Fractures, faults and earthquakes - Introducing Tectonics, Rock Structures and Mountain Belts - page 45

zones such as the San Andreas fault
zone of Western USA, which marks a
plate boundary ( see Figure 3.6), repeated
earthquake events have occurred in the
past and can confidently be predicted in
the future, for as long as there is relative
motion between the Americas plate and
the Pacific plate to the west. Sections of
the fault zone that have recently expe-
rienced movement are unlikely to fail
again soon, but may transmit stress to
adjoining sections. The longer a specific
segment of the fault zone is dormant,
the larger the earthquake event is likely
to be when it occurs.
It is possible to monitor sections
of a fault zone that lie within or near
inhabited areas to detect the small
changes in the ground that precede
failure. As explained above, the rocks
contract or expand slightly due to
increases or decreases in stress. Pores
and cracks may open up, affecting the
flow of fluids, and these changes are
accompanied by changes in physical
properties, such as electrical resist-
ance. However, the exact time of the
earthquake is impossible to predict
with sufficient accuracy as yet, and of
course failure might come in a part of
the zone that has not been monitored.

1 minute

5

surface

secondary

44 45

primary

Figure 5.14 Earthquake waves. A.
Typical earthquake-wave recording
(seismograph), showing primary,
secondary and surface wave traces.
B. Method of locating the epicentre
of an earthquake detected at
recording stations X, Y and Z; the
radii of the circles represent the
respective distances from these
stations to the epicentre.

A

Y

X

epicentre

Z

B

direction of movement on the fault; this
is termed the ' fault-plane solution '.
Unfortunately there are two possible
solutions in each case, so that it is ne-
cessary to make an assumption about
which of two possible orientations of
the fault is the correct one. From this
data it is possible, for example, to distin-
guish between dip-slip and strike-slip

motion and to determine which of
several possible fault sources is respon-
sible for a given earthquake.

Earthquake prediction
Much research has been carried out
in an attempt to accurately predict
major earthquakes. This has been only
partially successful. Along large fault