The vertical lines are not part of the seismogram but are present to indicate equal intervals of time. Time is indicated at the left end of some of the lines in local Pacific time (PST or PDT) and at the right end of some lines in Universal (or Greenwich) time (UTC).
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. Graphical example of seismogram display..
On these seismograms you may see local earthquakes in southern California or even earthquakes elsewhere in the world. Almost any earthquake in the World having a magnitude greater than 5.5 will be seen on these seismograms. Distant earthquakes will typically have long period waves lasting tens of minutes or more.
Not all the wiggles seen on the seismograms are due to earthquakes. Anything that produces ground vibrations could be recorded, for example passing cars or trains, sonic booms, thunder and ocean waves (this is why we try to locate most of our seismometers well away from noise sources). Electronic interference and mechanical problems with the instruments may also produce signals.
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