[Physltest] [Phys-L] Re: earthquake

[Leigh Palmer <palmer@SFU.CA>]

> Anthony Lapinski asked:
>
> I read a Yahoo news article from yesterday (Sunday) from the Asian
> Associated Press. In it the head of Italy's National Geophysics
> Institute
> said that "all the planet is vibrating" from the quake, and that the
> quake
> even disturbed the Earth's rotation. Nothing more specific regarding
> this
> phenomena was mentioned.
>
> Can any internal force disrupt the Earth's rotation? And if the quake
> really changed the Earth's movement, is it large enough to detect? I
> know
> that tidal friction is slowing the Earth's rotation over time, but
> that's
> an external force from the Moon causing the water to rise/fall and hit
> the
> land masses.

I had a physical moment today. I realized that what is reported here is
not only reasonable, but it should be an expected consequence of most
earthquakes. I hope this is in time for teachers to answer their
students' questions today.

First, let's acknowledge that this is the consequence a geophyicist
expects from his detailed consideration of the process, not a physical
observation.

When an event like this earthquake occurs the moments of inertia of
Earth change. The angular momentum is instantaneously unchanged by the
event itself, though of course the angular momentum changes gradually
with time due to tidal torques. Because of the instantaneous change in
the moments of inertia, the angular velocity also changes
instantaneously. For an event localized on Earth's surface near the
equator this could result in an appreciable change in the direction of
the angular velocity which would then precess about the direction of
the angular momentum vector, the "wobble" mentioned in other reports.

Oh, yes, my physical moment. Why would one expect the rotation of Earth
to speed up in general when a large earthquake occurs? Earthquakes are
powered by Earth's self gravitation. The release of that gravitational
energy is a consequence of net mass motion toward the center of Earth,
so one would expect changes in the diagonal components of its inertia
tensor to be negative. That's why Earth spins faster. I think it's that
simple!

(By the way, the same thing may happen with pulsars. That is an
explanation of glitches.)

Leigh
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