1. What is an earthquake?
2. What causes earthquakes?
3. How are earthquakes measured?
4. What areas are more susceptible to
an earthquake? Why?
5. What does an earthquake feel like?
The earthquake that caused the most
destruction in history occurred in the Shansi
province of China on January 23,1556. An
estimated 830,000 people were killed.
The second most destructive earthquake also
occurred in China--in July, 1976--and killed
255,000 people.
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The definition of an earthquake is…
vibrations that cause the breaking of
rocks.
These vibrations move in all directions
through the earth. They begin at a point
along a fault.
The earth’s crust is constantly
experiencing pressure from forces
within and around it. This pressure
builds up over time, and eventually
causes the crust to break. This
becomes a fault.
An earthquake begins along a fault (a
crack in the earth’s surface) at a point
called the focus.
Directly above the focus is a point on
the earth’s surface called the epicenter.
Epicenter
Focus
Seismologists have stations all over the world
that continuously collect information about
earthquakes. This kind of information can help
scientists figure out where larger, more
destructive earthquakes may strike by mapping
out the location of smaller ‘quakes. They also
get a greater understanding of the changes the
earth’s crust makes as the earthquakes occur.
How do they do this???
When the fault ruptures with a sudden movement
energy is released that has built up over the
years. This energy is released in the form of
vibrations called 'seismic waves'… earthquakes!
It is actually when these seismic waves reach the
surface of the earth that most of the destruction
occurs, which we associate with earthquakes.
Earthquakes can cause landslides, sudden eruptions as
in the case of a hot lava flow from a volcano or giant
waves called tsunamis. Sometimes new land mass are
also formed. Such earthquakes are attributed with the
creation of the greatest undersea mountain range and
the longest land mountain range.
…And that was
just a 7.2 on
the Richter
scale!
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Earthquakes are measured using the Richter
Scale. The strongest earthquake ever
measured was a 9.5 on the Richter Scale. This
is a measurement of the amount of energy
released from the earthquake.
9.5
9.2
9.2
9.1
9.0
8.8
8.7
8.6
8.5
8.5
8.5
Chile, May 22, 1960
Indian Ocean (Sumatra tsunami) Dec 26,2004
Prince William Sound, Alaska, March 28, 1964
Andreanof Islands, Aleutian Islands, Pacific,
March 9, 1957
Kamchatka, Russia, November 4, 1952
Off the Coast of Ecuador, January 31, 1906
Rat Islands, Aleutian Islands, Pacific,
February 4, 1965
India-China Border, August 15, 1950
Kamchatka, Russia, February 3, 1923
Banda Sea, Indonesia, February 1, 1938
Kuril Islands, Pacific, October 13, 1963
Magnitude is a number that characterizes the relative
size of earthquakes & is proportional to energy released
In the lower 48 states, there is a tie
between the February 1812, New
Madrid, Missouri earthquake and the
January 1857, Fort Tejon, California
earthquake--both magnitude 7.9--for
the strongest earthquake recorded.
Mercalli Scale
Generally, during an earthquake you first will
feel a swaying or small jerking motion, then a
slight pause, followed by a more intense rolling
or jerking motion. The duration of the shaking
you feel depends on the earthquake's
magnitude, your distance from the epicenter,
and the geology of the ground under your feet.
• For minor earthquakes, ground shaking usually lasts
only a few seconds.
• Strong shaking from a major earthquake usually lasts
less than one minute. For example, shaking in the 1989
magnitude 7.1 Loma Prieta (San Francisco) earthquake
lasted 15 seconds;
• For the 1906 magnitude 8.3 San Francisco
earthquake it lasted about 40 seconds.
• Shaking for the 1964 magnitude 9.2 Alaska
earthquake, however, lasted three minutes.
Aftershocks are smaller earthquakes which
occur in the same general area during the days
to years following a larger event or
"mainshock". As a general rule, aftershocks
represent minor readjustments along the
portion of a fault that slipped at the time of the
main shock. The frequency of these
aftershocks decreases with time.
Often, earthquakes
can be a sign of
things to come, like
in the case of Mt. St.
Helen’s in
Washington.
This photograph
was taken
during the 1980
eruption.
Is this volcano
active again???
The seismic
data says…YES!
After
Before
Notice, this
volcano
really blew
its top…
literally!
Mt. St. Helens Today