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[Phys-L] Re: earthquake



In the context of:

* * * <quotation from Tom Skilling> * * *
Incredibly, the magnitude 9.0 earthquake that struck off Sumatra on
Sunday morning caused a vertical displacement of so much material that
the rotation period of the Earth has been permanently altered. By a tiny
but measurable amount, the Earth is now rotating more quickly on its
axis, and the 24-hour day is now one ten-thousandth second
shorter.That's the result of calculations based on preliminary data made
by Oak Park astronomer Dr. Leslie M. Golden. It's analogous to the
increase in rotational speed that a twirling ice skater experiences when
he or she draws in their arms. It is estimated that during the Sumatran
quake, a block of material roughly 600 miles in length and 100 miles in
width fell 30 feet closer to the Earth's axis of rotation. The planet
has responded by rotating more rapidly, albeit ever so slightly, and our
24-hour days are now one ten-thousandth second shorter.

Hugh Haskell wrote:

While moving a chunk of matter this size could have an effect on the
earth's rotation, I seriously doubt that it is anywhere near as large
as 100 microseconds.

I agree that Golden's numbers are seriously implausible.

That number--100 microseconds--is one to two orders of
magnitude larger than others i have seen, which range from one to
three microseconds. I wonder how a large chunk of matter can move
closer to the earth's center without pushing a nearly equal amount
farther away from the earth's center, and thus cancelling out the
first effect, or very nearly so. I would guess that on a scale of 10
meters, the material of the earth is pretty nearly incompressible,
and i doubt that the chunk that was moved fell into a void in the
earth's interior.

I agree with about half of that.

We agree that there's lots of cancellation. Indeed it would be overly
bold to predict the sign of the effect! At least some earthquakes must
increase the earth's moment of inertia. For starters, the Himalayas
were produced by processes that include a series of earthquakes.

First of all, "pretty nearly incompressible" is not the same as
incompressible; it just means there's a lot of energy involved in
the deformation ... which there is. Also, there's tension as
well as compression, which means that sometimes an earthquake
produces a graben and sometimes produces a horst (among other
possibilities). A graben has pretty much the same effect as
dropping into a void, without there ever having been much of a
void directly underneath. A diagram can be found at
http://www.consrv.ca.gov/CGS/information/publications/teacher_features/images/fltblocc.gif
and its associated explanation at
http://www.consrv.ca.gov/CGS/information/publications/teacher_features/faults.htm

On a larger scale, there _are_ voids in the solid earth, namely the
ocean basins. The formation of the Himalayas involved enlarging
the Indian Ocean basin, as the Indian plate moved north and buckled
against the Asian plate. So stuff does enter/leave voids.

Also keep in mind that the launching of a tsunami commonly involves
not just the classic earthquake picture of motion along a fault;
a big additional factor is _underwater landslides_. That directly
does involve stuff that "falls into a void". The energy involved
in that is negligible compared to the energy of the fault-slip, so
it doesn't show up in the Richter number ... but the coupling to
the tsunami mode can be very significant.