PSN-L Email List Message

Subject: 6 second microseisms
From: sean@...........
Date: Wed, 6 Dec 2000 13:41:03 -0600 (CST)


Regarding the 6 second microseisms:

Nothing special should be needed to observe the microseisms in the
data from a long period instrument that is properly damped, has 
sufficient protection from noise, and sufficient gain to record
moderate quakes. The Ms 5.7 in Mexico on Dec. 04 had P-wave amplitudes
about 10x the (unfiltered) 6-second microseisms at the time (at 
St. Louis).

They are very clear and monochromatic in period, so much so that
I have used passive twin-T notch filters for years to remove them
from analog monitor records at the observatory. An analysis of the
average period at St. Louis showed 6.2 seconds to be the optimum 
period to design the filter for. The notch filter reduces the 
microseisms by over 40 db (factor of 100) to improve the readability
of the record. A notch here between the VBB-STM and the analog drum
makes them disappear for most of the time.

They are very clear sine waves on a long period record: simply count 
the peaks or number of sine waves between minute marks: sometimes there 
are 9, mostly 10, or sometimes 11.  A FFT of the LP data should show 
the microseism peak an average of about 40 db above the noise level 
at 20 seconds or so.


ps: Jeff,
BIGQUAKE is working; just received the report on the Mw 7.2 this AM
in Turkmenistan. A very nicely dispersed surface wave train arrived
here, starting at about 90 seconds period and rising to 20 seconds 
over about 15 minutes.
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Larry Cochrane <cochrane@..............>