Introduction to the earthquake
The depth where the earthquake begins to rupture.
This depth may be relative to the WGS84 geoid,
mean sea-level, or the average elevation of the
seismic stations which provided arrival-time
data for the earthquake location. The choice of
reference depth is dependent on the method used
to locate the earthquake, which varies by seismic
network. The depth is the least-constrained parameter
in the earthquake location, and the error bars are generally
larger than the variation due to different depth determination methods.
Sometimes when depth is poorly constrained by available
seismic data, the location program will set the depth at a fixed value.
An earthquake begins to rupture at a hypocenter which is defined by a
position on the surface of the earth (epicenter) and a depth below this
point (focal depth). We provide the coordinates of the epicenter in units
of latitude and longitude. The latitude is the number of degrees north (N)
or south (S) of the equator and varies from 0 at the equator to 90 at the poles.
The longitude is the number of degrees east (E) or west (W) of the prime meridian
which runs through Greenwich, England. The longitude varies from 0 at Greenwich to
180 and the E or W shows the direction from Greenwich.