I was looking over the data I recorded overnight from the 6.5 quake off 
Japan.  In the process I applied a band-pass filter (0.4 - 2.0 hz) to 
see what, if anything, would show up.  What showed up were two strange 
"chirps" that are all but hidden in the noise.  I wouldn't have noticed 
them except for the fact that the first occurred a little before the 
arrival time of the S wave.  I don't believe that they have anything to 
do with the quake.  But I have no idea and no theory for what did cause 
them.

They are substantially identical, lasting for about a minute and 20 
seconds each, and decending from around 2 hz to .4 hz.  (Hmmmm.... just 
noticed the obvious correlation with my filter limits.)  Trouble is, 
without the filter, the signals are indistinguishable in the overall 
background.  But, I made sound files from both the filtered and 
unfiltered data, and the chirps are obvious in both versions and sound 
virtually identical.  So, the filter may be truncating the ends a 
little, but looks like it is more or less a lucky fit to the data.

I'd be interested in whatever speculative explanations anybody may have 
to offer.  My system is an SG sensor based on Larry's electronics board, 
and the data was recorded from the high frequency channel.  I've seen 
some similar artifacts in my data before, but didn't recognize how odd 
they were.

If anyone is sufficiently curious, I'd be happy to e-mail them copies of 
the filtered and unfiltered event file data, along with the .wav files I 
made.

Larry Conklin
lconklin@............
__________________________________________________________

Public Seismic Network Mailing List (PSNLIST)

To leave this list email PSNLIST-REQUEST@.............. with 
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.