Transcript What is an earthquake?

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?
An aerial view of the San Andreas fault in the Carrizo
Plain, Central California. Picture from www.usgs.gov
• 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.
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Locatin' the Shakin'
• Focus: the place on the Earth’s crust where
the pressure was released. Underground.
• 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.
The Hanshin expressway in Kobe,
Japan collapsed due to an
earthquake in 1995.
• Most earthquakes occur
near plate boundaries.
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.
• Sierra Nevada fault
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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.
• Cascadia
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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 (Pwaves) 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 (Swaves) 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 and more
destructive.
• AKA Transverse waves
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L Waves
• Surface waves (LWaves) or land
waves move rock
particles in a backward,
rolling motion and a
side-to-side, swaying
motion.
• Surface Waves reach the
earth's surface after P and
S waves.
• Very destructive
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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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http://www.dropcoverholdon.org/be
atthequake/game/