Maps
Maps at various scales show the location of the event. Some maps for
locations in California and Nevada also show faults with fault names
revealed as you move the mouse over the fault (if you have Javascript
enabled in your browser). Links to additional location maps are often
listed farther down in the event description page.
Magnitude
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:
Magnitude type
|
Applicable magnitude range
|
Distance range
|
Comments
|
Duration (Md)
|
<4
|
0-400 km
|
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. |
Local (ML)
|
2-6
|
0-400 km
|
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)
|
5-8
|
20-180 degrees
|
A magnitude for distant earthquakes based on the amplitude
of Rayleigh surface waves measured at a period near 20
sec. |
Moment (Mw)
|
>3.5
|
all
|
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
slipped. |
Body (Mb)
|
4-7
|
16-100 degrees (only deep earthquakes)
|
Based on the amplitude of P body-waves. This scale is
most appropriate for deep-focus earthquakes. |
Date and Time
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.
We provide time in UTC (Coordinated
Universal Time). Seismologists use UTC to avoid confusion caused
by local time zones and daylight savings time.
On the individual event text pages, times are also provided in
local US timezones
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. Coordinates are
given in the WGS84
reference frame. The position uncertainty of the hypocenter location
varies from about 100 m horizontally and 300 meters vertically for
the best located events, those in the middle of densely spaced
seismograph networks, to 10s of kilometers for global events in many
parts of the world.
Depth
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.
Sometimes when depth is poorly contrained by available seismic data,
the location program will set the depth at a fixed value.
For example, 33 km is often used as a default depth for earthquakes
determined to be shallow, but whose depth is not satisfactorily
determined by the data, whereas default depths of 5 or 10 km are often used
in mid-continental areas and on mid-ocean ridges since earthquakes
in these areas are usually shallower than 33 km.
Region
The region name is an automatically generated name from the Flinn-Engdahl (F-E)
seismic and geographical regionalization scheme, proposed in 1965, defined in 1974 and
revised in 1995. The boundaries of these regions are defined at one-degree intervals
and therefore differ from irregular political boundaries. For example, F-E region 545 (Northern
Italy) also includes small parts of France, Switzerland, Austria and Slovenia and F-E region 493
(Chesapeake Bay Region) includes all of the State of Delaware, plus parts of the
District of Columbia, Maryland, New Jersey, Pennsylvania and Virginia. Beginning with January 2000,
the 1995 revision to the F-E code has been used in the QED and PDE
listings. This revision includes 28 additional regions, which were defined by
subdividing larger regions to provide better coverage for Northwest Africa, Southeast Asia and seismic
zones along oceanic ridges. (More info here.)
Distances
We provide distances and directions from nearby geographical
reference points to the earthquake. The reference points are
towns, cities, and major geographic features
(gazetteer info).
We realize that these distances are uncertain both because of the
errors inherent in locating earthquake (typically one or more kilometers) and because of
the impossibility of describing the location of a city by a single longitude-latitude entry
in a table. For places in the US, rather than rounding off distances to, say, the nearest 10 kilometers,
we chose to trust the user's common sense in interpreting the accuracy of these distances.
For places outside the US, distances are rounded depending on the
location uncertainty.
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 try to always provide at least one widely
recognized reference point in the list on the event page, even if the earthquake
occurs in a remote location.
Location Uncertainty
The horizontal and vertical uncertainties in an event's location
are based on values Ehro and Erzz described below.
We assign an "unknown" value if the contributing seismic
network does not supply the necessary information to generate
a uncertainty estimates.
The position uncertainty of the hypocenter location
varies from about 100 m horizontally and 300 meters vertically for
the best located events, those in the middle of densely spaced
seismograph networks, to 10s of kilometers for global events in many
parts of the world.
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.
Nst |
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. |
Nph |
Number of P and S arrival-time observations used to
compute the hypocenter location. Increased numbers of
arrival-time observations generally result in improved
earthquake locations. |
Dmin |
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.
|
Rmss |
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.
|
Erho |
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. |
Erzz |
The depth error, in km, defined as the largest projection
of the three principal errors on a vertical line. See
Erho |
Gp |
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
Erzz values. |
M-type |
Magnitude type, discussed at greater length above under
Magnitude |
Version |
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. |
Source
The organization supplying the information provided here.
Event ID
A combination of a 2-letter
Seismic Network Code
and a number assigned by the contributing seismic network.
Additional Information
Depending on the magnitude of the earthquake, additional information
is sometimes available. Location map links point to maps 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. |