[Phys-L] Re: earthquake

[jsd at AV8N.COM (John Denker)]

I had a little discussion with Richard Gross at JPL.  He said:

> I have modeled the coseismic effect on the Earth's rotation of the
> December 26 earthquake in Indonesia by using the PREM model for the
> elastic properties of the Earth and the Harvard centroid-moment
> tensor solution for the source properties of the earthquake. The
> result is:
>
> change in length of day: -2.676 microseconds
> polar motion excitation X : -0.670 milliarcseconds
> polar motion excitation Y: 0.475 milliarcseconds
>
> The source properties of earthquakes are computed by a number of groups.
> For the Indonesian earthquake I used the solution of the Harvard group
> which can be found at their web site:
>
> http://www.seismology.harvard.edu/projects/CMT/
>
> It is the global shift in mass that is important when computing the effect
> on the Earth's rotation, and that is what my model computes. I actually
> don't save the mass displacement field itself, but integrate the mass
> displacements to get the change in the Earth's inertia tensor and hence
> in the Earth's rotation.
>
> Since the length of the day can be measured with an accuracy of about
> 20 microseconds, this model predicts that the change in the length-of-day
> caused by the earthquake is much too small to be observed. And, since
> the location of the earthquake was near the equator, this model predicts
> that the change in polar motion excitation is also rather small, being
> about 0.82 milliarcsecond in amplitude. Such a small change in polar
> motion excitation will also be difficult to detect.


On the other hand ... the uncertainty in day-length is not cumulative, so
if we wait a few weeks the rate-change should become readily observable.