PSN-L Email List Message

Subject: Re: Caution on low pass filtering emphasis
From: George Bush ke6pxp@.......
Date: Wed, 30 Dec 2009 13:46:22 -0800


Thanks for this reminder. I try to not filter, as a general rule and 
only resort to it when I can't see the signal without filtering.


At 12:45 PM 12/30/2009, you wrote:
>Hello All,
>I sometimes think that we amateur seismologist over-emphasize the 
>need for low pass filtering, with the unexpected result that we 
>destroy the fidelity of the earthquake signal.  To illustrate what 
>we might be losing, I posted my recording of this mornings Baja 
>quake using both raw data and HIGH PASS filtered data in the 
>WinQuake volume format.  The high pass was done using WinQuake 
>filters set at 1 Hz, two pole.
>Here is a link to the posting: 
>The trace on the bottom is the raw data.
>As you can see, the quake was about 1677 km from this station but 
>there was still lots of high frequency (> 1 Hz) in the data.   If 
>filtered low pass below 1 Hz, this information would be compromised, 
>and as you can see, there is a lot of information to be lost.
>Something to think about,
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George Bush
Sea Ranch, CA, USA
38.73775N, 123.48882W 


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