Glossary of Terms

COMMENTS

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

DATE

An earthquake begins at a given date and time, the date is given in the form y/m/d where y is the last two digits of the year, m is the month starting at 1 for January, and d is the date starting at 1 for the first day in the month.

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 mean sea-level.

LAT, LATITUDE

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.

LON, LONGITUDE

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 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.

MAG, 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 Northern California (NC) earthquake listings include:

ML
local magnitude, the original scale defined by Richter and Gutenberg based on the maximum amplitude of the waves.
MD
coda magnitude, based on the duration of shaking.
MW
moment magnitude, based on inverting the waveforms for the moment of the earthquake which is equal to the rigidity of the fault times the average amount of slip on the fault times the amount of fault area that slipped.

The methods used in Southern California (SC) earthquake listings include:

MGN
empirically calibrated local magnitude based on readings from high-gain components
MLG
local magnitude based on synthetic Wood-Anderson response from low-gain components
ML
local magnitude based on synthetic Wood-Anderson response from TerraScope stations

Q

The quality of the location. Two characters are used. Either a letter from A-D or a number from 0-4 are used to show that the location quality goes from good to bad respectively. If the location was determined automatically, without review by a person, then this character is followed by an "*".

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