Geoscience Reference
In-Depth Information
Human Interactions: Earthquakes
as Natural Hazards
North American Plate
As discussed at the beginning of this chapter, examples
abound of recent earthquakes that have severely impacted
people in many places around the world. The 2010 earth-
quake in Haiti is, unfortunately, an excellent example of
such a natural disaster. This earthquake measured 7.0 on
the Richter scale and occurred along the Enriquillo-Plantain
Garden fault system. It was particularly horrific because the
epicenter was near Port-au-Prince, which is a densely popu-
lated and impoverished city that is ill equipped to handle
such an event. As the 1906 San Francisco earthquake at-
tests, such earthquakes have even occurred in the United
States. Another big earthquake (magnitude 7.1), called the
Loma
Prieta
earthquake, occurred along a branch of the San
Andreas Fault south of San Francisco in 1989.
You may be surprised to learn that one of the largest events
recorded in the continental United States occurred in 1811 and
1812 in the central part of the country, far from the active con-
tinental margin. This event took place within the New Madrid
Seismic Zone, which extends through southeastern Missouri,
western Tennessee, and northeastern Arkansas. At least four
earthquakes occurred from December 1811 to February 1812
that had reconstructed magnitudes of about 8.0 on the Richter
scale. These earthquakes were felt over a huge area, with even
reports of church bells ringing 1800 km (~1100 mi) away in
Boston
due to the Earth shaking! Fortunately, the central United
States was lightly populated at that time, and so the damage was
minimal. If such an event occurred today, however, it would
be absolutely devastating. According to a 2008 report by the
Federal Emergency Management Agency, a serious earthquake
in this area could result in the highest economic losses by a
natural disaster in the United States, with potentially cata-
strophic damage in states ranging from Kansas to Kentucky,
Illinois, Indiana, and Alabama. Missouri and Tennessee would
be particularly hard it, with as many as 4000 fatalities projected
in Memphis alone.
In this context, an earthquake is viewed as a
natural hazard
that has the potential of having a negative effect on people. Al-
though many different kinds of natural hazards occur on Earth,
such as hurricanes, tornadoes, landslides, and floods, to name
a few, earthquakes are probably the most intense because they
occur without warning and can kill many thousands of people
in a very short period of time. Given the devastating potential
impact of earthquakes on people, it is no wonder that geologists
are working hard to better predict when and where earthquakes
will occur. Unfortunately, with current knowledge, the best
predictions are nothing more than statements of probability. In
northern California, for example, geologists now believe a 62%
San Andreas Fault
Pacific Plate
Figure 13.25 The San Andreas Fault.
In Southern California,
the Pacific plate (foreground) is moving toward the northwest
(left) relative to the North American plate. Note the change in
stream direction that occurs at the fault boundary. This distinct
pattern evolved because the stream flows onto the Pacific plate
from the North American plate. Because the Pacific plate has
been gradually moving in a northwest direction relative to the
North American plate, the stream has slowly changed direction
as well.
the fault geography, including the major subsidiary faults in
the San Francisco region. Many of these faults, such as the
Hayward and Garlock faults, are strike-slip faults. In many
areas, the San Andreas Fault is clearly visible as a large trough
on the Earth's surface, as pictured in Figure 13.25. In this pho-
tograph, the Pacific plate is in the foreground, whereas the
North American plate is the hilly ground in the background.
You can also see how the movement of the fault has occurred
through time by looking at the stream that flows from the
North American plate onto the Pacific plate. Notice that the
stream bends sharply at the fault and then flows directly on
it for a short distance before turning onto the Pacific plate. In
other words, the Pacific plate has moved at a sufficiently slow
speed to allow the stream to bend with it. Geologists say that
the total accumulated displacement along this fault is at least
560 km (~350 mi) since it came into being about 15 to 20 mil-
lion years ago.
Given the immense size of the San Andreas Fault, the
earthquakes associated with it are sometimes quite violent
and destructive. The largest earthquake on the San Andreas in
recorded history is the 1906 San Francisco earthquake, which
had a magnitude of 7.9 on the Richter scale and caused a rup-
ture along 470 km (290 mi) of the fault. This extensive earth-
quake caused as much as 8.5 m (28 ft) of near instantaneous
slip in some places and resulted in extensive damage in San
Francisco and the surrounding area. Between the effects of the
earthquake and the catastrophic fire that occurred afterward,
700 to 800 people were killed and about 28,000 buildings
were destroyed.
Natural hazard
An actual or potentially occurring natural
event, such as an earthquake or volcanic eruption, that has a
negative effect on people or the environment.
