1996 AUG03 Pushmataha County, Oklahoma Earthquake

Origin time 06:25:20.57 UTC/GMT, Latitude 34.524degrees N, Longitude 95.343degrees W

Magnitude(s): mbLg=1.8, MDUR=1.8

SHORT PERIOD VERTICAL SEISMOGRAM AND SPECTROGRAM OF THE 1996 AUG 03 OKLAHOMA EARTHQUAKEjl

The top figure (black) is a seismogram showing the vertical short period ground
velocity recorded near Leonard by a Geotech GS-13 short period seismometer in
the underground walk in vault. The numbers below the seismogram are the
number of seconds after the earthquake occurred 159 kilometers away in
Pushmataha County. The P and S arrivals (not labelled) occur at about 25 and
46 seconds.

The lower figure (color) is a spectrogram of the seismogram above. It shows
the distribution of frequencies of earth motion. The left hand scale (0-20)
is frequency in Hertz. The bottom scale is the same as the time scale on the
seismogram above. The colors indicate power at that frequency from blue (least)
through green and red to purple (highest).

Frequencies below one Hertz are attenuated by the GS-13 seismometer. The
signal was only digitized (sampled) at 40 samples/second. This makes it
impossible to see frequencies above 20 Hertz. Twenty Hertz (half the sample
frequency) is called the Nyquist frequency. Frequencies above this are only
sampled at random spots, and they would cause alias signals to appear at low
frequencies, when the 40 numbers per second were turned into visible plotted
seismograms or spectrograms. To avoid aliasing, digital signal acquisition
systems have anti-aliasing filters to cut off signals above the Nyquist
frequency. In this system the anti-alias frequency filter is at 16 Hertz,
which is slightly below Nyquist.

The seismometer and anti-alias filter cause the power to fall to zero 
below about 3 Hertz and above about 17 Hertz. Between these limits there 
is a wide frequency content in both P and S pulses. S has a higher power 
than P in general, and the S power is concentrated at lower frequencies
than the power in the P pulse.


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