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(a)
(c)
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(d)
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Figure 4.27. (a) A normal-faulting earthquake. (b) The fault-plane solution for the
normal faulting earthquake in (a). The white region represents locations at which the
first motion is dilatational (negative); black regions represent locations at which the
first motion is compressional (positive). (c) A thrust-faulting earthquake. (d) The
fault-plane solution for the thrust faulting earthquake in (c).
Figs. 4.25(c) and (d).Inthese cases, there is no ambiguity about the strike of
the nodal planes but there is ambiguity about the dip of the fault plane. For a
thrust fault, the first motion at the centre of the projection is always compres-
sional, whereas for a normal fault the first motion at the centre of the projection is
always dilatational. A vertical fault plane plots as a straight line passing through
the origin, and the auxiliary plane plots around the circumference. Figure 4.26(a)
shows an example of a predominantly normal-faulting event from the East African
Rift. Figure 4.26(b) shows an example of a predominantly thrust-faulting event
from the Macquarie Ridge (southwest of New Zealand). Both of these events
had a small strike-slip component in addition to the main normal or thrust com-
ponent, which means that there is ambiguity about the normal and auxiliary
planes. Note that motion on a fault can have both normal and strike-slip com-
ponents or both thrust and strike-slip components, but never normal and thrust
components.
The slip vector u of the earthquake is the relative displacement which occurred
between the two sides of the fault plane. It always lies in the fault plane
(Fig. 4.22). The horizontal component of the slip vector u h gives the azimuth
of the relative horizontal motion occurring at the epicentre (Fig. 4.29). Although,
for a pure strike-slip event such as that illustrated in Figs. 4.20(b) and 4.26, u h
is parallel to the fault plane, this is not the case in general. Consider the thrust-
faulting earthquake shown in Fig. 4.25(a);inthis case, u h is perpendicular to the
strike of the fault plane. For every fault-plane solution, however, the strike of u h
is normal to the strike of the auxiliary plane . Thus, the strike of u h can be found
by adding 90 to the strike of the auxiliary plane. The fault-plane solution cannot
tell us anything about the magnitude of u h , simply its direction.
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