From: Brett Nordgren 
To: psnlist 
Sent: Sun, 6 Oct 2013 19:23
Subject: Why we like verticals


Hi All,

Dave N. has been playing with an old Guralp 24-bit digitizer.
Yesterday he hooked it up to his Trillium 120 to do some testing
and today he recorded a quake that he said was a 4-something
in Mexico, though I don't seem to find it on the USGS site.

http://bnordgren.org/seismo/T120ZNE.JPG

It gives one of the best illustrations I have seen recently of why we
prefer verticals for quake watching.  The top trace is the vertical
axis and the other two are horizontals from the same bedrock-mounted
instrument.  The quake is difficult to see above the noise on the
horizontals though it shows up nicely in the vertical channel.  From
what I can see of the LA weather I think the wind may have been
blowing pretty strongly.

Hi Brett,

     It would be great if you could put some time markers on the
trace, please ? It is difficult to get a good idea of the wave periods
without this. Maybe just tell us the total trace time ?
     Could we have a 'good' trace from a low wind / low sea day, for
comparison, please ?
     Did Dave leave the door to the seismometer vault open, or
something ? I get signals like this if I leave the insulating cover off
my Lehman / don't seal the case to the floor with foam tape.

    Regards,

    Chris Chapman

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