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EXAMPLE 19.2
Suppose the vertical displacement in Example 19.1 is 500 m. How
much extension has occurred? (Answer: 660 m.) Eureka

19.3 EARTHQUAKES Reno
Nevada
This section is concerned with the nature and origin of earth-
San Francisco
quakes. The use of seismic waves to determine Earth’s internal
structure was discussed in chapter 18. In this section, seismic waves California
of earthquakes will be considered in greater detail, along with how
the quakes are measured and located by measuring the waves. The
effects of earthquakes, such as ground motion, ground displace-
ment, damage, and tsunamis (“tidal waves”), will also be described.
Los Angeles

CAUSES OF EARTHQUAKES San Andreas fault
What is an earthquake? An earthquake is a quaking, shak- Arizona
ing, vibrating, or upheaval of the ground. Earthquakes are the
result of the sudden release of energy that comes from stress
on rock beneath Earth’s surface. In the section on diastrophism,
you learned that rock units can bend and become deformed in re-
sponse to stress, but there are limits as to how much stress rock FIGURE 19.12 The San Andreas fault, with the Pacific Plate
can take before it fractures. When it does fracture, the sudden moving on one side and the North American Plate moving on the
movement of blocks of rock produces vibrations that move out other.
as waves throughout Earth. These vibrations are called seismic
waves. It is strong seismic waves that people feel as a shaking,
quaking, or vibrating during an earthquake. are felt as an earthquake. The elastic rebound and movement tend
Seismic waves are generated when a huge mass of rock to occur along short segments of the fault at different times rather
breaks and slides into a different position. As you learned in than along long lengths. Thus, the resulting earthquake tends to
the diastrophism section, the plane between two rock masses that be a localized phenomenon rather than a regional one.
have moved into new relative positions is called a fault. Major Most earthquakes are explained by the movement of rock
earthquakes occur along existing fault planes or when a new blocks along faults, but there are also other causes. Earthquakes
fault is formed by the fracturing of rock. In either case, most have occurred in the eastern United States and without any
earthquakes occur along a fault plane when there is displace- apparent relationship to known faults at or near the surface.
ment of one side relative to the other. One of the largest earthquakes on record in the United States
Most earthquakes occur along a fault plane, and they oc- occurred not in California but in the region of New Madrid,
cur near Earth’s surface. You might expect this to happen since Missouri, in 1811. This quake toppled chimneys 400 miles away
the rocks near the surface are brittle, and those deeper are more in Ohio and was felt from the Rocky Mountains to the Atlantic
ductile from increased temperature and pressure. Shallow-focus Coast and from Canada to the Gulf of Mexico. The New Madrid
earthquakes are typical of those that occur at the boundary of the fault zone is theorized to represent a failed rift that is being reac-
North American Plate, which is moving against the Pacific Plate. tivated by compressional stress from the west and east.
In California, the boundary between these two plates is known Some of the few earthquakes that are difficult to explain seem
as the San Andreas fault (Figure 19.12). The San Andreas fault to be associated with deeply buried anticlines or other deeply
runs north-south for some 1,300 km (800 mi) through California, buried structures. Earthquakes are also associated with the move-
with the Pacific Plate mov ing on one side and the North Ameri- ment of magma that occurs beneath a volcano before an eruption.
can Plate moving on the other. The two plates are tightly pressed Earthquakes associated with volcanic activity, however, are always
against each other, and friction between the rocks along the fault relatively feeble compared to those associated with faulting.
prevents them from moving easily. Stress continues to build along
the entire fault as one plate attempts to move along the other.
Some elastic deformation does occur from the stress, but eventu- LOCATING AND MEASURING EARTHQUAKES
ally the rupture strength of the rock (or the friction) is overcome. Most earthquakes occur near plate boundaries. Occurrences
The stressed rock, now released of the strain, snaps suddenly into do happen elsewhere, but they are rare. The actual place where
new positions in the phenomenon known as elastic rebound seismic waves originate beneath the surface is called the focus
(Figure 19.13). The rocks are displaced to new positions on either of the earthquake. The focus is considered to be the center of the
side of the fault, and the vibrations from the sudden movement earthquake and the place of initial rock movement on a fault.

484 CHAPTER 19 Building Earth’s Surface 19-8