[irised] Another example of filtering

[Chris Midden]

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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