Terrenmotos

CHAPTER

10
Ear thquakes
and the Ear th's
Structure

C

ontinents glide continuously around the globe, but we
cannot feel the motion because it is too slow. Occasionally, however, the Earth trembles noticeably. The ground
rises and falls and undulates back and forth, as if it were
an ocean wave. Buildings topple, bridges fail, roadways and
pipelines snap. An earthquake is a suddenmotion or
trembling of the Earth caused by the abrupt release of energy that is stored in rocks.
Before the plate tectonics theory was developed, geologists recognized that earthquakes occur frequently in
some regions and infrequently in others, but they did not
understand why. Modern geologists know that most earthquakes occur along plate boundaries, where huge tectonic
plates separate,converge, or slip past one another.

The January 1995 Kobe earthquake destroyed this portion of
the Kobe-Osaka Highway in western Japan and killed nearly 5000
people. (Atsushi Tsukada/AP Wide World)

155

156

CHAPTER 10

EARTHQUAKES AND THE EARTH’S STRUCTURE

10.1 WHAT IS AN EARTHQUAKE?

Elastic limit

Plastic deformation

Ela
sti
c

de
for
m

ati
on

Brittle fractureIncreasing stress

How do rocks store energy, and why do they suddenly
release it as an earthquake?
Stress is a force exerted against an object.1 You
stress a cable when you use it to tow a neighbor’s car.
Tectonic forces stress rocks. The movement of lithospheric plates is the most common source of tectonic
stress.
When an object is stressed, it changes volume and
shape. If a solidobject is stressed slowly, it first deforms
in an elastic manner: When the stress is removed, the object springs back to its original size and shape. A rubber
band exhibits elastic deformation. The energy used to
stretch a rubber band is stored in the elongated rubber.
When the stress is removed, the rubber band springs
back to its initial size and shape and releases the stored
energy. Rocksalso deform elastically when tectonic
stress is applied (Fig. 10–1).
Every rock has a limit beyond which it cannot deform elastically. Under certain conditions, when its elastic limit is exceeded, a rock continues to deform like
putty. This behavior is called plastic deformation. A
rock that has deformed plastically retains its new shape
when the stress is released (Fig. 10–2). Earthquakes donot occur when rocks deform plastically.
Under other conditions, an elastically stressed rock
may rupture by brittle fracture (Fig. 10–3). The fracture
releases the elastic energy, and the surrounding rock
springs back to its original shape. This rapid motion creates vibrations that travel through the Earth and are felt
as an earthquake.
Earthquakes also occur when rock slips alongpreviously established faults. Tectonic plate boundaries are
huge faults that have moved many times in the past and
will move again in the future (Fig. 10–4).
Although tectonic plates move at rates between 1
and 16 centimeters per year, friction prevents the plates
from slipping past one another continuously. As a result,
rock near a plate boundary stretches or compresses. When
its accumulatedelastic energy overcomes the friction
that binds plates together, the rock suddenly slips along

Ultimate strength

Increasing strain (deformation)
(a)

Stress is applied

Rock deforms to its
elastic limit
(amount of elastic
deformation is
exaggerated)

1More

precisely, stress is defined as force per unit area and is measured in units of newtons per square meter (N/m2).

Figure10–1 The behavior of a rock as stress increases in
graphical form (a), in schematic form (b). At first the rock deforms by elastic deformation in which the amount of deformation is directly proportional to the amount of stress. Beyond
the elastic limit, the rock deforms plastically and a small
amount of additional stress causes a large increase in distortion. Finally, at the yield point, the...