Harry Matheson; A Nomogram for Determining Azimuth and Horizontal Trace Velocity from Tripartite Measurements. Seismological Research Letters ; 37 (1): 33–37. doi: https://doi.org/10.1785/gssrl.37.1.33
The direction normal to a traveling plane-wave front, and the wave’s horizontal trace velocity at the earth’s surface, are easily determined by observing the wave at three noncollinear points. Reduction of the data involves the solution of several trigonometric equations. This note describes a nomogram which reduces this task to 30 sec with ruler and pencil.