USGS Experimental Spectrograms, page 1

While trolling through seismograph readings last month, I stumbled upon USGS�s experimental spectrograms. Early on, there was very little data, usually delayed by at least a week and nothing on interpretation.

The USGS has made good progress, as the graphs are getting closer to real-time. A new feature is an explanation of the graphs and how to read them. It is obviously a work in progress as half the links are dead. The pages they do have completed have answered several of my questions about reading the graphs and I think these graphs may be of some assistance for the WWS and Yellowstone threads. While these instruments are looking at low-end frequencies (0 to 10 Hz), there could be some delayed interplay between HAARP and these instruments.

www.geophys.washington.edu...

How to read the spectograms.

quake.usgs.gov...

What's a Spectrogram?
A spectrogram is a means for viewing the frequency content of a seismogram as it changes with time. Once each minute, we calculate the frequency spectrum of the seismogram between 0 and 10 Hz. The spectral amplitude values are converted to color with deep blues representing low values, ranging through greens and yellows to deep red for the high values. Each minute is thus displayed as a horizontal colored line representing by its changing color the differences in shaking intensity at different frequencies from 0 to 10 Hz. By plotting these horizontal lines adjacent to one another as they are calculated we can see a time sequence of the frequency spectrum.

How to read the display
The spectrograms displayed are from a few of the seismograph stations routinely recorded by the Northern California Seismograph Network. The spectrograms show a record of the frequency content of ground motion at a particular seismograph station in Northern California during a 24-hour period. The spectrogram is "read" from top to bottom (this is the direction that time increases). Each horizontal line represents in color the amount of ground motion at frequencies ranging from 0 to 10 Hz. Each horizontal line represents the frequency spectrum for 1 minute of data. The corresponding data trace is plotted along the right-hand axis. The vertical lines are not part of the spectrogram but are present to indicate equal intervals of frequency. Time is indicated at the left side of the plot in local Pacific Time and at the right side in Universal (or Greenwich) time.

Interpretation
When an earthquake occurs the spectrogram will show ground motions that typically last from several tens of seconds to many minutes depending on the size of the earthquake and the sensitivity of the seismograph. On these spectrograms you may see local earthquakes in Northern California and earthquakes elsewhere in the world. Almost any earthquake in the world having a magnitude greater than 5.5 will be seen on these spectrograms.

Remember, these are experimental. On some displays you may see tracer lines like these. I�m not sure what causes them but they are not related to the data.

There are no examples of earthquakes on the info page. This is the best example of a micro-quake I could find.

There is also no example of a harmonic tremor. This the best example I could find of a harmonic tremor, except its too brief to be truly harmonic. It gives you an idea though.