Page 25 - Global Tectonics

12 CHAPTER 2

accommodated by the rock (Fig. 2.3b). Eventually,
however, the strain reaches the level at which it exceeds
the frictional and cementing forces opposing movement
along the fault plane (Fig. 2.3c). At this point fault move-
ment occurs instantaneously (Fig. 2.3d). The 1906 San
Francisco earthquake resulted from a displacement of
6.8 m along the San Andreas Fault. In this model, fault-
ing reduces the strain in the system virtually to zero,
but if the shearing forces persist, strain would again
build up to the point at which fault movement occurs.
The elastic rebound theory consequently implies that
earthquake activity represents a stepwise response to
persistent strain.

2.1.6 Focal mechanism
solutions of earthquakes

The seismic waves generated by earthquakes, when
recorded at seismograph stations around the world, can
be used to determine the nature of the faulting associ-
Figure 2.3 Elastic rebound mechanism of earthquake ated with the earthquake, to infer the orientation of the
generation. fault plane and to gain information on the state of stress
of the lithosphere. The result of such an analysis is
referred to as a focal mechanism solution or fault plane
solution. The technique represents a very powerful
100 km as the P–pP time separation becomes very small.
method of analyzing movements of the lithosphere, in
The focal depths of local earthquakes can be deter-
particular those associated with plate tectonics. Infor-
mined if a network of seismographs exists in the vicin-
mation is available on a global scale as most earthquakes
ity of the epicenter. In this case the focal depth is
with a magnitude in excess of 5.5 can provide solutions,
determined by triangulation in the vertical plane, using
and it is not necessary to have recorders in the immedi-
the P–S time difference to calculate the distance to the
ate vicinity of the earthquake, so that data are provided
focus.
from regions that may be inaccessible for direct study.
According to the elastic rebound theory, the strain
2.1.5 Mechanism energy released by an earthquake is transmitted by the
seismic waves that radiate from the focus. Consider the
of earthquakes fault plane shown in Fig. 2.4 and the plane orthogonal

to it, the auxiliary plane. The first seismic waves to arrive
Most earthquakes are believed to occur according to the at recorders around the earthquake are P waves, which
elastic rebound theory, which was developed after the San cause compression/dilation of the rocks through which
Francisco earthquake of 1906. In this theory an earth- they travel. The shaded quadrants, defi ned by the fault
quake represents a sudden release of strain energy that and auxiliary planes, are compressed by movement
has built up over a period of time. along the fault and so the first motion of the P wave

In Fig. 2.3a a block of rock traversed by a pre-existing arriving in these quadrants corresponds to a compres-
fracture (or fault) is being strained in such a way as sion. Conversely, the unshaded quadrants are stretched

eventually to cause relative motion along the plane of or dilated by the fault movement. The first motion of
the fault. The line AB is a marker indicating the state of the P waves in these quadrants is thus dilational. The
strain of the system, and the broken line the location region around the earthquake is therefore divided into
of the fault. Relatively small amounts of strain can be four quadrants on the basis of the P wave fi rst motions,