What Happens When an Earthquake Occurs?

The earthquakes displayed on these maps and associated web pages have been detected and located by the combined seismographic networks of the Advanced National Seismic System (ANSS).

Seismic signals are telemetered in real-time by radio and land lines from over remote seismic stations in the various regions to a network center. Real-time computer systems at each center continuously monitor the incoming signals 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 message is sent to the computers that update 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. Several different formulas are used 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 3.5 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 last seven days 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!)