azimuth - The direction in which a ray path departs the source. North corresponds to zero degrees.

back azimuth - The in coming direction of a raypath to a receiving station. North corresponds to zero degrees. Because of the Earth's spherical shape, the back azimuth is not usually 180 degrees from the azimuth.

body wave - A seismic wave which travels deep into the Earth, turns and eventually returns to the surface. Body waves travel through the Earth instead of along the surface.

broadband - This terms is used subjectively to describe seismometers which are sensitive to a wide range of frequencies. In earthquake seismology, this usually means that an instrument can record waves with periods from at least 0.1 to 100 seconds.

compressional waves - Commonly called P waves, these waves travel as as a series of compressions and rarefactions(opposite of compression). The direction of particle motion is along the ray path. Sound travels through air as acompressional wave. Compressional waves can travel through both fluids and solids.

epicenter - This is the projection of the hypocenter onto the surface of the earth. It is the point on the surface of the earth, beneath which a given earthquake originated.

focal mechanism - Any set of parameters which describe the fault motion during an earthquake. The most common way to present this is in terms of the strike, dip and rake. For more information, see any basic seismology text including "Earthquakes" by Bruce Bolt.

hypocenter - This is the point in the earth, well below the surface, where an earthquake originates.

Jullian day - The day of year. January 20 is Jullian day 20; February 20 is JD 51; December is JD 354 except during leap year when it is JD 355.

moment tensor - A 2nd order tensor quantity which describes the nature of a fault's motion in terms of the amount of seismic moment release. For more information on moment tensors consult any seismology text including "Modern Seismology" by Lay and Wallace.

regional event - Term used to describe seismic events recorded at distances of less than 3000 KM. Regional events are complicated to interprete as they typically include phases from both the crust and mantle.

shear wave - Commonly called S waves, these waves exhibit side to side vibrations similar to a ripple traveling down a rope. Particle motion is perpendicular to the ray path.

short period - This terms is used subjectively to describe seismometers which are only sensitive to higher frequencies. In earthquake seismology, this usually means in instrument is not very sensitive to waves with periods above roughly 10 seconds.

surface wave - Any seismic phase which travels along the surface of the Earth. Rayleigh and Love waves are the two common surface waves.

teleseism - Term used loosely to describe seismic events recorded at distances of at least 30 degrees (~3000 KM). At these distances, body waves must travel deep into the mantle before reaching the receiver.

traveltime table - A plot of distance (usually in degrees) versus traveltime (usually in minutes) (FIG I). Each phase has a unique traveltime curve reflecting its path through the Earth. These curves are typically calculated from models of the Earth. The IASPEI model is the most common reference. One notable exception is the IRIS poster (fig II) which was created by compiling phases from thousands of seisograms. This table has the distinct advantage of showing the relative strength of each phase. Note: Traveltime tables depend on the depth of an Earthquake. Depth may affect traveltime by as much as 2-3 minutes for very deep Earthquakes. Different table are usually provided for a range of depths.


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