Larry,

At 10:11 AM 10/22/2009 -0400, you wrote:


>The other thing I would love to hear an explanation for is why the 
>microseism noise is so narrow band.  When it is strong, it looks virtually 
>sinusoidal on the monitor, and an FFT of the data shows a very narrow 
>peak.  Does anyone know why that is?

This still appears to be an active subject among seismologists, but they 
seem to generally agree on the basics.  Low frequency microseisms 
(generally 8-12 sec) are caused from pressure variations on the ocean floor 
from large ocean swells.  These then get transmitted over great distances 
through the crust as surface waves.  In the Northern hemisphere there are 
two places where these effects appear mostly to originate, one in the 
Northeastern Pacific off the Alaska and Canada coasts, and one in the 
Atlantic, between Newfoundland and Greenland.  It sounds to me like those 
two regions are somehow acting as "resonators" for the ocean wave energy 
generated by large ocean storms.

Since ocean swells are sort of sinusoidal, often with a single predominant 
frequency, I'm not surprised that their microseisms would be too, and then 
like other surface waves, as they travel through the crust, any higher 
frequency harmonics will disappear.

One popular theory has it that the double frequency (4-8) second waves 
result from the interaction of the main swell with a rebounding swell, 
generated when it hits a coast.  The resulting vibration will be at twice 
the frequency of the original and similarly, somewhat sinusoidal.

Another thing to watch when looking at microseisms is where any Low Pass 
filtering is set.  In order for a wave to be non-sinusoidal it must contain 
harmonic frequencies which could get removed by filtering too low.

Regards,
Brett


Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


__________________________________________________________

Public Seismic Network Mailing List (PSN-L)