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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