Geoscience Reference
In-Depth Information
Figure 4.26.
A fault-plane
solution for a strike-slip
earthquake. The strike and
dip of the nodal planes
are given by
φ
and
δ
. The
east-striking nodal plane
was chosen as the fault
plane because the
earthquake epicentre lies
on an east-striking
transform fault. The fault
plane is not vertical in this
instance; it is dipping at
79
◦
. (After Sykes (1967).)
or a stereographic projection. It is obvious from Fig. 4.25 that the first P-wave
arriving at a seismograph close to the earthquake focus must travel almost hori-
zontally. Thus, this ray intersects the lower focal hemisphere almost at its equator,
i
90
◦
.Incontrast, the direct P-wave to a station on the opposite side of the Earth
to the earthquake travels almost vertically down from the focus and intersects the
lower focal hemisphere near its centre (pole),
i
=
0
◦
. This means that close seis-
mograph stations plot around the edge of the projection, and distant stations plot
towards the centre. The azimuth of each seismograph station is easily measured
geographically. The polarity (positive
=
=
=
dilata-
tional) of the first motion recorded by each seismograph station is then plotted
on the projection. In this way, data from seismograph stations around the world
can be plotted on a graph that is a projection of the lower focal hemisphere. The
right-handed or dextral strike-slip earthquake illustrated in Fig. 4.24 would have
the
fault-plane solution
shown in Fig. 4.23(c). The four quadrants are separated
by two orthogonal planes, or nodal surfaces, one of which is the
fault plane
. The
other is called the
auxiliary plane
. There is no way of telling from the fault-plane
solution alone which plane is the fault plane and which is the auxiliary plane.
The radiation pattern shown in Fig. 4.23(c) could have been generated either by
dextral strike-slip motion on a vertical fault plane striking 090
◦
or by sinistral
strike-slip motion on a vertical plane striking 000
◦
.
If the fault plane is not vertical, the fault-plane solution for strike-slip motion
still has four quadrants, but the fault plane and auxiliary plane do not plot as
orthogonal straight lines passing through the origin. Instead, they plot as orthog-
onal great circles offset from the origin by 90
◦
compressional, or negative
is their dip. An
example is shown in Fig. 4.26.Again the fault-plane solution does not distinguish
between the fault plane and the auxiliary plane. A normal-faulting earthquake
(Fig. 4.25(a))would have the fault-plane solution shown in Fig. 4.25(b).In
contrast, a thrust-faulting earthquake and its fault-plane solution are shown in
−
δ
where
δ
