9. Earthquakes

On March 27, 1964, thousands of people in Anchorage, Alaska, were shaken as
the ground rocked under them. A strong earthquake, possibly the most powerful one
ever recorded (9.1 on the Richter scale), knocked down houses, broke up roads, and
cut water, gas, and power lines all over the area.
An earthquake is a
shaking of the ground
caused by the sudden
release of energy in
Earth’s crust. The energy
released as plates crush
together, scrape past each
other, or bend along
jagged boundaires can cause great damage. Earthquakes are very common. More
than a million of them occur each year. However,
most are too small to be felt or to cause damage.
Many earthquakes occur along the boundaires
of the Pacific plate. Earthquakes also occur along
faults in the crust. You have read that Earth’s crust
can bend or break in the middle of a plate as
forces press in on it. These breaks can form
faults, or places where pieces of the crust
The underwater earthquake that triggered the December 2004, Sumatra
Tsunami was the longest earthquake ever recorded, scientists have announced. Most
earthquakes last a few seconds or perhaps a minute. The Sumatra quake lasted
between 500 and 600 seconds. In addition, scientists say, the earthquake created the
longest fault ever seen, a rupture in the sea floor nearly 800 miles long. Land was
ripped apart as far as 50 feet.
The massive quake also released an
unheard-of amount of energy, the amount
that would be found in a 100-gigaton bomb.
In effect, scientists say, "the whole planet
shook." Sophisticated instruments recorded
movement everywhere around the world.
An earthquake sends out energy in the form of seismic waves. Seismic waves
are like ripples that form on a pond when a stone is
tossed in. Scientists measure and record seismic
waves on an instrument called a seismograph. These
measurements can then be used to compare the
relative strengths
of earthquakes.
√What is an earthquake?