Predicting Earth's position in space as a result of earthquakes

[Greenfaerie]

After the devastating earthquake in Japan, a Sky report was issued yesterday, claiming the earth has tilted some 17 degrees on it's axis, consequently affecting the length of a day.

 

If further quakes should occur of a similar magnitude, what then could this mean for the planet geographically and how will this affect climate worldwide?

 

GF x

 

 

 

[lemur]

After the devastating earthquake in Japan, a Sky report was issued yesterday, claiming the earth has tilted some 17 degrees on it's axis, consequently affecting the length of a day.

 

If further quakes should occur of a similar magnitude, what then could this mean for the planet geographically and how will this affect climate worldwide?

 

GF x

I don't understand how an Earthquake could change the tilt of the Earth's axis.

[insane_alien]

17 degrees? yeah, BS-o-meter just melted on that one.

 

if it was 17 degrees then you currently wouldn't be alive right now as 1000mph winds would have scoured the surface of earth.

 

earthquakes tend to affect the rotational velocity rather than the tilt and even then for a massive earthquake like this it is only going to be a matter of a few nanoseconds per day difference.

 

this will not impact the climate

[swansont]

The axis shift was about 10 cm. Which is a tad short of 17 degrees.

 

The earth isn't a perfect sphere, so it wobbles as it rotates. When you shift the mass around, as happened with the quake, you change the wobble and the location of the axis.

[D H]

The axis shift was about 10 cm. Which is a tad short of 17 degrees.

10 cm is about 3 milliarcseconds, so yes, a tad short of 17 degrees.

 

Greenfaerie, there are 3.6 million milliarcseconds in a degree.

 

The claim is that the Earth's principle axes shifted by about that much (actually, 14 cm). Since angular momentum is a conserved quantity,with all other things being equal, this shift in the Earth's inertia tensor means that there has to be a corresponding counter-shift in the Earth's angular velocity.

 

The problem is that all other things are not equal. Similar but slightly smaller shifts were predicted for the Feb 27, 2010 Chilean earthquake and the Sumatran quake of Christmas 2004. There were no detectable shift that rose above measurement and process noise from these previous quakes. Some experimentalists explain this lack of an observable shift in a way that is kind to the theorists. There are other confounding conditions, etc. etc. Other experimentalists aren't so kind. Effects as large as these should be observable (measurement error is about 0.1 mas for polar motion) -- but only if the effects are real.

[lemur]

The axis shift was about 10 cm. Which is a tad short of 17 degrees.

 

The earth isn't a perfect sphere, so it wobbles as it rotates. When you shift the mass around, as happened with the quake, you change the wobble and the location of the axis.

Why? Because the pole moved relative to the equator? How does a planet's tilt change without having something else to push against?

[swansont]

Why? Because the pole moved relative to the equator? How does a planet's tilt change without having something else to push against?

 

The same way a skater's spin changes by moving her arms. If she moves them in, she speeds up. If she moves just one in, she wobbles — her spin axis shifts. Conservation of angular momentum, as D H mentioned.

[lemur]

The same way a skater's spin changes by moving her arms. If she moves them in, she speeds up. If she moves just one in, she wobbles — her spin axis shifts. Conservation of angular momentum, as D H mentioned.

So there's also a conservation of wobble-geometry then, I'm guess.

[D H]

So there's also a conservation of wobble-geometry then, I'm guess.

No, there is conservation of angular momentum.

 

There is no such thing as conservation of angular velocity. The angular momentum of a solid body is given by [math]\vec L = \mathbf I \vec{\omega}[/math]. That [math]\mathbf I[/math] is a tensor. Because angular momentum remains constant, a change in any of the components of that tensor means there has to be a corresponding change in the angular velocity.

[lemur]

No, there is conservation of angular momentum.

 

There is no such thing as conservation of angular velocity. The angular momentum of a solid body is given by [math]\vec L = \mathbf I \vec{\omega}[/math]. That [math]\mathbf I[/math] is a tensor. Because angular momentum remains constant, a change in any of the components of that tensor means there has to be a corresponding change in the angular velocity.

Right, but what I meant was that if the wobble of the axis changes by 1 degree in some direction, that change would have to be offset by an appropriate amount of offset in some other direction. I.e. the mean tilt of the axis can't change, only the amount and patterns of deviation from that mean. Would that be incorrect to assume?

[swansont]

The axis precesses and nutates.

http://en.wikipedia.org/wiki/Precession

http://en.wikipedia.org/wiki/Nutation

[Greenfaerie]

Thank you kindly Ladies & Gentlemen.

 

GFxxx