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Welcome
to the NCSN* Spectrogram Displays
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Real-time
Views of Selected Spectrograms
(The
spectrograms are updated every 5 minutes.)
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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.
Some Illustrative Examples
* NCSN stands for Northern California Seismic Network
Please direct comments or questions about the Spectrogram pages to
Jim Luetgert, USGS.
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