as solid as it seems. Several factors cause the entire Earth to shake and wobble. The firmness and immutability of the ground under our feet is an illusion created by our limited point of view. Our planet rotates on its axis every 23 hours 56 minutes and 4 seconds. It also revolves around the sun, the solar system revolves around the center of the Milky Way, and the galaxy rushes through the universe in the direction of the Great Attractor. The speeds involved in all this action are dizzying.
Even if all this is not taken into account, the Earth is far from stable. Somewhere below us, huge pieces of rocks are constantly breaking each other, forming valleys, pushing mountains out. Collide and drag each other to form rivers and oceans. The earth below us is constantly and always changing, stretching and wobbling.
For the most part, this is fine. However, our growing understanding of these phenomena allows us to learn more about the inner workings of our planet. It is also convenient for anyone trying to navigate and land a spacecraft. There are seven things that make the earth move. “Eppur si muove!” Galileo said. And yet it turns.
Under pressure
A desk globe is a perfect sphere, so it rotates smoothly around a fixed axis. Nevertheless, the Earth is not a sphere, and the mass in it is unevenly distributed and tends to move. Therefore, both the axis around which the planet rotates and the poles of this axis move. Moreover, since the axis of rotation is different from the axis around which the mass is balanced, the Earth wobbles as it rotates.
This oscillation was predicted by scientists back in the era of Isaac Newton. And to be precise, this oscillation consists of several.
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One of the most important is the Chandler oscillation, which was first observed by the American astronomer Seth Chandler Jr. in 1891. It causes the poles to move 9 meters and completes a full cycle in 14 months.
Throughout the 20th century, scientists have put forward many different reasons, including changes in the storage of continental waters, atmospheric pressure, earthquakes, interactions at the boundaries of the Earth's core and mantle.
Geophysicist Richard Gross of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, resolved the mystery in 2000. He applied new meteorological and oceanic models to observations of the Chandler oscillation in 1985-1995. Gross calculated that two-thirds of these fluctuations are caused by pressure fluctuations on the seabed and one-third by changes in atmospheric pressure.
"Their relative importance changes over time," says Gross, "but at present this cause, a combination of changes in atmospheric and oceanic pressure, is considered the main one."
Water wears away the stone
The seasons are the second largest factor related to the Earth wobble. Because they lead to geographic changes in rain, snow and humidity.
Scientists were able to determine the poles using the relative positions of the stars as early as 1899, and since the 1970s they have been assisted by satellites. But even if you eliminate the influence of seasonal and Chandler fluctuations, the north and south poles of rotation still move relative to the earth's crust.
In a study published in April 2016, JPL's Surendra Adikari and Eric Ivins highlighted two critical pieces of the Earth wobble puzzle.
Until 2000, the Earth's axis of rotation was moving towards Canada by two inches a year. But then measurements showed that the axis of rotation changed direction to the British Isles. Some scientists have suggested that this may be the result of ice loss due to the rapid melting of the Greenland and Antarctic ice sheets.
Adikari and Ivins decided to test this idea. They compared GPS measurements of pole positions with data from GRACE, a study that uses satellites to measure mass changes across the Earth. They found that the melting of the Greenland and Antarctic ice accounts for only two-thirds of the recent shift in the direction of the poles. The rest, according to scientists, should be explained by the loss of water on the continents, mainly on the Eurasian land area.
The region suffers from aquifer depletion and drought. Nevertheless, at first the volume of water involved in this seems too small to lead to such consequences.
Therefore, scientists looked at the position of the affected areas. “We know from the fundamental physics of rotating objects that the movement of the poles is very sensitive to changes within 45 degrees of latitude,” says Adikari. That is, exactly where Eurasia lost water.
This study also identified continental water storage as a plausible explanation for another wobble in the Earth's rotation.
Throughout the 20th century, scientists could not understand why the axis of rotation shifts every 6-14 years, leaving 0.5-1.5 meters east or west of its general drift. Adikari and Ivins found that from 2002 to 2015, dry years in Eurasia corresponded to swings to the east, and wet years to movements to the west.
“We found the perfect match,” says Adikari. "This is the first time someone has successfully identified the perfect match between interannual polar motion and global interannual drought-humidity."
Technogenic impact
The movements of water and ice are caused by a combination of natural processes and human actions. But there are other effects that affect the wobble of the earth.
In 2009, Felix Landerer, also of JPL, calculated that if carbon dioxide levels doubled from 2000 to 2100, the oceans would warm and expand so that the North Pole would move 1.5 centimeters a year towards Alaska and Hawaii for the next century. …
Likewise, in 2007 Landerer modeled the effects of ocean warming caused by the same increase in pressure and circulation from carbon dioxide on the ocean floor. He found that these changes could shift mass at higher latitudes and shorten the day by about 0.1 milliseconds.
Earthquake
It is not only large volumes of water and ice that affect the rotation of the Earth as it moves. The displacement of rocks also has this effect if they are large enough.
Earthquakes occur when the tectonic plates that make up the Earth's surface suddenly begin to "rub in" as they pass by. This could also contribute. Gross measured a powerful 8.8-magnitude earthquake that hit the Chilean coast in 2010. In an as-yet-unpublished study, he calculated that the movement of the plates shifted the Earth's axis relative to mass balance by about 8 centimeters.
But this is only based on the model's assessment. Since then, Gross and others have tried to observe the actual shifts in the Earth's rotation from earthquake data from GPS satellites.
So far, this has been unsuccessful, because it is rather difficult to remove all other factors that influence the rotation of the Earth. “The models are not perfect and there is a lot of noise masking small earthquake signals,” says Gross.
The movement of masses, which occurs when tectonic plates pass nearby, also affect the length of the day. Gross calculated that the magnitude 9.1 earthquake that hit Japan in 2011 reduced the length of the day by 1.8 microseconds.
Trembling earth
When an earthquake occurs, it sets off seismic waves that carry energy through the bowels of the earth.
There are two types of them. "P-waves" several times compress and expand the material through which they pass; vibrations travel in the same direction as the wave. Slower "S-waves" rock the rocks from side to side, and the vibrations are at right angles to their direction of travel.
Intense storms can also create weak seismic waves, similar to those that cause earthquakes. These waves are called microseisms. Until recently, scientists could not determine the source of S-waves in microseisms.
In a study published in August 2016, Kiwamu Nishida from the University of Tokyo and Ryota Takagi from Tohoku University reported using a network of 202 detectors in southern Japan to track P- and S-waves. They traced the origin of the waves to a major North Atlantic storm called the "weather bomb": in this storm, atmospheric pressure in the center drops unusually quickly.
Tracking microseisms in this way will help researchers better understand the Earth's internal structure.
The influence of the moon
Not only earthly phenomena affect the movements of our planet. Recent studies have shown that large earthquakes occur with full and new moon. Perhaps this is because the Sun, Moon and Earth are aligned, thus increasing the gravitational force acting on the planet.
In a study published in September 2016, Satoshi Ida of the University of Tokyo and his colleagues analyzed tidal stresses over two weeks prior to major earthquakes in the past twenty years. Of the 12 largest earthquakes of 8.2 magnitude or greater, nine occurred during a full moon or a new moon. For small earthquakes, no such correspondence was found.
Ida concluded that the additional gravitational influence that occurs at these times can increase the effect of forces on tectonic plates. These changes should be small, but if the slabs are already energized, the additional force may be sufficient to trigger large fractures in the rocks.
However, many scientists are skeptical about Ida's findings, since he only studied 12 earthquakes.
Trembling sun
Even more controversial is the idea that vibrations that originate deep within the Sun can explain a number of shaking phenomena on Earth.
When gases move inside the sun, they give rise to two different types of waves. Those that are born in the process of pressure changes are called p-modes, and those that form when dense material is sucked in by gravity are called g-modes.
P-mode takes several minutes to complete a full vibration cycle; g-mod takes from ten minutes to several hours. This amount of time is called the "period" of the mod.
In 1995, a team led by David Thomson of Queen's University in Kingston, Canada, analyzed patterns of the solar wind - the flow of charged particles that emanate from the sun - from 1992 to 1994. They noticed oscillations that had the same periods as the p- and g-modes, suggesting that the solar vibrations were somehow related to the solar wind.
In 2007, Thomson again reported that unexplained voltage fluctuations in undersea utility cables, seismic measurements on Earth, and even cutoffs in phone calls have frequency patterns consistent with the waves within the Sun.
However, scientists believe that Thomson's claims have shaky ground. According to simulations, these solar vibrations, especially the g-modes, should be so weak by the time they reach the surface of the Sun that they could not affect the solar wind in any way. Even if this is not the case, these patterns must have been destroyed by the turbulence of the interplanetary medium long before reaching Earth.
Perhaps Thomson's idea is wrong. But there are many other reasons why our planet shakes and sways.
ILYA KHEL
