Latest Earthquakes - 10 of 13
Latest Earthquakes FAQs - 13 Found
Seismic signals are transmitted in real-time by radio, satellite, telemetry, and land lines from remote seismic stations 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 system triggers the website to update the earthquake map and create an "event page" for the new earthquake. Initially the magnitude may not be known, in which case the updated maps may show the earthquake location with a white box. 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. 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 the web site.