PSN-L Email List Message

Subject: Re: Seismic Waves motion effects of Earths Rotation
From: Brett Nordgren brett3nt@.............
Date: Sat, 05 Oct 2013 12:59:11 -0400


Yes it does, but the effects are very subtle. They can be seen with a 
good instrument following a large quake, like >M 8.0
Those quakes excite the earth's free oscillation modes, causing the 
earth to ring like a bell, except that the periods of the many 
vibration modes tend to range from 2000 to about 200 seconds.  After 
a really big quake, the earth will ring for weeks.

If you make a spectrum from a long enough recording there will be 
clear peaks at each of the various modes' frequencies.  Several 
things effect those frequencies a little, things like the fact that 
the earth isn't spherical, but slightly elipsoidal, and it isn't 
completely smooth inside, but has regions and directions of faster 
and slower wave velocity.  One of the other things that affects them 
is the Coriolis effect from the earth's rotation.  The result is 
that, if your measurements are good enough, some of the peaks will be 
split into two, (or more?) very close peaks.  From observations of 
the splitting, after removing the effects of the earth's rotation and 
other known factors, computations can be made which help provide more 
insight into the structure deep inside the earth.

Following M7+ quakes, we regularly see earth modes down to about 1 
milliHertz, though we're still looking to see the splitting 
effect.  Another 'great' quake would give much better data for trying that.


At 09:44 AM 10/5/2013, you wrote:
>Does anyone know whether or not the Earths Rotation
>Affects the behavior of Seismic waves as they
>travel through the Earth's lithosphere ?_


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