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Logo Recent Earthquakes in the Intermountain West

Glossary of Terms

AUTHORITATIVE REGION

The earthquakes displayed on these maps come from several different networks and, for one earthquake, it is possible that more than one network will submit a location and magnitude. When this happens, we need to determine which location to use. To do this, each network is assigned an authoritative region as shown on this map in which it has priority. If an event falls between regions and has multiple locations, we use the solution that has the larger magnitude.

MAGNITUDE

The size of an earthquake is given by it's magnitude which is often referred to as Richter Magnitude. On this scale, the amplitude of shaking goes up by a factor of 10 for each unit on the scale. Thus, 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. There are many different ways that magnitude is measured from seismograms, partially because each method only works over a limited range of magnitudes and with different types of seismometers. But, all of the methods are designed to agree well over the range where they overlap.

The methods used in University of Utah (UU) earthquake listings include:
ML local magnitude, the original scale defined by Richter and Gutenberg based on the maximum amplitude of the waves. This is the preferred magnitude, when available. It is determined on several types of instruments, including synthetic Wood-Anderson from digital broad-band instruments and electronically simulated Wood-Anderson from low-gain short-period instruments. MC is coda magnitude, based on measurements of the duration of the seismic waves. A good fit for earthquakes up to about magnitude 5.

DATE AND TIME

An earthquake begins at a given date and time. The date is given in the form yyyy/mm/dd where yyyy is the year, mm is the month starting at 01 for January, and dd is the date starting at 01 for the first day in the month.

The time is given in the form h/m/s where h is the hour starting at 00 at midnight and going to 23 as the day passes, m is the number of minutes in the hour from 00 to 59, and s is the number of seconds in the minute from 00 to 59. On the individual text pages for each earthquake we provide time in UTC (Coordinated Universal Time) as well as local time. The local time zone is either Mountain Standard Time or Mountain Daylight Time depending on the time of year when the earthquake occurred. Seismologists use UTC to avoid confusion caused by local time zones and daylight savings time.

DISTANCE

The distance field is primarily used to give the distance and direction from a local town or other known landmark.

COORDINATES

An earthquake begins at a given point (hypocenter) which is defined by a position on the surface of the earth (epicenter) and a depth below this point (focal depth). The epicenter is given by the 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

An earthquake begins at a given point (hypocenter) which is defined by a position on the surface of the earth (epicenter) and a depth below this point (focal depth). This depth is given as kilometers (km) below a datum of 1500 m above sea level.

QUALITY

To evaluate the reliability of an earthquake location, a "quality" is assigned to each location. The quality 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".

PARAMETERS

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 the University of Utah Seismograph Stations' two-letter Seismic Network Code (uu) and a number assigned by the seismic network. A seismic network code other than "uu" would indicate that the event is not in the authoritative region for our 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.

Recent Earthquakes home page.





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Salt Lake City, Utah 84112-0111  «»   Phone 801-581-6274  «»  Fax 801-585-5585
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