News 8.8 earthquake in Chile

[ex-orbinaut]

What happened to the other earthquake related posts of today? I could have sworn that there were some more posts regarding the earth axis tilt? Maybe a glitch in the Matrix...

@topic Even if the day/year is shortened by a few milli- or microseconds, it could have a major impact. Correct me if I'm wrong, but I think some atomic clocks probably need to be readjusted.

Aha, so there had been some posts related to this day shortening thing that were not in evidence when I posted.

I see, there is some precession involved, but it still does not make sense to me why it SHORTENS the day.

My way of thinking, initially:

Radial inertia, like the well used analogy of the spinning ice skater swinging his (or her) arms out and drawing them in to change the rotational velocity. To shorten the day by this method, one of the plates would have to have SUBSIDED considerably. The crust is very low mass in comparison to the core and would need to "drop" a great deal to have any effect....

Now, precession...

If it is this, then, that tilts the axis of the planet (the plate moves north/south) and the energy is then exerted at 90 degrees around the rotational plane from where it was originally produced (the point of the earthquake) and tips the axis slightly. Some of the rotational energy is used up "tipping" the axis by precession. This, then, would slow down the rotation, ie; lengthening the days slightly.

I do not get it... either way.

 

[tblaxland]

Some of the rotational energy is used up "tipping" the axis by precession.

"used"? Where would it go?

This, then, would slow down the rotation, ie; lengthening the days slightly.

If the axis moves slightly, it might lengthen or increase the length of the day, depending on the direction it moved. What you are considering is slightly changing the moment of inertia tensor for the Earth. If the principle axis moves toward the angular momentum vector then angular velocity around the principle axis will increase and the day will get shorter.

 

[ex-orbinaut]

"used"? Where would it go?

In tipping the axis. I think I am forgetting that there is not a physical "axis" as such, so no bearing friction when it is precessed, like a bicycle wheel.

What you are considering is slightly changing the moment of inertia tensor for the Earth.

That's what I am considering in the first instance, yes...

If the principle axis moves toward the angular momentum vector then angular velocity around the principle axis will increase and the day will get shorter.

Hmmmm. :hmm: Something to do with the fact that the focus of the earthquake (the applied force) is not on the equator, ie; more or less around the center of gravity of the revolving mass?

 

[tblaxland]

That's what I am considering in the first instance, yes...

Actually it is what you are considering in both instances. In the first instance you were only considering changing the magnitude of the moment of inertia about the major principle axis. In the second instance, you were considering rotating the moment of inertia tensor (if I understood you correctly).

Hmmmm. :hmm: Something to do with the fact that the focus of the earthquake (the applied force) is not on the equator, ie; more or less around the center of gravity of the revolving mass?

It doesn't matter where the focus is really. What matters is how the mass of the earth is redistributed.

By the way, back to a point from your other post:

To shorten the day by this method, one of the plates would have to have SUBSIDED considerably

Note that a transform fault (ie, one plate moving north or south) will also reduce or increase the moment of inertia about the major principle axis, in addition to rotating the moment of inertia tensor (any object that moves north or south gets closer or further from the Earth's axis).

 

[ex-orbinaut]

Actually it is what you are considering in both instances. In the first instance you were only considering changing the magnitude of the moment of inertia about the major principle axis. In the second instance, you were considering rotating the moment of inertia tensor (if I understood you correctly).

I see. This manages to marry both concepts into one (the spinning ice skater analogy) through different routes, so I imagine this is the theory behind the idea of an earthquake modifying the rotational velocity of a planet. It makes some sense, but does not explain the axis tilt change, which I can only seem to explain to myself by invoking the effects of precession.

It doesn't matter where the focus is really. What matters is how the mass of the earth is redistributed.

So it must be happening gradually and constantly all the time as a result of convection in the outer mantle, which is a much larger and far from even redistribution of mass than relatively low density (and comparatively small) crustal movements. Granted, plate movement is a symptom of this convection, yes, and I can see the point that way. However, the articles are sort of suggesting it is the jolt of the crust (the earthquake) itself that is causing the axis shift and rotational speed.

In any case, thank you for the explanation. It has clarified the machinations of the mechanism somewhat.

 

[Urwumpe]

The rotation axis and the precession of it depends on the moments of inertia - just try it on a spinning chair and just pull one arm to you...

 

[ex-orbinaut]

The rotation axis and the precession of it depends on the moments of inertia - just try it on a spinning chair and just pull one arm to you...

:thumbup:Yeah, you know what I mean exactly, and it's a workable example. Now, spin yourself around clockwise, and hold a big, heavy cast iron 4x4's brake disk at arm's length in your left hand while you spin. Hold it way out to the left, then lift it briskly. It will tend to tip you backwards while you spin. And if you then bring it down briskly, it will tend to tip you forwards. This is the only way I can see (so far) that a rotating body, by its own action, can affect its own axis without the assistance of external forces to topple it.

 

[Urwumpe]

This is the only way I can see (so far) that a rotating body, by its own action, can affect its own axis without the assistance of external forces to topple it.

Yes, just conservation of rotational momentum...

L = I * w

L = momentum vector (R³)
I = matrix with the moments of inertia (R^3x3)
w = rotation velocity vector. (R³)

If you change the moments of inertia by changing the mass distribution, your rotation velocity vector will also change. Which is pretty much what the Earthquake did - it did move a few hundred thousand tons of rock closer to the center of gravity.

Earthquakes are just the stronger example - guess which ordinary event every autumn also changes the rotation velocity of Earth a bit.

 

[ex-orbinaut]

- guess which ordinary event every autumn also changes the rotation velocity of Earth a bit.

The invernal equinox, perhaps? It suggests that one hemisphere (north / south) of the planet is more massive than the other, like the Moon's synchronous rotation suggests that the hemisphere (east / west) facing us is denser than the other. Is this a known fact, by any chance? I only go about plotting the planet's movement and watching them, but their physics are quite fascinating, I am finding.

Now, why I have been skeptical about the earthquake thing. I live in an earthquake prone region, and there have been several reasonably strong ones not more than 150 miles away over the years, some even closer, and what did I feel from them, that close? Zilch. Nut'n. If the whole Nazca, Continental South American or Pacific plates were shifting to the amount that they could affect the planet's rotation, I am certain that I would have felt a bit more than zilch. The volcanoes have been more impressive, much more impressive...

It is not that I completely dis-believe the explanations put forward, it just needs its credibility testing, from my point of view. That's all. I will let it rest now.

Many thanks!

 

[Urwumpe]

Actually, it is just the trees dropping their leaves on the northern hemisphere. :rofl:

 

[ex-orbinaut]

Actually, it is just the trees dropping their leaves on the northern hemisphere. :rofl:

:rofl: .. and shortening the inertia tensor!

What will be the effect of deforestation on that?
:uhh:

 

[Urwumpe]

What will be the effect of deforestation on that?
:uhh:

As long as we only cut evergreen plants, the effect on the inertia tensor of Earth is non-periodic.