PSN-L Email List Message
Subject: Re: microseisms
From: Brett Nordgren brett3nt@.............
Date: Thu, 22 Oct 2009 12:37:23 -0400
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.
Watch our wiggles
or watch some very very good wiggles
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