PSN-L Email List Message

Subject: Re: Caution on low pass filtering emphasis
From: Brett Nordgren brett3nt@.............
Date: Wed, 30 Dec 2009 16:35:19 -0500

Hi Roger,

Thanks for the thoughts.  You got me thinking about how I have been 
approaching the filtering problem.

For me, what's appropriate in the way of filtering depends a lot on the 
nature of the quake.  For local ones, (to Los Angeles) such as the Baja, I 
open up the low pass to 20Hz.  Normally that would let through a huge 
amount of noise, but the signal was so large that noise was no issue and 
when it reached LA it still had considerable energy even above 1Hz.

With distant quakes, where the signal may be very weak, the only way to see 
anything is to use low pass filtering, around 0.08 Hz, to get rid of the 
microseisms and other background noise, even at the expense of losing some 
of the earthquake signal.  For large telesisms, where noise isn't so much 
of an issue, it would certainly make sense to raise the filter cutoff to 
let you see the important 1Hz components at full strength.

Matching the filter to the quake seems to work the best.


At 12:45 PM 12/30/2009 -0800, 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,

Watch our wiggles

or watch some very very good wiggles


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