[irised] Filtering - November 14, 2007 M7.7 N. Chile Earthquake

[L Braile]

Just after the November 14, 2007 M7.7 N. Chile Earthquake I noticed an 
unusual high-frequency arrival overlapping the long period surface waves 
on the seismogram for this event that was recorded on the WLIN AS-1 
station.  I high-pass filtered the seismogram and assumed that the 
resulting arrival was from an aftershock about 29 minutes after the main 
shock.  I checked with the USGS earthquake list for the next few days 
and didn't find an aftershock at the appropriate time so I looked for 
another explanation for the high frequency arrival.  I gathered some 
additional data and eventually figured out the clues to the explanation 
for the mystery arrival - it's quite surprising. 

All this led me to write up some information on filtering seismograms.  
The discussion of examples is located at: 
http://web.ics.purdue.edu/~braile/edumod/as1lessons/filtering/filtering.htm.  
An MS Word version and a pdf version (easier printing) is also 
available.  At the end of the write-up, there are several suggested 
activities for further exploration.

The filtering write-up is also linked from the list of additional AS-1 
activities and resources at: 
http://web.ics.purdue.edu/~braile/edumod/as1lessons/as1lessons.htm

Have fun trying out the seismogram filtering activities (all you need is 
AmaSeis installed on your computer)!  If you have a seismogram of this 
earthquake from your station, also try filtering it to find the mystery 
arrival (and some aftershocks).

Happy New Year!

-Larry Braile

-- 
Lawrence W. Braile, Professor and Department Head
Department of Earth and Atmospheric Sciences
550 Stadium Mall Drive
Purdue University
West Lafayette, IN 47907-2051
(765) 494-5979 (O), (765) 496-1210 (Fax)
E-mail: braile at purdue.edu <mailto:braile at purdue.edu>
Web page: http://web.ics.purdue.edu/~braile 
<http://web.ics.purdue.edu/%7Ebraile>
Departmental web page: http://www.purdue.edu/eas/ 
<http://www.purdue.edu/eas/>
SAGE web page: http://www.sage.lanl.gov/ <http://www.sage.lanl.gov/>
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