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Earthquakes
INSTANT CHAOS
Destruction can be so swift and
sudden that people have no
time to escape. Falling
masonry crushes cars
and blocks roads.ONCE EVERY 30 SECONDS, somewhere in the world, Earth shakes slightly.
These earth tremors are strong enough to be felt, but cause no damage.
However, every few months a major earthquake occurs. The land shakes so
violently that roads break up, forming huge cracks, and buildings and
bridges collapse, causing many deaths. Earthquakes are caused by the
movements of huge plates of rock in Earth’s crust. They occur in places that
lie on the boundaries where these plates meet, such as the San Andreas
fault, which runs 270 miles (435 km) through
central California. In some cases, scientists
can tell in advance that an earthquake
is likely to occur. In 1974, for example,
scientists predicted an earthquake in China,
saving thousands of lives. But earthquake
prediction is not always accurate.
In 1989, a major earthquake
struck San Francisco
without warning,
killing 67 people.
CAUSES OF
EARTHQUAKES
Earth’s crust consists of
several vast plates of solid
rock. These plates move very
slowly and sometimes slide past
each other. Most severe earthquakes
occur where the plates meet.
Sometimes the edges of the plates grip
each other and cannot move, so pressure
builds up. Suddenly, the plates slip and
lurch past each other, making the land
shake violently.
RICHTER SCALE
The severity of an earthquake
is measured on the Richter
scale, which runs from 0 to 9.
An earthquake reaching 8
on the scale can flatten a city.
The Richter scale measures
the movement of the ground,
rather than the damage an
earthquake causes, which varies
from place to place.EARTHQUAKE BELTS
Earthquakes occur only in certain parts of the world. This map
shows the world’s earthquake belts, which also extend through
the oceans. Most severe earthquakes happen near boundaries
between plates in Earth’s crust, so the belts follow the edges
of the plates.FAULT
A deep crack, or fault,
marks the boundary
of two plates.SEISMOLOGY
Sensitive equipment can
pick up vibrations far
from an earthquake. This
is because the sudden slip
of rocks produces shock
waves that move through
Earth. The study of
earthquakes and the
shock waves they cause
is called seismology.Find out more
Continents
Earth
Geology
VolcanoesThe place within Earth where
an earthquake occurs is called
the focus. An earthquake is usually
strongest at the epicentre, the
point on Earth’s surface directly
above the focus.The rocks suddenly
slip along the fault:
a movement of a
few feet is enough
to cause a severe
earthquake.Rocks grip
along the fault.An earthquake in the Indian Ocean on
December 26, 2004, caused tsunamis that
devastated the coasts of parts of South
East Asia, India, and Africa. It was one of
the worst natural disasters of recent times.TSUNAMIS
Earthquakes that occur on the ocean
floor, can produce a wave called a
tsunami that races toward the shore.
The wave is not very high in mid-
ocean. But it begins to rise as it nears
the coast, sometimes growing to
about 250 ft (75 m) high. The
tsunami smashes on to the shore,
destroying buildings and carrying
boats far inland. Tsunamis, which are
often wrongly called tidal waves, are
also caused by volcanic eruptions.