The earthquakes displayed on these maps and associated web pages have been detected
and located by the combined seismographic networks of the U.S. Geological Survey National Seismic Network and University of Memphis CERI Seismic Network; the Saint Louis University Earthquake Center, the South Carolina Seismic Network and other participants in the Advanced National Seismic System Mid-America Region.
Seismic signals are telemetered in real-time by radio and land lines from over 600
remote seismic stations in the region to one or more of the four centers. Real-time computer systems at each
center continuously monitor the Earth for the occurrence of earthquakes.
When an earthquake occurs, seismic waves are created, which propagate away from the
focus or hypocenter. The fastest waves, the P-wave, travels outward at a speed of
about 3 to 5 miles/second. As the P-wave passes each seismic station, its arrival
time is detected and noted by the real-time computers. The computers use the list
of arrival times to determine the location of the earthquake. The location is typically available within a minute or less after the occurrence
of the earthquake.
Once the location of the earthquake is known, a signal is sent to the computer that
updates these web pages. Initially the magnitude may not be known, in which case the
updated maps may show the earthquake location with a white box with an X through it.
Only the index map and the zoomed-in or special maps displaying this new earthquake
are updated at this time.
The magnitude of an event is determined from the strength of the seismic waves detected
at each station. We use several different formulas to determine the magnitude. Most
formulas depend on a measure of the shear, or S-waves, which have the largest amplitude
and carry most of the seismic wave energy. S-waves travel more slowly than the P-waves
used to locate the earthquake, at about 2 to 3 miles/second, so a particular magnitude
may not be available until a few minutes after the earthquake.
Once a reliable magnitude is available, the relevant maps and text files are updated
to replace preliminary magnitude estimates. This process is typically completed within
about five minutes of the occurrence of the earthquake.
When a potentially significant earthquake occurs, as determined by pre-defined criteria,
the real-time computer alerts the duty seismologist by radio pager. The seismologist
logs-in to the real-time computer, reviews the automatically determined location and
magnitude, corrects any problems that are detected, and notifies the appropriate emergency
response agencies, if necessary. An earthquake of about magnitude 2.5 or larger typically
generates a human response. Following the review by the duty seismologist, the revised
location and magnitude information replaces the automatically-determined values on
this web site.
All reliably located earthquakes from the past six months are shown on these maps.
The index map and relevant zoomed-in maps are updated whenever new information about
an event becomes available. Approximately once an hour, the time stamp on all maps
containing earthquakes is updated, whether or not an earthquake has occurred. If the
time stamp on a particular map is much older than one hour, you should try to reload
the maps into your browser using the reload button (possibly holding down the shift
key at the same time). If a new map does not appear, there may be computer problems
at our end. (We hope not, but they do happen!)