
Environmental Encyclopedia 3
Earthquake
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ESOURCES
O
RGANIZATIONS
Earth Pledge Foundation, 122 East 38th Street , New York , NY USA
10016 (212) 725-6611, Fax: (212) 725-6774, , <http://
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Earth Summit
see
United Nations Earth Summit (1992)
Earthquake
Earthquakes have been around for as long as the planet and
have plagued humans throughout history. With no warning,
major earthquakes strike populated areas of the world every
year, killing hundreds, injuring thousands, and causing hun-
dreds of millions of dollars in damage. Yet despite millions
of dollars and decades of research, seismologists (scientists
who study earthquakes) are still unable to predict precisely
when and where an earthquake will happen.
An earthquake is a geological event in which rock
masses below the surface of the earth suddenly shift, releasing
energy and sending out strong vibrations to the surface.
Most earthquakes are caused by movement along a fault
line, which is a fracture in the earth’s crust. Thousands of
earthquakes happen each day around the world, but most
are too small to be felt.
Earth is covered by a crust of rock that is broken into
numerous plates. The plates float on a layer of molten (liquid)
rock within the earth called the mantel. This molten rock
moves and flows, and this movement is thought to cause
the shifting of the plates. When plates move, they either
slide past, bump into, overrun, or pull away from each other.
The movement of plates is called
plate tectonics
. Bound-
aries between plates are called faults.
Earthquakes can occur when there are any of the four
types of movement along a fault. Earthquakes along the San
Andreas and Hayward faults in California occur because of
two plates sliding past one another. Earthquakes also occur
if one plate overruns another. When this happens one plate
is pushed under the other plate, as on the western coast of
South America, the northwest coast of North America, and
in Japan. If plates collide but neither is pushed downwards,
as they do crossing Europe and Asia from Spain to Vietnam,
earthquakes result as the plates are pushed into each other
and are forced upwards, creating high mountain ranges.
Many faults at the floor of the ocean are between two plates
moving apart. Many earthquakes with centers at the floor
of the ocean are caused by this kind of movement.
The relative size of earthquakes is measured by the
Richter Scale, which measures the energy an earthquake
releases. Each whole number increase in value on the Richter
405
scale indicates a ten-fold increase in the energy released
and a thirty–fold increase in ground motion. An earthquake
measuring 8 on the Richter scale is ten times more powerful,
therefore, than an earthquake with a Richter magnitude of
7. Another scale, called the Mercalli Scale uses observations
of damage (such as fallen chimneys) or people’s assessments
of effects (such as mild or severe ground shaking) to describe
the intensity of a quake. The Richter Scale is open-ended,
while the Mercalli scale ranges from 1–12.
Catastrophic earthquakes happened just as often in
past human history as they do today. Earthquakes shattered
stone-walled cities in the ancient world, sometimes has-
tening the ends of civilizations. Earthquakes destroyed
Knossos, Chattusas, and Mycenae, ancient cities in Europe
located in tectonically active mountain ranges. Scribes have
documented earthquakes in the chronicles of ancient coun-
tries. An earthquake is recorded in the Bible in the Book
of Zachariah, and the Apostle Paul wrote that he escaped
from jail when the building fell apart around him during an
earthquake.
Many faults are located in California because two large
plates are sliding past each other there. Of the 15 largest
recorded earthquakes ever to hit the continental United
States, eight have occurred in California, according to the
United States
Geological Survey
(USGS). The San Fran-
cisco earthquake of 1906 is perhaps the most famous. It
struck on April 4, 1906, killing an estimated 3,000 people,
injuring thousands, and causing $524 million in property
loss. Many of the casualties and much of the damage resulted
from the ensuing fires. This earthquake registered a 7.7
magnitude on the Richter Scale and 11 on the Mercalli
Scale. Four other devastating earthquakes have occurred in
California in the twentieth century: 1933 in Long Beach,
1971 in the San Fernando Valley, 1989 in the San Francisco
Bay area, and 1994 in Los Angeles.
The Long Beach earthquake struck on March 10,
1933, killing 120, injuring hundreds, and causing more than
$50 million in property damage. It led to the passage of
the state’s Field Act, which established strict building code
standards designed to make structures better able to with-
stand strong earthquakes.
Centered about 30 mi (48 km) north of downtown
Los Angeles, the San Fernando earthquake killed 65, injured
more than 2,000, and caused an estimated $505 million in
property damage. The quake hit on February 9, 1971, and
registered 6.5 on the Richter Scale and 11 on the Mercalli
Scale. Most of the deaths occurred when the Veterans Ad-
ministration Hospital in San Fernando collapsed.
The Loma Prieta earthquake occurred on October 18,
1989, in the Santa Cruz Mountains about 62 mi (100 km)
south of San Francisco. It killed 63, injured 3,757, and
caused an estimated $6 billion in property damage, mostly