Seismic signals are telemetered in real-time by radio and land lines from over 70 seismic stations across the islands of Hawai`i. Real-time computer systems continuously monitor the seismic 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 couple minutes after the occurrence of the earthquake.

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.

Earthquakes are routinely reviewed by a seismic data analyst and updates posted to this web site. Due to limitations of our automatic processing programs and manpower constraints, we will endeavor to maintain a web display of earthquakes with magnitude of 1.7 and greater. Automatically processed earthquakes that drop below magnitude 1.7 upon review will be deleted from the display, and missing earthquakes determined to be at least magnitude 1.7 will be added to the web page, provided they fall within the confines of our maps.