The dots presented in this map represent earthquake location calculated by
the Hawaiian Volcano Observatory and its partners.
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.
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.