1994 DEC23 Garvin County, Oklahoma Earthquake

Origin time 21:56:16.89 UTC/GMT, Latitude 34.671degrees N, Longitude 97.463degrees W

Magnitude(s): mbLg 2.2

SHORT PERIOD VERTICAL SEISMOGRAM AND SPECTROGRAM OF THE 1994 DEC23 OKLAHOMA EARTHQUAKEjl 

The above seismogram and spectrogram are from short period vertical earth
velocity measured at the foot of Bald Mountain on Bald Mountain Ranch near
Vivian, Mc Intosh County Oklahoma. Ranch owner Boots Adams kindly allows the
location of this station on his ranch. The seismometer is a Geotech S-13 in
a shallow tank vault. Analog (continuously varying, rather than in steps
like digital signals) are transmitted from an antenna on top of Bald Mountain
to the OGS Observatory near Leonard. In the walk in vault, this signal is
digitized 40 times per second.

The seismogram (black and white) shows the earth velocity over a period of
140 seconds (scale at bottom). The spectrogram (color) shows the variation
in power of the signal with frequency (Hertz scale on left) over the 140
seconds. 

P and S are marked, as is a steady 11.1 Hertz vibration from a gas compressor
some distance from the seismometer.

SHORT PERIOD VERTICAL SEISMOGRAM AND SPECTROGRAM OF 1994 DEC23 OKLAHOMA EARTHQUAKE, BAND REJECT FILTERED 11.0 to 11.2 Hertzjl 

These are the same as the first seismogram and spectrogram, except that a 
simple digital band reject filter is attenuating frequencies from 11.0 to 11.2 
Hertz. The Garvin County earthquake is seen more clearly, the earth noise is
reduced. The reduction makes the seismogram to the left of the P arrival
smaller (narrower), and erases the 11.1 Hertz line from the spectrogram.

SHORT PERIOD VERTICAL SEISMOGRAM AND SPECTROGRAM OF THE 1994 DEC23 EARTHQUAKE WITH ADDITIONAL 11.1 HERTZ NOISEjl 

This seismogram and spectrogram show the same signal as the above figures,
but a large artificial computer generated 11.1 Hertz narrow band noise (in
the form of a sine wave) has been added. By comparing with the first two 
figures, the P and S arrivals from the earthquake are barely visible, but 
their arrival times could not be measured.

This figure simulates a very small earthquake made all but undetectable by
large narrow band earth noise.

SHORT PERIOD VERTICAL SEISMOGRAM AND SPECTROGRAM OF THE 1994 DEC23 EARTHQUAKE WITH 11.1 HERTZ NOISE ADDED THAN REMOVED BY BAND REJECT FILTERjl 

In this figure the 11.0 to 11.2 digital band reject filter (Butterworth four
pole) has been applied to nearly eliminate the large artificial noise. The
blips from zero to 15 seconds, are artifacts from the start of filtering
such a large noise. Otherwise the earthquake is clearly seen, and the P and
S arrivals are measurable.

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