Origin time: is calculated for each earthquake on the basis of multi-station arrival times. Time is given in Coordinated Universal Time (UTC), in hours:minutes:seconds. To convert to Pacific Standard Time (PST) subtract eight hours, or to Pacific Daylight Time subtract seven hours.
North latitude: of the epicenter, in degrees and minutes.
West longitude: of the epicenter, in degrees and minutes.
Depth: given in kilometers, is usually freely calculated from the arrival-time data. In some instances, the depth must be fixed arbitrarily to obtain a convergent solution. Such depths are noted by an asterisk (*) in the column immediately following the depth. A $ or a # following the depth mean that the maximum number of iterations has been exceeded without meeting convergence tests and both the location and depth have been fixed.
Coda-length magnitude, Mc: an estimate of local magnitude ML (Richter, C.F., 1958, Elementary Seismology: W.H. Freeman and Co., 768p), calculated using the coda-length/magnitude relationship determined for Washington (Crosson, R.S., 1972, Bull. Seism. Soc. Am., v. 62, p. 1133-1171). Magnitudes may be revised as we improve our analysis procedure.
NS: the number of station observations, and NP: the number of P and S phases used to calculate the earthquake location. A minimum of three stations and four phases are required. Generally, more observations improve the quality of the solution.
Azimuthal gap: The largest angle (relative to the epicenter) containing no stations.
Root-mean-square residual: (observed arrival time minus predicted arrival time) at all stations used to locate the earthquake. It is only useful as a measure of the quality of the solution when 5 or more well-distributed stations are used in the solution. Good solutions are normally characterized by RMS values less than about 0.3 sec.
Quality factors: Two factors indicate the general reliability of the solution (A is best quality, D is worst). Similar quality factors are used by the USGS for events located with the computer program HYPO71.
First Quality factor is a measure of the hypocenter quality based on travel-time residuals. For example:
A quality requires an RMS less than 0.15 sec.
D quality has an RMS of 0.5 sec.
Second Quality factor depends on the spatial distribution of stations around the epicenter, i.e. number of stations, their azimuthal distribution, and the minimum distance DMIN from the epicenter to a station. A quality requires a solution with 8 or more phases, GAP <= 90 degrees and DMIN <= 5 km or depth, whichever is greater. If the number of phases, NP, is 5 or fewer or GAP > 180 degrees or DMIN > 50 km the solution is assigned quality D.
Crustal velocity model: Layered velocity models appropriate to different geographic areas are used in location calculations (Ludwin, R.S., et al., 1994, Earthquake hypocenters in Washington and northern Oregon, 1987-1989, and Operation of the Washington Regional Seismograph Network, Information Circular 89, Washington State Dept. of Natural Resources).
P3
-
C3 - Cascade model
S3 -
N3 - northeastern model
E3 - southeastern model
O0
-
K3
-
R0 and J1 - Regional and Offshore models
Flagging: Events flagged in Table 4 use the following code:
F- earthquake reported to have been felt
P - probable explosion
L - low frequency earthquake (e.g. glacier movement, volcanic activity)
H - handpicked from helicorder records
S - Special event (e.g. rockslide, avalanche, volcanic steam emission, harmonic tremor, sonic boom), not a man-made explosion or tectonic earthquake
X - known explosion