Geoscience Reference
In-Depth Information
Figure 4.32.
How to determine the fault plane, auxiliary plane and azimuth of the
slip vector for an earthquake.
is measured clockwise around the outside of the projection from N, 238
◦
and 348
◦
(Fig. 4.30(e)).
This earthquake was therefore a combination of normal faulting and strike-
slip on either a fault plane striking 78
◦
and dipping at 60
◦
with the strike of the
horizontal component of slip 238
◦
,orafault plane striking 147
◦
and dipping at
60
◦
with the strike of the horizontal component of slip 348
◦
.
For more details on using lower-focal-hemisphere projections, consult an intro-
ductory structural geology textbook or Cox and Hart (1986).
To distinguish between the nodal planes, it is often necessary to have some
additional information. Sometimes local geology can be used to decide which
nodal plane is most likely to have been the fault plane. On other occasions, the
earthquake may show a surface break or fault, in which case the strike of that
fault and the offset along it should agree with one of the nodal planes. Often
though, this information is unavailable. McKenzie and Parker (1967)faced this
problem in studying earthquakes in the Pacific. They plotted both of the possible
horizontal components of the slip vector for a set of earthquakes that occurred
