Unformatted text preview: Treaty . Graphical
The data for an
earthquak e is plotted
us ing a lower- hemis phere s tereographic projec tion. Theaz imuth and tak e-off angle are us ed to plot the
pos ition of an individual s eis mic rec ord. The tak e-off angle is the angle from the vertic al of a
s eis mic ray as it emerges from the earthquak e foc us . Thes e angles are c alc ulated from a
s tandard s et of tables that des c ribe the relations hip between the tak e-off angle and the
dis tanc e between the foc us and the obs erving s tation. By c onvention, filled s y mbols are us ed
to plot data from s tations where the P-wave firs t motion rec orded was up (a c ompres s ive
wave), hollow s y mbols for down (a tens ional wave), with dots for s tations with arrivals too
weak to get a s ens e of motion. If there are s uffic ient obs ervations , one may draw two wellc ons trained orthogonal great c irc les that divide the c ompres s ive from the tens ional
obs ervations and thes e are the nodal planes . Obs ervations from s tations with no c lear firs t
motion normally lie c los e t...
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This document was uploaded on 02/04/2014.
- Spring '14