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 U.S.
Geological Survey, Menlo Park and USGS,
Pasadena, CA; the
Seismological Laboratory, University of California, Berkeley; the
Seismological Laboratory, California Institute of Technology; and the
Seismological Laboratory, University of Nevada, Reno.
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 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!)