Glossary for Recent Earthquakes Pages

Recent Earthquakes

Glossary of Terms

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.

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.

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.

Distance from

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.

Coordinates

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.

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.

Quality

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 a quality.

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.

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.

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. "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.

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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.