Description
These displays digitally simulate actual seismograms that are routinely recorded at the University of Washington earthquake laboratory. The seismograms show a record of how the ground moved at a particular seismograph station in the Pacific Northwest during a 24-hour period. 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. The colors of the horizontal lines have no particular significance.

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 and at the right end of some lines in Universal (or Greenwich) time.

When an earthquake occurs the seismogram will show ground motion fluctuations that typically last from several tens of seconds to many minutes depending on the size of the earthquake. The height of the recorded waves on the seismogram (wave amplitude) is a greatly magnified representation of the actual ground motion. The magnification is roughly 100,000 times or more. A recording of an earthquake has recognizable characteristics. Typically, once can recognize the arrival of different wave types: P (the fastest traveling waves), S, and Surface waves.

On these seismograms you may see local earthquakes in the Pacific Northwest 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. For distant earthquakes be sure to look at one of the "broad band" seismograms. Broad-band seismic instruments are particularly sensitive to 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 on broad band seismograms.

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), or man-made blasts (for example the quarry blasts near Centralia, WA each day).

For an annotated seismogram see XXXXX.

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#### Data Sources:

Northern California - USGS = U.S. Geological Survey, Menlo Park
Northern California - UCB = University of California, Berkeley
Southern California - USGS = U.S. Geological Survey, Pasadena
Southern California - Caltech = California Institute of Technology
Nevada - UNR = University of Nevada, Reno
Washington, Oregon - UW = University of Washington
Hawaii - HVO = U.S. Geological Survey, Hawaii Volcano Observatory