Recent Earthquakes in California and Nevada
|Recent Earthquakes - Glossary|
VersionComputers 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.
MagnitudeSeismologists 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:
Time and DateWe 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 fromWe 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.
CoordinatesAn 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.
DepthThe 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.
QualityTo 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 ParametersThese parameters provide information on the reliability of the earthquake location. Zero values usually indicate that the contributing seismic network did not supply the information.
Event ID#A combination of a 2-letter Seismic Network Code and a number assigned by the contributing seismic network.
Additional InformationDepending 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.