December 2004 changes - Stations have been filtered and decimated to eliminate aliasing. This produces an apparent (and sometimes, real) gain change. We also do adjust the gains from time to time on the webicorders (and more rarely at the stations).
The vertical lines are not part of the seismogram but are present to indicate equal intervals of time. Each vertical line is equal to one minute. Time is indicated at the left end of some of the lines of the plot in local Pacific time and at the right end in Universal (or Greenwich) time. The horizontal lines are colored in order to differentiate one line from the ones above and below it.
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. The magnification is 50,000 times or more depending on the site. 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 the Pacific Northwest and earthquakes elsewhere in the world. Almost any earthquake in the world having a magnitude greater than 5.5 will be seen on these seismograms.
For distant earthquakes be sure to look at one of the stations like LON (Longmire) that has a seismograph that is particularly sensitive to the low-frequency ground motions generated by large earthquakes. Some types of seismic waves (for example surface waves) from a distant earthquake will only be visible seismograms from instruments "tuned" to the low-frequency ground motions.
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 typcally transmitted to the University of Washington over telephone wires any electrical noise on the telephone lines will also show up on the seismogram. Such noise is usually easy to distinguish from earthquake generated signals because the the noise is often "spikey" in appearance.
Other interesting signals may be recorded. For example rockfalls (see a station near Mount St Helens), icequakes in glaciers (see station RCS), ground motion induced by high wind conditions or man-made blasts (for example, the quarry blasts near Centralia, WA each day).