Geoscience Reference
In-Depth Information
Figure 4.31.
How to find the great circle which joins two points on the surface of the
Earth.
orthogonal. This means that we need to find two orthogonal great circles that
separate the positive first motions from the negative first motions.
1. Pin the tracing paper onto the base projection through its centre.
2. Put the first motions onto the tracing paper (Fig. 4.30(a)).
3. Rotate the tracing paper until you find a great circle that separates positive and negative
first motions.
4. Trace that great circle; it is nodal plane 1.
5. The dip of nodal plane 1,
δ
1
,ismeasured along the equator of the base projection from
the outside edge to the great circle. (A horizontal plane with zero dip plots around the
edge of the projection. A vertical plane, with 90
◦
dip, plots as a straight north-south
line.)
6. Count 90
◦
along the equator of the base projection from its intersection with nodal
plane 1.
7. Mark that point P
1
on the tracing paper; it is normal to nodal plane 1 (Fig. 4.30(b)).
(Sometimes the normal to a great circle is called the pole.)
8. Nodal plane 2 must separate the remaining positive and negative first motions, and,
since it is also normal to nodal plane 1, point P
1
must lie on it. So, rotate the tracing
paper until you find such a great circle.
9. Trace that great circle; it is nodal plane 2 (Fig. 4.30(c)). If you cannot find nodal plane
2, go back to step 3 and check that nodal plane 1 was correct.
10. Repeat steps 5-7 to find the dip of nodal plane 2,
δ
2
, and point P
2
, the normal to nodal
plane 2 (Fig. 4.30(c)).
11. Rotate the tracing paper so that N is again at the top of the projection.
12. The strike of the nodal planes is measured clockwise around the outside of the pro-
jection from N, 78
◦
and 147
◦
(Fig. 4.30(d)).
13. The slip vector is the normal to the auxiliary plane. Thus, if nodal plane 2 is the fault
plane, point P
1
is the slip vector; and if nodal plane 1 is the fault plane, point P
2
is the slip vector. The strike of the horizontal component of the possible slip vector
