PSN-L Email List Message

Subject: Re: Pendulum Q
From: Larry Cochrane lcochrane@..............
Date: Sat, 20 Jan 2007 22:52:23 -0800

 From Randall. -Larry

Notice, Bob, what I said about the matter.  If the 'purity' of the events is not
what you are most interested in, then give it a try.  The 'carrier' (a misnomer)
you mention is not 20 Hz but 50 mHz (much lower frequency than body wave
values).  There is useful information concerning the energy of the earthquake by
doing the power spectral density (corrected for the transfer function of the
instrument; i.e., the transient, or 'ringing' features)-- independent of the
'events'.  My guess is that most people don't really use the details of the event
information quantitatively anyway.  You go find out where the earthquake occurred
by consulting USGS webpages after you've seen it on your seismograph.  It is not
the time difference between P and S events that serves as the primary means for
locating an earthquake by USGS anyway.  The errors of a calculation based on the
difference in speeds of the longitudinal and transverse components are much worse
than a calculation using only the P event from two or more instruments
geographically separated in azimuth by a large amount relative to the
earthquake.  In other words, USGS uses triangulation as the means for localizing
earthquakes.  Yes it is nice to observe the S wave delay relative to the P wave
as a rough measure of how far away the earthquake was.  I figured, however, that
some of you would like to just 'see' the surface waves from weaker, distant
earthquakes.  I predict that by the means I mentioned you will see some of them
that otherwise would go unnoticed.  Before anybody makes sweeping generalizations
about my heresy, I suggest that you first take a look at the graphs that I have
posted on my webpage at    It shows
that the undamped pendulum (in spite of ringing, following arrival of each of the
P and S waves for the case shown-- nevertheless contains decent information
concerning the spectral features of the most important piece of the record; i.e.,
the surface waves, whose frequencies are distributed between approximately 7 s
and 70 s.  The very longest periods differ between the VolksMeter record and the
undamped pendulum record (PSD's), but this might be due in part to the different
locations of the instruments.  The undamped instrument was on a home-built 'pier'
in my home basement, whereas the VolksMeter was on the concrete-slab floor of the
Willett Science Center here at Mercer University (about two miles from my
house).  The building may be hindered with respect to the tilts occurring at the
longest periods.  For you 'purists', note the fact that the first-disturbance
arrival times of both P and S are abundantly clear, including the delta time
between them--closely correlated with the 'proper' instrument, our VolksMeter.  I
expect similar results with an undamped, tuned garden-gate pendulum with a period
5 times longer at 20 s--the big difference being that the surface waves that
characterize the teleseismic earthquake record will be enhanced significantly
because their frequencies are close to the high-Q resonance of the instrument.



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