principles and applications of microearthquake networks

6.2. Fuul t- P ~N lie Solu tiori 143
Fig. 28. Equal-area plot of a plane and its pole (modified from Ragan, 1973, p. 94). (a)
Perspective view of the inclined plane to be plotted (shaded area). (b) The position of the
overlay and net for the actual plot. (c) The overlay as it appears after the plot.
illustrates how a fault plane (strike N 30" E and dip 40" SE) may be
projected. The intersection of this fault plane with the focal sphere is the
great circle ABC (Fig. 28a). On a piece of tracing paper we draw a circle
matching the outer circumference of the equal-area net (Fig. 28b) and
mark N, E, S, and W. We overlay the tracing paper on the net, rotate the
overlay 30" counterclockwise from the north for the strike, count 40" along
the east-west axis from the circumference for the dip, and trace the great
circle arc from the net as shown in Fig. 28b. The resulting plot is shown on
Fig. 28c. Thus A'B'C' is an equal area plot of the fault plane ABC. On
Fig. 28b, if we count 90" from the great circle arc for the fault plane, we
find the point P which represents the pole or the normal axis to the fault
plane. By reversing this procedure, we can read off the strike and dip of a
fault plane on an equal-area projection with the aid of an equal-area net.
Similarly, Fig. 29 illustrates how to plot an axis OP with strike of S 42" E,
and plunge 40". Again by reversing the procedure, we can read off the
azimuth and plunge of any axis plotted.
(3a) To determine manually the nodal planes from a P-wave first motion
plot, we need an equal-area net (often called a Schmidt net) as shown
in Fig. 27b. If we fasten an equal-area net (normally 20 cm in diameter to
match plots by the HYPO71 computer program) on a thin transparent
plastic board and place a thumbtack through the center of the net, we can