PSN-L Email List Message

Subject: Re: Detecting micro-earthquakes
From: Roger Sparks rsparks@..........
Date: Mon, 06 Feb 2006 06:23:36 -0800

Hi Jon,

I live about 150 km east of Seattle, Washington where we have a lot of 
quakes less than magnitude 3.0.   I wanted to see some of those quakes.

When I first began this hobby, I filtered hard to remove the high 
frequency noise but soon found that I was also removing the signals from 
the small local quakes.  That was because the small quakes were 
generating signals at a frequency above 1 Hz, up to 10 Hz and higher.

I found my seismic noise floor to be the ocean wave generated noise near 
0.2 Hz.   Therefore, I expected that the small local quakes would be 
seen as high frequency deviations from the fairly smooth curve generated 
by the 0.2 Hz background noise.   This expectation turned out to be correct.

I presently have my  computer screen filters set to high pass above 0.7 
Hz.  Using a sample rate of about 10.8 Hz, I can see quakes from 
magnitude 1.2 on up depending upon distance.   I also have the ability 
to low pass the data which is helpful on the larger teleseismic 
quakes.   Nearly all the teleseismic quakes also have energy at 
frequencies above my 0.7 Hz high pass filter cutoff so I can usually see 
them without the low pass filtering.

Your problem might be different from mine if your noise floor is not 
from the ocean wave background.   If your noise floor is from your 
electronics, seismometer hinge or wind, then your filtering needs would 
be different.  

You may have noticed that you can download the data from the quakes 
posted on PSN so that you can examine it carefully with Winquake.   You 
can  see what data operations have been used, what equipment the station 
is using, and examine the background noise for frequency.   This can be 
very helpful in comparing the results from your station with the results 
of other station operators.

I post on PSN from Ellensburg, Wash., and use the station signature "ebgz".

When I began this hobby, I spent a lot of time looking for known events 
in my data.   Now I mostly manually look for the event only if I think 
it is within my sensitivity and distance range but it is not obvious on 
my screen.

Best wishes,


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Subject: Detecting micro-earthquakes
From:    =?ISO-8859-1?Q?J=F3n_Fr=EDmann?= 
Date:    Sun, 05 Feb 2006 13:49:16 +0000

Hi all

I am wondering what is the best way for me to detect micro-earthquakes
that i might record, manually or automatic. Since i have to have a way
to detect them from the noise. I already know that i have to filter the
data down to at least 3Hz to get rid of the noise. But i am not sure how
to detect the wave pattern of such small earthquake (not smaller then
2Ml) after such filtering.

Thanks and regards.
-- Jón Frímann Jónsson

Public Seismic Network Mailing List (PSN-L)

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