Phase Data Information
Explanation of Phase Data Listing
These lines give information about the hypocenter or location of the earthquake.
- The top line shows the UTC date, followed by the day of the year in parentheses.
Origin time in Coordinated Universal Time (UTC) and the 90% marginal
confidence interval for time. If the marginal confidence interval is
exactly 0, the solution has been held to a solution computed by some
other agency or by some other special processing at NEIC. The source
of this solution is then shown in angle brackets, as in <PAS>. Most of
these source codes are network codes shown on the Networks page of
the on-line Seismograph Station Codes and Coordinates. Other codes
which may appear here include:
- GM - U.S. Geological Survey, Menlo Park, California
- GS - U.S. Geological Survey, NEIC, Golden, Colorado
solution held to the approximate center of the area where the
earthquake was reported felt.
The final item on this line is the name of the geographic region name using the system of Flinn, Engdahl and Hill (1974) with occasional name changes. The boundaries of these regions are defined at one-degree intervals and differ slightly from irregular political boundaries.
- Geographic latitude in decimal degrees (north positive, south negative) and the 90% marginal confidence interval for latitude.
Geographic longitude in decimal degrees (east positive, west negative)
and the 90% marginal confidence interval for longitude.
The NEIC "official" preliminary magnitude for the earthquake is printed on the right side of this line, in the form, MAGNITUDE n.n XX (SRC) - for example, MAGNITUDE 5.4 MS (GS).
n.n - is the preliminary magnitude value determined for this quake. It may be changed over the next days, weeks or months as additional data are received from other observatories. Note that the "official" magnitude will NOT get updated on this listing, because the solution is loaded to the web only once.
(SRC) - the source (contributor) of the magnitude computation. GS means the USGS/NEIC.
Depth in kilometers below the
surface of the Earth
and the 90%
marginal confidence interval on depth OR one of the following phrases
(geophysicist) - depth assigned by the analyst who reviewed the solution.
(normal depth) - depth assigned by the computer or the analyst to 33 kilometers, defined as "normal" depth in the Jeffreys-Bullen travel time tables.
(depth phases) - depth restrained by the computer program based on 2 or more compatible pP-P time intervals.
The next line gives a distance in miles and kilometers from a town,
city or other reference point. The distances are rounded to the nearest
5 miles and nearest 5 kilometers. EACH is computed independently, to avoid
compounding the rounding errors. For example, 7.6 miles (about 12.2
kilometers) will be rounded to both 10 miles AND 10 km, even though 10 miles is
approximately 16 km.
This line may be followed by other lines of text, giving felt or damage information that was available at the time the solution was prepared. Note that this information does NOT get updated on this listing, because the solution is uploaded to the website only once.
- The number of phases (arrival times) used and associated to the solution. This is followed by the standard error (se) of the observations used (in seconds). On the right side of this line is one of the following words describing the quality of the epicentral solution:
- error ellipsoid
- The components of the 90% confidence error ellipsoid. In other words, statistically there is a 90% chance that the "true" location of the earthquake is within this ellipsoid. For each component (separated by semi-colons), the first number is the strike (azimuth, in degress clockwise from north) of the semi-axis, the next number is the dip of the semi-axis (in degrees downward, with 0 meaning a horizontal axis), and the third number is the value (in kilometers) of the semi-axis. The length and orientation of the three semi-axes specify the orientation, size and shape of the error ellipsoid. If the last component is all zeroes, the depth of the event has been held by the location program, and the remaining components refer to the 90% confidence horizontal ellipse instead of a full ellipsoid.
Station Data Lines:
The stations and their readings associated with the event are listed in increasing time order, according to the first arrival time for each station, one phase per line. The contents of each column is described below.
- The international station code for this station. The code is given only for the first arrival, and remains in effect until a new station code is given.
- The phase code, using standard seismological nomenclature, with two major exceptions. Codes ending in "?" (the phases P? and ?) are phases picked automatically by the computer and have not been reviewed or repicked by a human analyst. The phase LR is a dispersed surface wave, and therefore has no true "arrival time." The LR time shown is an approximation, assuming that the wave packet is traveling at 3.9 km/sec, and is used simply as means to associate the surface wave magnitude data. The LR times themselves are not used in the hypocenter computation.
- The arrival time (onset, or beginning time) of the phase, in UTC. See note above about "arrival times" of LR.
- The residual, or time difference, between the observed arrival time of the phase and the theoretical arrival time, in seconds. A positive residual means the observed time is later than the time predicted by the travel-time tables. The 1940 Jeffreys-Bullen P and 1968 Bolt PKP tables are used. An X following the residual indicates that the arrival time was not used (ie. given a weight of zero) in the solution.
- The central angle in degrees of arc between the hypocenter and the station. One degree of arc on the surface of the Earth equals approximately 111.2 kilometers or about 69.5 miles.
- The azimuthal angle in degrees, measured clockwise from due north, from the epicenter to the station.
- The ground amplitude (center-to-peak) of the wave used for the station magnitude identified by the single character ahead of each amplitude value. The type character matches the last letter of the magnitude types given in the last line of the header information (b = mb, L = ML, etc). The amplitudes are expressed in micrometers (microns) for MS data; in nanometers for all others except md. For duration magnitudes (md), the value given in this column is not an amplitude, but the time interval, in seconds, from the onset of the wave until the amplitude of the signal never again exceeds twice the amplitude of the background noise just prior to the onset of the event.
- The period of the wave used for the magnitude calculation, in seconds. This column is blank for duration magnitude data.
- The magnitude value computed using the duration or amplitude and period shown in the two columns immediately to the left. An X after this value indicates it has been completely removed from the calculation of the average magnitude for that type. All other magnitude data are used to compute averages using the 25% trimmed mean procedure of Rosenberger and Gasko (1983).
See also: Magnitude Definitions Used by the NEIC
Bolt, B.A., 1968, Estimation of PKP travel times, Bulletin of the Seismological Society of America, v. 58, pp. 1305-1324.
Choy, G. L., and Boatwright, J. L., 1995, Global patterns of radiated seismic energy and apparent stress: Journal of Geophysical Research, v. 100, p. 18205-18228.
Flinn, E.A., Engdahl, E.R. and Hill, A.R., 1974, Seismic and geographical regionalization, Bulletin of the Seismological Society of America, v. 64, pp. 771-993.
Gutenberg, B. and Richter, C.F., 1956, Magnitude and energy of earthquakes, Annali di Geofisica, v. 9, no. 1, pp. 1-15.
Jeffreys, H. and Bullen, K.E., 1940, Seismological Tables, British Association for the Advancement of Science, Gray Milne Trust. Reprinted 1970.
Nuttli, O.W., 1973, Seismic wave attenuation and magnitude relations for eastern North America, Journal of Geophysical Research, v. 78, no. 5, pp. 876-885.
Richter, C.F., 1935, An instrumental earthquake scale: Bulletin of the Seismological Society of America, v. 25, pp. 1-32.
Rosenberger, J.L. and Gasko, M., 1983, Comparing location estimators: trimmed means, medians and trimean, Understanding Robust and Exploratory Data Analysis, ed. D.C. Hoaglin, F. Mosteller and J.W. Tukey, John Wiley, NY.