Transcript Quaking, Shaking, Earth

Quaking, Shaking, Earth
All about Earthquakes
What is an earthquake?
• Simply put:
• An earthquake is the shaking of the earth.
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Many buildings in
Charleston,
South Carolina,
were damaged or
destroyed by the
large earthquake
that occurred
August 31, 1886.
Picture from the United States Geological Service www.usgs.gov
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San Francisco, California, Earthquake
April 18, 1906. East side of Howard
Street near Seventeenth Street. All
houses shifted toward the left. The
tall house dropped from its south
foundation wall and leaned against its
neighbor. 1906.
Picture from USGS.GOV
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San Fernando,
California,
Earthquake
February 1971.
Collapsed
overpass
connecting
Foothill
Boulevard and
the Golden State
Freeway. Feb 10,
1971.
Photo by R.E.
Wallace, USGS.
www.usgs.gov
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What causes an earthquake?
• Earthquakes are the Earth's natural
means of releasing stress.
• Due to the constant motion of the
Earth’ plates, this put stress on the
edges of the plates.
• To relieve this stress, the rocks tend
to bend, compress, or stretch.
An aerial view of the San Andreas fault in the Carrizo
Plain, Central California. Picture from www.usgs.gov
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Locatin' the Shakin'
• Focus: the place on the Earth’s crust where the pressure was
released.
• Epicenter: the spot on the Earth’s surface directly above the focus.
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Faults
• If the force is great enough, the rocks will
break.
• An earthquake is the vibrations produced
by the breaking of rock.
• Most earthquakes occur near plate
boundaries.
The Hanshin expressway in Kobe,
Japan collapsed due to an
earthquake in 1995.
Picture from
http://www.ce.washington.edu/~liquefaction/html/quakes/kobe/kobe.html
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Normal Fault
•Rock above the fault
surface moves
downward in relation
to rock below the fault
surface.
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Reverse Fault
• Reverse faults result from
compression forces that
squeeze rock.
• If rock breaks from forces
pushing from opposite
directions, rock above a
reverse fault surface is forced
up and over the rock below
the fault surface.
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Strike-slip Fault
• At a strike-slip fault, rocks on
either side of the fault are
moving past each other
without much upward or
downward movement.
• The San Andreas Fault is the
boundary between two of
Earth’s plates that are moving
sideways past each other.
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How does energy created by an earthquake
move through the Earth?
•Seismic waves are energy
waves that travel outward from
the source of the earthquake.
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Surfin the Waves
•When earthquakes
occur, three different
types of seismic
waves are produced.
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P Waves
• Primary waves (P-waves)
cause particles in rocks to
move back and forth in the
same direction that the wave
is traveling.
• P-Waves are the fastest
waves and are felt first,
usually as a bang or a thump.
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S Waves
• Secondary waves (S-waves)
move through Earth by
causing particles in rocks to
move at right angles to the
direction of wave travel.
• These waves are slower than
P-Waves.
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L Waves
• Surface waves cause most of
the destruction resulting
from earthquakes.
• Surface waves (L-Waves) or land
waves move rock particles in a
backward, rolling motion and
a side-to-side, swaying
motion.
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How are seismic waves measured?
• Seismic waves from earthquakes
are measured with an instrument
known as a seismograph.
• Seismographs register the waves
and record the time that each
arrived.
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Measuring Earthquake Magnitude
•Magnitude is a
measure of the
energy that is
released during an
earthquake.
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Richter Scale
• The Richter magnitude scale is
used to describe the strength of
an earthquake and is based on
the height of the lines on the
seismogram.
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Richter Scale
• For each increase of 1.0 on
the Richter scale, the height
of the line on a seismogram is
ten times greater.
• However, about 32 times as
much energy is released for
every increase of 1.0 on the
scale.
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Earthquake Severity
•
•
•
•
Richter Earthquake Magnitudes Effects
Less than 3.5 Generally not felt, but recorded.
3.5-5.4 Often felt, but rarely causes damage.
Under 6.0 At most slight damage to well-designed buildings. Can cause
major damage to poorly constructed buildings over small regions.
• 6.1-6.9 Can be destructive in areas up to about 100 kilometers across
where people live.
• 7.0-7.9 Major earthquake. Can cause serious damage over larger areas.
• 8 or greater Great earthquake. Can cause serious damage in areas
several hundred kilometers across.
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