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Volcanic: Mount Hood 07/21/2014(UTC)

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Magnitude

Measure of the energy released in an earthquake, obtained from interpretation of seismograms.  For technical reasons several different magnitude scales are in common use.  At PNSN we use the following:  Md (Duration Magnitude) - based on the duration of shaking.  Ml (Local Magnitude) - based on the peak amplitudes of high frequency seismograms, and Mw (Moment Magnitude) - based on matching waveforms of the lowest frequency ground motions in broad-band seismograms.  More information at: http://earthquake.usgs.gov/earthquakes/glossary.php#magnitude and http://neic.usgs.gov/neis/phase_data/mag_formulas.html

(Origin) Time

Date and Time when the earthquake rupture initiated.  Large earthquake ruptures can take many seconds to finish.  Seismologists usually use Grennwich Mean Time (GMT) to avoid confusion ariasing from mixing observations from different time zones.  However, the local time is also given as a reference for what local residents experience.

Distance From

Distances and directions from hearby geographical reference points to the earthquake.  The reference points are towns, cities, and major geographic features.  The accuracy of these distances are limited both because of earthquake location uncertainties (typically for PNSN earthquakes, less than 1 km.) and because of the geographic spread of reference points such as cities.

Coordinates

Location in geographic coordinates (as Latitude, Longitude in decimal degrees) of the position on Earth's surface directly above where an earthquake rupture initiated.  PNSN coordinates are referenced to the WGS84 ellipsoid.

Depth

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.

Location Quality

To assist non-seismologists in evaluating the reliability of an earthquake location, we assign a "quality" to each location.  The quality types are (in decreasing order of reliability) "excellent", "good", "fair", "poor" and "unknown".  This description is determined from the formal uncertainties produced by the earthquake location program.

Horizontal Uncertainty

The horizontal uncertainty of the earthquake location, given in km, is an estimate of how well the observed data constrain the location.  The estimate includes information about data quality and the arrangement and proximity of stations to the earthquake.

Depth Uncertainty

The uncertainty in depth of the earthquake location, given in km.  The depth is often the least well constrained of the location parameters, and trades off with uncertainties in the Origin Time.

Azimuthal Gap

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.

Number of Phases

Number of P and S arrival-time observations used to compute the hypocenter location.  In general, more arrival-time observations result in improved earthquake locations.

RMS Misfit

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.

Number of P First Motions

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.

Plane A

Orientation of first possible fault plane

Strike

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.

Dip

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

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).

Plane B

Orientation of second possible fault plane

Uncertainty

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.

Station

Code, or name, to designate a particular seismic station

Network

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

Quality

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).

Channel

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

Polarity means the direction of motion, in this context it means whether it is up (U) or down (D).