Helicorder Description

A complete list of stations operated by CERI is available here.

These displays digitally simulate actual seismograms that are routinely recorded at CERI. The seismograms show a record of how the ground moved at a particular seismograph station in the Southeastern U.S. 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 Eastern time and at the right end of some lines in Universal (or Greenwich) time.

Each channel is designated by three groups of letters, sometimes referred to as "scn": Station, Channel, and Network. The Station (e.g. BRBC) identifies geographically which seismic station generated the data. Because a particular station may have multiple sensors and sensor may have multiple channels, the Channel code is used to identify a particular data stream from the station (e.g. HHZ is a broadband, high sample rate, high gain, vertical component data stream). While network seismologists attempt to uniquely name their stations, this is not always possible. To ensure the uniqueness of the data stream, a network code is also used that identifies which seismic network is responsible for the particular station (e.g. SE is the Southeast Seismic Network operated by CERI, TVA, VA Tech, Auburn, Charleston Southern, Delaware GS, Georgia Tech, Maryland GS, Univ of FL, Univ. of SC, VA Dept. of Mines, and Westinghouse Savannah River Co.) US is the U.S. National Seismic Network operated by the U.S. Geological Survey). A list of recognized network codes is online at the IRIS Data Management Center.

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 Southeastern U.S. or even earthquakes elsewhere in the world. Almost any earthquake in the World having a magnitude greater than 6.5 will be seen on these 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 Memphis over FM radio electro-magnetic interference may 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. Telemetry dropouts may cause sustained periods of high amplitude noise.

Other interesting signals may be recorded. For example trains or man-made blasts (for example the quarry and mining activity in Central and Eastern Tennessee regularly seen on most stations).

This description was plagiarized from Pete Lombard at UW Seattle and modified for CERI. TYP.