[irised] Another example of filtering

Thu May 3 13:33:57 PDT 2007

Thanks John,

This makes sense and will help me view my seismograms in a whole new way.
It is exactly what I needed.

Chris

-----Original Message-----
From: irised-bounces at iris.washington.edu
[mailto:irised-bounces at iris.washington.edu] On Behalf Of John or Jan Lahr
Sent: Thursday, May 03, 2007 11:25 AM
To: irised at iris.washington.edu
Subject: RE: [irised] Another example of filtering

Hi Chris,

Good questions.

For the helicorder view AmaSeis only allows one filter to filter out 
high frequencies and one to filter out low frequencies.

For example, if you want to enhance the high frequency signals from 
local events on your helicorder record, you might filter out 
frequencies below 1.5 Hz, which leaves frequencies between 1.5 Hz and 
3 Hz. (Note that because we are only sampling at 6 Hz, the highest 
frequency that can be resolved is 3 Hz.)  The bottom of the 
helicorder record would indicate, in this case, "High pass cutoff 
period: 0.67 s"

To enhance the surface wave signals from distant earthquakes, one 
could filter out frequencies above 1/10 Hz (shorter than 10 s 
period).  The AS-1 system doesn't record signals with frequencies 
lower that 1/25 Hz (periods longer than 25 seconds), so I would also 
filter out frequencies lower than 1/25 Hz.  The bottom of the 
helicorder screen would indicate, in this case, "Band pass cutoff 
periods:  10 s 25 s"

When a highlighted period of time is first viewed in the event viewer 
window, no filtering is applied, even if the helicorder record is 
being viewed with a filter.  Data in the event viewer window can be 
filtered with the Control/Filter/Low pass.. and Control/Filter/High 
pass... options.  Each time one of those filters is selected the data 
is filtered again.  For example, to achieve a band pass from 10 s to 
20 s, one would first apply a high pass filter with a corner at 20 s 
and then apply a low pass filter with a corner a 10 s.

For my local event, I applied a high pass filter with the corner set 
to 1 Hz.  Then I applied the same filter twice more for a total of 3 
applications.  Each time the filter was applied it further reduced 
the low frequency waves and enhanced the local event.

For a given event, I often try out various filter settings.  If they 
don't workout, I use use the Control/Unfilter option to return to the 
original seismogram and then try again.

Note that in the helicorder view, along with filtering to reduce the 
unwanted frequencies, the gain will need to be increased so that the 
signal of interest is large enough to be clearly visible.  I usually 
increase the gain by about a factor of 10 when applying a filter.  In 
the event viewer window, the amplitudes are automatically adjusted to 
fill the screen.    This can make it a bit more difficult to adjust 
the filter settings;  the Control/Scale ... option can be used to 
turn off the automatic scaling feature.

Hope this helps.
John

At 06:37 AM 5/3/2007, you wrote:
>What would the low pass and high pass filter parameters be to highlight the
>higher frequency for the local events?
>
>Living near the New Madrid fault I was hoping to be able to see the micro
>earthquakes that occur there regularly but so far I have not successfully
>been able to identify them.
>
>I saw on your web site that the low pass would be 1.5mhz. Should I enable
>the high pass filter too? Or does the high pass filter not have an effect?
>
>Then do you also filter again after the event is extracted? How is the
>filter applied 3 times?
>
>
>Thanks!
>
>Chris Midden
>
>-----Original Message-----
>From: irised-bounces at iris.washington.edu
>[mailto:irised-bounces at iris.washington.edu] On Behalf Of John or Jan Lahr
>Sent: Thursday, May 03, 2007 1:30 AM
>To: irised at iris.washington.edu
>Subject: [irised] Another example of filtering
>
>A while back I showed how to set up a helicorder filter in AmaSeis so
>that the surface waves from distant earthquakes would be enhanced.
>(See:  http://jclahr.com/science/psn/as1/filtering/index.html)
>
>The other day I received an earthquake notification message* from the
>USGS about a 3.1 earthquake not too far from my station in Corvallis,
>Oregon.  Although I couldn't see anything on my helicorder record,
>especially with the filter I was using to enhance low frequencies, by
>switching to a filter that would bring out the high frequencies the
>earthquake was clearly visible above the background noise.  This
>example clearly demonstrates how the frequency content of seismic
>waves depends on the distance to the event.   Local and regional
>events will be rich in frequencies above 1 Hz, while distant events
>are often best identified by surface waves with periods of 18 to 20
seconds.
>
>See:  http://jclahr.com/science/psn/as1/filtering/high_pass.html
>
>*To subscribe to receive earthquake messages, go to this page:
>http://earthquake.usgs.gov/eqcenter/ens/
>
>Cheers,
>John
>
>
>
>#################################/ John C. Lahr
>################################/ Emeritus Seismologist
>###############################/ U.S. Geological Survey
>==========================/ Central Region Geologic Hazards Team
>#############################//#################################
>############################//##################################
>                               PO Box 548
>/###################################
>               Corvallis, Oregon 97339 /===============================
>               Phone: (541) 758-2699 /####################################
>                 Cell: (541) 740-4844
/#####################################
>               Fax: (413) 658-2699 /######################################
>                   johnjan at lahr.org
/#######################################
>                                   http://jclahr.com/science/
>
>
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