# Technical Terms used on Event Pages

## 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. Preliminary magnitudes based on incomplete but available data are sometimes estimated and reported. For example, the Tsunami Centers will calculate a preliminary magnitude and location for an event as soon as sufficient data is available to make an estimate. In this case, time is of the essence in order to broadcast a warning if tsunami waves are likely to be generated by the event. Such preliminary magnitudes, which may be off by one-half magnitude unit or more, are sufficient for the purpose at hand, and are superseded by more exact estimates of magnitude as more data become available. 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.
Energy (Me) > 3.5 all Based on the amount of recorded seismic energy radiated by the earthquake.
Moment (Mi) 5-8 all Based on the integral of the first few seconds of P wave on broadband instruments (Tsuboi method).
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.
Surface wave (MLg) 5-8 all A magnitude for distant earthquakes based on the amplitude of the Lg surface waves.

## Event 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 pages, times are also provided for the time at the epicenter, and your local time based on the time your computer is set.

## 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 constrained 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.

## Nearby Cities

We provide distances and directions from nearby geographical reference points to the earthquake. The reference points are towns, cities, and major geographic features derived from US Census data, such as from http://www.census.gov/geo/www/gazetteer/places2k.html. International places were gathered from a specially created USGS catalog. Selected places were based on minimum population values that were specified for each particular region.

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 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. The horizontal and vertical uncertainties in an event's 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. We assign an "unknown" value if the contributing seismic network does not supply the necessary information to generate uncertainty estimates.

## Depth Uncertainty

The depth error, in km, defined as the largest projection of the three principal errors on a vertical line.

## Azimuthal Gap

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 location and depth uncertainties.

## Number of Stations Used

Number of seismic stations which reported P- and S-arrival times for this earthquake. This number may be larger than the Number of Phases Used 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 Phases Used

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.

## Minimum Distance

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.

## Travel Time Residual

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.

## Event ID

A combination of a 2-letter Seismic Network Code and a number assigned by the contributing seismic network.