# Has the Earth shuddered: Offset axis

Chilean earthquake on the map. February 27, 2010 Center - at a depth of 35 km. Magnitude 8.8

Regular fluctuations in the length of the day that cause changes in the Earth's rotation associated with tides, winds, ocean currents and other factors

Normal oscillation of the Earth's rotation axis, starting in January 2009. The grid cells correspond to a value of 1 angular millisecond

To begin with, we recall that the Chilean earthquake of February 27 had a magnitude of 8.8 and became really very, very powerful. Just because its hypocenter was located far from the populated area (and deep beneath the surface) saved us from a multitude of human victims. However, we wrote more about this event in hot pursuit, in the article "Chilean shock."

Already a few days later, some scientists made statements that such a strong concussion could change the tilt of the axis of our entire planet. NASA geophysicist Richard Gross says: “If our calculations are correct, the Earth’s own axis has shifted about 8 cm.” It is important to pay attention to the fact that this is not about tilting the axis of rotation. “The self axis does not characterize how tilted the Earth is, ” adds Gross, “but how it is balanced.”

This can be explained as follows. Our planet, as you know, is not an ideal sphere. Firstly, the globe is slightly flattened from the poles - the exact geometry of this should be established by the GOCE mission sent to space a couple of years ago (read: “Non-circular Earth”). Secondly, the distribution of mass on the planet is heterogeneous, if only because part of its surface is made up of oceans, and part - continents. In the northern hemisphere there is much more land than in the southern, less in the western than in the eastern. The Earth’s own axis is the axis on which this heterogeneous ball of the planet is “balanced”, and the real axis of rotation of the planet oscillates around it.

Now we understand what Richard Gross and his colleagues had in mind. The Chilean earthquake was so powerful that it caused the movement of colossal volumes of matter. This movement changed the distribution of mass over the surface of the planet - not too noticeable, but quite enough for the "axis of balance" of the globe to slightly deviate.

However, this deviation is far from the first and not the last. The Earth’s own axis is shifted by itself, without any catastrophic events, as a result of slow geological processes. The last ice age ended about 11 thousand years ago, and huge masses of ice disappeared from the surface of continents and oceans. This not only led to the redistribution of mass, but also “unloaded” the earth's mantle, allowing it to take a closer to a spherical shape. This process has not yet been completed, and as a result, the axis on which our planet “balances” naturally shifts by about 10 cm per year.

But it is worth saying that if Gross’s calculations are correct, then as a result of the earthquake in a few minutes the axis has shifted by almost the same amount as in the year. This is really impressive. However, to date, all this is only theoretical calculations and, as they say, speculation. No practical measurements were taken, although a group of Richard Gross intends to address this issue in the near future. And the key tool for these measurements should be ... a GPS global positioning system.

“Using a network of GPS receivers across the planet, we can monitor the Earth’s rotation with high accuracy, ” the scientist explains. “Changes in its characteristics affect both the phase of the signals coming from satellites and the time it takes them to get from orbit.” For years, GPS has been used by scientists to track seasonal and annual changes in Earth's rotation. Thanks to these accurate observations, it has been shown that such factors as tides and winds, currents in the oceans and in the molten bowels of the planet influence it. These factors have a periodic effect (see the illustration on the left), at different time scales - weekly, annual, and seasonal. For example, the average day in January is about 1 millisecond longer than in June.

Against this regular background, the Chilean earthquake should look like a sharp jump - and Richard Gross and colleagues really hope to find this jump in the data of the monitoring system. He says: “We take GPS data on the Earth’s rotation, subtract the characteristic periodic effects of tides, winds, currents and so on, and then the earthquake should manifest itself.”

Note that shortly after the earthquake - simultaneously with screaming headlines about the "displacement of the earth's axis" - some media noted that the duration of the day as a result of this event was reduced by 1.26 microseconds. This is true, but this value does not represent anything dangerous or sensational. It is negligible in comparison with the normal change in the length of the day that causes tides or ocean currents. Their influence is thousands of times stronger.

In a word, it remains for us to wait for the final results of the work of the Richard Gross group. No one has investigated the displacement of the Earth’s own axis as a result of earthquakes. Gross himself first tried to do this in 2004, after an earthquake of magnitude 9.1, which occurred in Sumatra, but then did not get any decent results. According to the scientist, the location of the epicenter of the earthquake is to blame for this: despite the impressive power, the location near the equator did not allow him to exert sufficient influence on the rotation of the planet. Now the situation is different, and perhaps the effect of the Chilean earthquake will be more noticeable.

According to NASA