Welcome
to the NCSN* Drum Recorder Displays
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Real-time
Views of Selected Seismograms
(The
seismograms are updated every 5 minutes.)
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What's a Drum Recorder?
Drum recorders, like the ones pictured here, draw a seismogram
on a piece of paper wrapped around a slowly revolving drum.
As the drum revolves, the pen shifts across it, making a big
spiral or helical record that eventually fills the entire page.
When the paper is removed and laid flat, the record appears
to be many horizontal lines, like lines in a book. These mechanical
recorders, popular 30 years ago, have largely been replaced
by computers, which digitize the data (typically at 100 samples
per second) and store it in digital form. The digital data can
then be displayed in a variety of ways by the computer. Here,
we've displayed the data to look like the old-fashioned paper
records that drum recorders produced. Pictured here are Helicorders®,
a common type of drum recorder, manufactured by Geotech Instruments, LLC.
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How to read the display
The seismograms displayed are from a few of the seismograph stations
routinely recorded by the Northern California Seismograph Network.
Each is a record of how the ground moved at a particular seismograph
station in Northern California during a 24-hour period. The plots
are "magnified" according to the level of background or
ambient seismic noise, which is generated by wind, cultural disturbances
and oceanic microseisms. Thus, some sites appear noisier than others.
The seismogram is "read" like a book, from left to right and top
to bottom (this is the direction that time increases). As with a
book, the right end of any horizontal line "connects" with the left
end of the line below it. Each line represents 15 minutes of data;
four lines per hour. The colors of the horizontal lines have no
particular significance; they are used to make it easier to distinguish
lines from one another and to make it easier to recognize at which
quarter-hour of data we are looking. The vertical lines are not
part of the seismogram. They were added to indicate equal intervals
of time. Time is indicated on the left in local Pacific time, and
on the right end in Universal (or Greenwich) time.
Interpretation
When an earthquake occurs the seismogram 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. The height of the recorded waves on the seismogram
(wave amplitude) is a greatly magnified representation of the actual
ground motion. A recording of an earthquake has recognizable characteristics.
Typically, one can recognize the arrival of different wave types:
P (the fastest traveling waves), S (shear waves), and surface waves.
On these seismograms you may see local earthquakes in Northern California
and earthquakes elsewhere in the world.
Not all the wiggles seen on the seismograms are due to earthquakes.
Anything that produces ground vibrations could be recorded, for
example a car that passes by the seismometer (this is why we try
to locate most of our seismometers well away from roads). Since
the electrical signals from the seismometers are typically transmitted
to the USGS by microwave or over telephone wires, electrical and
radio noise on the transmission path may also show up on the seismogram.
Such interference is usually easy to distinguish from earthquake-generated
signals because it is often erratic or "spikey" in appearance.
Some
Illustrative Examples
How the data channels are named
Each data channel has a three part name such as MSL VHZ NC. Read
more about what these codes mean.
* NCSN stands for Northern California Seismic
Network
Please direct comments or questions about the Drum Recorder pages
to Jim Luetgert, USGS.
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