300 CHAPTER 12 Mineral and Soil Resources
Earthquakes
Forces inside Earth sometimes push and stretch rocks in
the lithosphere. The rocks absorb this energy for a time,
but eventually, as the energy accumulates, the stress is too
great, and the rocks suddenly shift or break. The energy—
released as seismic waves, vibrations that rapidly spread
through rock in all directions—causes one of the most
powerful events in nature, an earthquake. Most earth-
quakes occur along faults, fractures in the crust where rock
moves forward and backward, up and down, or from side
to side. Fault zones are often found at plate boundaries.
The site where an earthquake begins, often far below
the surface, is the focus (Figure 12.3). Directly above the fo-
cus, at Earth’s surface, is the earthquake’s epicenter. When
seismic waves reach the surface, they cause the ground to
shake. Buildings and bridges may collapse, and roads may
break. One of the instruments used to measure seismic
waves is a seismograph, which helps seis mologists (scientists
who study earthquakes) determine where an earthquake
started, how strong it was, and how long it lasted.
Seismologists record more than 1 million earth quakes
each year. Some of these are major, but most are too small
for humans to feel, equivalent to readings of about 2 on
the Richter scale, a measure of the mag nitude of energy re-
leased by an earthquake. In popu lated areas, a magnitude
5 earthquake usually causes some property damage, and
quakes of 8 or higher cause massive property destruction
and kill large num bers of people. Although the public is
familiar with the Richter scale, most seismologists prefer
to use a more accurate scale, the moment magnitude scale,
to measure earthquakes, especially those larger than
magnitude 6.5 on the Richter scale. The moment magni-
tude scale calculates the total energy that a quake releases.
In January 2010, an earthquake with a moment magni-
tude of 7.0 struck in an area approximately 25 km (16 mi)
from the capital of Haiti. About 230,000 people were
killed, making it one of the deadliest earthquakes on
record. Most of these people died due to the structural
collapse of poorly constructed buildings. About one
million people whose homes were destroyed became
refugees. The Caribbean region is prone to earthquakes
due to movements between the North American, South
American, and Caribbean plates. Puerto Rico, Jamaica,
Dominican Republic, Martinique, and Guadeloupe have
had earthquakes greater than magnitude 7 in the past.
Side effects of earthquakes include landslides and
tsunamis. A landslide is an avalanche of rock, soil, and
other debris that slides swiftly down a moun tainside. In
2008 a powerful earthquake in a mountainous area in
Sichuan Province, China, and its aftershocks, triggered
massive landslides and the structural collapse of many
buildings. About 70,000 people were killed, and more
than 1.5 million were left homeless.
A tsunami, a giant sea wave caused by an underwa ter
earthquake, volcanic eruption, or landslide, sweeps across
the ocean at more than 750 km (450 mi) per hour. Al-
though a tsunami may be only about 1 m (3 ft) high in
deep-ocean water, it can build to a wall of water 30 m (about
100 ft)—as high as a 10-story building—when it comes
ashore, often far from where the original earth quake trig-
gered it. Tsunamis have caused thousands of deaths, par-
ticularly along the Pacific coast. Colliding tectonic plates
in the Indian Ocean triggered tsunamis in 2004 that killed
more than 230,000 people in South Asia and Africa. Not
only did the tsunamis cause catastrophic loss of life and
destruction of property, but they resulted in widespread
environmental damage. Salt water that moved inland as
far as 3 km (1.9 mi) polluted soil and groundwater. Oil
and gasoline from overturned cars, trucks, and boats con-
taminated the land and poisoned wildlife. Coral reefs and
other offshore habitats were also damaged or destroyed.
In March 2011, an earthquake with a magnitude
of 8.9 hit Japan. This earthquake, which was the stron-
gest ever recorded in Japan, also triggered a disastrous
tsunami. Other tsunamis generated by the earthquake hit
coastal areas of several Pacific Rim countries, although
they caused substantially less damage than in Japan.
The death toll in Japan numbered in the thousands and
would have been worse except that Japan is one of theFault
Seismic
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Earthquakes occur when plates along a fault suddenly move
in opposite directions relative to one another. This movement
triggers seismic waves that radiate through the crust.