[irised] Another example of filtering

[John or Jan Lahr]

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
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                              PO Box 548  /###################################
              Corvallis, Oregon 97339 /===============================
              Phone: (541) 758-2699 /####################################
                Cell: (541) 740-4844 /#####################################
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                  johnjan at lahr.org /#######################################
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