Computers automatically update the WWW pages as more reliable
information about the earthquake is computed, particularly in
the first 10 minutes following the earthquake. The highest version
number is always considered authoritative.
Seismologists indicate the size of an earthquake in units of
magnitude. There are many different ways that magnitude is measured
from seismograms because each method only works over a limited
range of magnitudes and with different types of seismometers.
Some methods are based on body waves (which travel deep within
the structure of the earth), some based on surface waves (which
primarily travel along the uppermost layers of the earth), and
some based on completely different methodologies. However, all
of the methods are designed to agree well over the range of
magnitudes where they are reliable.
Earthquake magnitude is a logarithmic measure of earthquake
size. In simple terms, this means that at the same distance
from the earthquake, the shaking will be 10 times as large
during a magnitude 5 earthquake as during a magnitude 4 earthquake.
The total amount of energy released by the earthquake, however,
goes up by a factor of 32.
Magnitudes commonly used by seismic networks include:
Applicable magnitude range
||Based on the duration of shaking as measured
by the time decay of the amplitude of the seismogram.
Often used to compute magnitude from seismograms with
"clipped" waveforms due to limited dynamic recording range
of analog instrumentation, which makes it impossible to
measure peak amplitudes.
||The original magnitude relationship defined by Richter
and Gutenberg for local earthquakes in 1935. It is based
on the maximum amplitude of a seismogram recorded on a
Wood-Anderson torsion seismograph. Although these instruments
are no longer widely in use, Ml values are calculated
using modern instrumentation with appropriate adjustments.
Surface wave (Ms)
||A magnitude for distant earthquakes based on the amplitude
of Rayleigh surface waves measured at a period near 20
||Based on the moment of the earthquake, which is equal
to the rigidity of the earth times the average amount
of slip on the fault times the amount of fault area that
16-100 degrees (only deep earthquakes)
||Based on the amplitude of P body-waves. This scale is
most appropriate for deep-focus earthquakes.
Time and Date
We indicate the date and time when the earthquake initiates
rupture, which is known as the "origin" time. Note that large
earthquakes can continue rupturing for many 10's of seconds.
On the individual text page for each earthquake we provide time
in UTC (Coordinated
Universal Time). Seismologists use UTC to avoid confusion caused
by local time zones and daylight savings time. In some fields
the origin time has been converted to the "Local Time" in which
the WWW site operates. For example, the origin times reported
for California earthquakes on the recent
earthquakes website are given in Pacific Daylight or Standard
Time to facilitate use by California citizens.
We provide distances and directions from several nearby geographical
reference points to the earthquake. The reference points are
towns, cities, and major geographic features
(gazetteer info). If the computed
location is close to an operating quarry which is known to use
explosives in its operations, we indicate that the event may
be a quarry explosion. We always provide at least one widely
recognized reference point in the list, even if the earthquake
occurs in a remote location.
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.
The depth where the earthquake begins to rupture. This depth
may be relative to 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.
To assist non-seismologists in evaluating the reliability of
an earthquake location, we assign a "quality" to each location.
It is based on the values of Nph, Dmin, Erho, Erzz, and Rmss
(described below) for the computed earthquake location. The
quality is given as "excellent", "good", "fair", "poor", and
"unknown" reflecting each contributing seismic network's definition
of how the quality relates to the above values. For example,
parameters for an earthquake located by a global seismic network
might result in the assignment of an "excellent" quality, whereas
the same parameters would result in the assignment of a "poor"
quality had they been calculated for an earthquake located by
a regional seismic network monitoring an area the size of Los
Angeles. We assign an "unknown" value if the contributing seismic
network does not supply the necessary information to generate
Location Quality Parameters
These parameters provide information on the reliability of the
earthquake location. Zero values usually indicate that the contributing
seismic network did not supply the information.
||Number of seismic stations which reported P- and S-arrival
times for this earthquake. This number may be larger than
Nph if arrival times are rejected because the distance
to a seismic station exceeds the maximum allowable distance
or because the arrival-time observation is inconsistent
with the solution.
||Number of P and S arrival-time observations used to
compute the hypocenter location. Increased numbers of
arrival-time observations generally result in improved
||Horizontal distance from the epicenter to the nearest
station (in km). In general, the smaller this number,
the more reliable is the calculated depth of the earthquake.
||The root-mean-square (RMS) travel time residual, in
sec, using all weights. This parameter provides a measure
of the fit of the observed arrival times to the predicted
arrival times for this location. Smaller numbers reflect
a better fit of the data. The value is dependent on the
accuracy of the velocity model used to compute the earthquake
location, the quality weights assigned to the arrival
time data, and the procedure used to locate the earthquake.
||The horizontal location error, in km, defined as the
length of the largest projection of the three principal
errors on a horizontal plane. The principal errors are
the major axes of the error ellipsoid, and are mutually
perpendicular. Erho thus approximates the major axis of
the epicenter's error ellipse.
||The depth error, in km, defined as the largest projection
of the three principal errors on a vertical line. See
||The largest azimuthal gap between azimuthally adjacent
stations (in degrees). In general, the smaller this number,
the more reliable is the calculated horizontal position
of the earthquake. Earthquake locations in which the azimuthal
gap exceeds 180 degrees typically have large Erho and
A combination of a 2-letter
Seismic Network Code
and a number assigned by the contributing seismic network.
Depending on the magnitude of the earthquake, additional information
is sometimes available. "Map" points to a 2-degree map on which
the earthquake appears. "Waveforms" are commonly available for
a number of instruments which detected the event. If the event
is large enough, focal mechanisms, aftershock probabilities
and other kinds of information may also be available.