M0.0 - 0.25 mi S from Mt St Helens, WA
Depth within the Earth where an earthquake rupture initiated. PNSN reports depths relative to sea level, so the elevation of the ground above sea level at the location of the epicenter must be added to estimate the depth beneath the Earth's surface.
A measure of how well network seismic stations surround the earthquake. Measured from the epicenter (in degrees), the largest azimuthal gap between azimuthally adjacent stations. The smaller this number, the more reliable the calculated horizontal position of the earthquake.
How well the given earthquake location predicts the observed phase arrivals (in seconds). Smaller misfits mean more precise locations. The best locations have RMS Misfits smaller than 0.1 seconds.
A P first motion is the direction in which the ground moves at the seismometer when the first P wave arrives. We distinguish between upward and downward first motions. This is the number of observations that were used to obtain the fault plane solution.
Orientation of first possible fault plane
The strike is the angle between the north direction and the direction of the fault trace on the surface, while keeping the dipping fault plane to your right.
The dip is the steepness of the fault plane measured as an angle between the fault plane and the surface. For example, 0 degrees is a horizontal fault and 90 degrees is a vertical fault.
Rake is the angle, measure in the fault plane, between the strike and the direction in which the material above the fault moved relative to the material on the bottom of the fault (slip direction).
Orientation of second possible fault plane
The orientation of the two possible fault planes is the best solution we can find to match the observed first motions at the seismometers using a grid search method. The uncertainty of the strike, dip, and rake indicate the number of degrees by which those values can vary and still match the observations satisfactorily.
Code, or name, to designate a particular seismic station
Network Code indicates the organization responsible for a particular station, the PNSN consists of UW=University of Washington, UO=University of Oregon, and CC=Cascade Volcano Observatory
The quality of an observed P arrival polarity indicates how well you can tell whether it is up or down and can range from 0 (poor) to 1 (good).
The channel name allows one to distinguish between data from different kinds of sensors. The first character indicates the sample rate of the data, examples are E=100Hz, B=40 or 50Hz, H=80 or 100 Hz. The second character indicates whether the channel is a high (H) gain or low (L) gain velocity channel or a strong-motion acceleration channel (N). The third character indicates the direction of motion measured, Z=up/down, E=east/west, N=north/south.
Polarity means the direction of motion, in this context it means whether it is up (U) or down (D).