Geoscience Reference
In-Depth Information
Figure 4.30.
How to plot a seismograph station on an equal-area projection of the
lower focal hemisphere.
Example 1
Plot seismograph station S (azimuth N20
◦
E from the earthquake; angle of first-
motion ray 40
◦
from the vertical) on an equal-area projection.
1. Pin the tracing paper onto your base equal-area projection through its centre.
2. Mark N, the north axis.
3. Mark the azimuth N20
◦
Eas
x
at the edge of the projection (Fig. 4.28(a)).
4. Rotate the tracing paper so that
x
is at the top of the projection.
5. Mark the point S, 40
◦
from the centre of the projection towards
x
(Fig. 4.28(b)).
6. Rotate the tracing paper so that N is again at the top of the projection (Fig. 4.28(c)).
Sisnow correctly plotted on the projection of the lower focal hemisphere.
Example 2
Find the great circle which joins the two points R and S.
1. Plot the points R and S as described in Example 1 (Fig. 4.29(a)).
2. Rotate the tracing paper until R and S both lie on the same great circle (i.e., they lie on
the same line of longitude).
3. Trace that great circle (Fig. 4.29(b)).
4. Rotate the tracing paper so that N is again at the top of the projection (Fig. 4.29(c)).
The great circle which joins points R and S is now drawn.
Example 3
Determine the fault plane, auxiliary plane and slip vector for an earthquake, given
the directions of first motion at several seismograph stations.
The fault plane and the auxiliary plane plot on the projection of the lower
focal hemisphere as great circles. The fault plane and the auxiliary plane are
