## Magnitude

Earthquake magnitude is a measure of the size of an earthquake at its source. It is a logarithmic measure. At the same distance from the earthquake, the amplitude of the seismic waves from which the magnitude is determined are approximately 10 times as large during a magnitude 5 earthquake as during a magnitude 4 earthquake. The total amount of energy released by the earthquake usually goes up by a larger factor: for many commonly used magnitude types, the total energy of an average earthquake goes up by a factor of approximately 32 for each unit increase in magnitude. There are various ways that magnitude may be calculated from seismograms. Different methods are effective for different sizes of earthquakes and different distances between the earthquake source and the recording station. The various magnitude types are generally defined so as to yield magnitude values that agree to within a few-tenths of a magnitude-unit for earthquakes in a middle range of recorded-earthquake sizes, but the various magnitude-types may have values that differ by more than a magnitude-unit for very large and very small earthquakes as well as for some specific classes of seismic source. This is because earthquakes are commonly complex events that release energy over a wide range of frequencies and at varying amounts as the faulting or rupture process occurs. The various types of magnitude measure different aspects of the seismic radiation (e.g., low-frequency energy vs. high-frequency energy). The relationship among values of different magnitude types that are assigned to a particular seismic event may enable the seismologist to better understand the processes at the focus of the seismic event. The various magnitude-types are not all available at the same time for a particular earthquake. Preliminary magnitudes based on incomplete but rapidly-available data are sometimes estimated and reported. For example, the Tsunami Warning Centers will calculate a preliminary magnitude and location for an event as soon as sufficient data are 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 Such preliminary magnitudes are superseded by improved estimates of magnitude as more data become available. For large earthquakes of the present era, the magnitude that is ultimately selected as the preferred magnitude for reporting to the public is commonly a moment magnitude that is based on the scalar seismic-moment of an earthquake determined by calculation of the seismic moment-tensor that best accounts for the character of the seismic waves generated by the earthquake. The scalar seismic-moment, a parameter of the seismic moment-tensor, can also be estimated via the multiplicative product rigidity of faulted rock x area of fault rupture x average fault displacement during the earthquake.

## Magnitudes commonly used by seismic networks include:

Magnitude type Magnitude Range Distance Range Comments
Duration (Md or 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 Ml, or ml) 2 - 7.5 0 - 600 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.
Short-period surface wave (mb_Lg, mb_lg, or MLg) 3.5 - 7 150 – 1100 km A magnitude for regional earthquakes based on the amplitude of the Lg surface waves as recorded on short-period instruments.
Short-period body wave (mb) 4 - 7 15 - 100 degrees Based on the amplitude of P body-waves as recorded on short-period instruments that are most sensitive to waves with a period of about 1 s.
Twenty-second surface wave (Ms or Ms_20) 5 - 8.5 20 - 160 degrees A magnitude for distant earthquakes based on the amplitude of Rayleigh surface waves measured at a period near 20 sec.
Moment (generic notation Mw or mw. Specific types denoted Mwb or mwb, Mwc or mwc, Mwr or mwr, and Mww or mww) > 3.5 all Based on the scalar seismic-moment of the earthquake, as determined by a moment-tensor inversion. Mwb – Mw based on moment tensor inversion of long-period (~10 - 100 s) body-waves (P- and SH). Mwc -- Moment magnitude derived from a centroid moment tensor inversion of intermediate- and long-period body- and surface-waves. Mwr -- Moment magnitude derived from a moment tensor inversion of complete waveforms at regional distances (less than ~13 degrees). Sometimes called RMT. Mww -- Moment magnitude derived from a centroid moment tensor inversion of the W-phase.
Moment (Mi or Mwp) 5 - 8 all Based on an estimate of moment calculated from the integral of the displacement of the P wave recorded on broadband instruments.
Energy (Me) > 3.5 all Based on the seismic energy radiated by the earthquake as estimated by integration of digital waveforms

## 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.

## Magnitude Uncertainty

The estimated standard error of the magnitude. The uncertainty corresponds to the specific magnitude type being reported and does not take into account magnitude variations and biases between different magnitude scales. We report an "unknown" value if the contributing seismic network does not supply uncertainty estimates.

## 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 report an "unknown" value if the contributing seismic network does not supply 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.

## Review Status

Review status is either automatic or reviewed. Automatic events are directly posted by automatic processing systems and have not been verified or altered by a human. Reviewed events have been looked at by a human. The level of review can range from a quick validity check to a careful reanalysis of the event.

## Event ID

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

## Moment Tensors

A mathematical representation of the movement on a fault during an earthquake. The tensor depends on the source strength and fault orientation. See also Focal Mechanisms