Earthquakes

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Earthquakes
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Earthquake Intro
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What is an Earthquake?

• Earthquakes vibrations (seismic waves) within
  Earth materials are produced by the rapid release
  of energy.
• Earths crust is in constant motion because of
  tectonic forces.
• Earths crust can store elastic energy.
• When forces exceed the elastic limits and
  structural strength of the rocks the rocks will
  break and/or move producing vibrations that
  travel outward in all directions.

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Earthquakes

• Focus The actual place
• underground where the
• rocks break producing
• Vibrations.
• .
• Epicenter The place on the surface directly
  above the focus.

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What types of forces are created?
Picture http//www.iris.washington.edu/gifs/anima
tions/faults.htm

• Tension force
• Stretching or pulling force.
• Makes a normal fault.

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Normal Fault
Picture http//www.see.leeds.ac.uk/structure/faul
ts/index.htm
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What types of forces are created?
Picture http//www.iris.washington.edu/gifs/anima
tions/faults.htm

• Compression Force.
• Force pushing something together.
• Makes a reverse fault.

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Reverse Fault
Picture http//www.see.leeds.ac.uk/structure/faul
ts/index.htm
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What types of forces are created?

• Shear Force
• Forces that operate against a body from different
  sides.
• Makes a strike-slip fault.

Picture http//www.see.leeds.ac.uk/structure/faul
ts/index.htm
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Strike Slip Fault
Picture http//www.uoregon.edu/millerm/LVSS.html
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What causes Earthquakes?

• Movement along faults occurs when the energy
  exceeds the friction holding the sides of the
  fault together and is suddenly released.
• Movement of magma (volcanic).
• Volcanic eruptions.

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Seismic Waves

• Originate at the focus and travel outward in all
  directions.
• Foreshocks small earthquakes that come before a
  major earthquake.
• Aftershocks are adjustments in the crust after
  an earthquake.
• Smaller than main earthquake but can cause as
  much or more damage. They can continue for weeks
  to months. Not every earthquake produces
  aftershocks.

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3 Types of seismic Waves

• P Waves (primary waves) Compressional Waves.
• Particles move back and forth in the same
  direction as the wave.
• Travels the fastest.
• Can pass through solids and liquids (gases also).
• Does not cause damage.

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P Wave
Picture http//www.cdli.ca/CITE/earth_waves.htm
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Types of Waves

• S Wave (secondary wave, shear wave).
• Particles move at right angles to the direction
  of the wave.
• Travels slower than P Waves.
• Can pass through solids only.
• Does not cause damage.

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S Wave
Picture http//www.cdli.ca/CITE/earth_waves.htm
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Types of Waves

• L Wave (long wave, surface wave, ground wave)
• Particles move in elliptical orbit.
• Originates on the surface after the P and S waves
  go straight up from the focus and reach the
  surface.
• The L wave causes the damage and will be the
  strongest at the epicenter.
• Travels the slowest.

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L Wave
Picture http//www.cdli.ca/CITE/earth_waves.htm
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How do we Measure Earthquakes?

• Earthquake waves are recorded by a seismograph
  and the recording of waves on paper is called
  seismogram.

Picture http//upload.wikimedia.org/wikipedia/en/
2/2d/Strong_motion_K2_seismometer.jpg
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Seismic Waves
Picture http//www.geo.mtu.edu/UPSeis/reading.htm
l
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How do we Measure Earthquakes?

• Intensity a measure of the effects on an
  earthquake at a particular location.
• Magnitude a measure of the strength or amount of
  energy released during an earthquake.

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How do we Measure Earthquakes?

• Richter Scale Measures the amplitude of
  earthquake waves on seismograms.
• Scale from 1-10
• Each number is 10 times the amplitude of the
  number below.

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Magnitude
Picture http//tremor.nmt.edu/faq/how.html
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How do we Measure an Earthquake?

• Modified Mercalli intensity scale
• An estimate of the intensity based on observation
  of actual damage.
• A 12 point scale using Roman Numerals.
• Very dependant upon the quality of structures.

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Modified Mercalli Intensity Scale

• I not felt
• II Felt only by persons at rest.
• III-IV Felt by persons indoors only.
• V-VI Felt by all some damage to plaster,
  chimneys.
• VII People run outdoors, damage to poorly built
  structures.
• VIII Well-built structures slightly damaged
  poorly built structures suffer major damage.
• IX Buildings shifted off foundations.
• X Some well-built structures destroyed.
• XI Few masonry structures remain standing
  bridges destroyed.
• XII Damage total waves seen on ground objects
  thrown into air.

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Measuring Earthquakes

• Locating the Epicenter
• Lag Time between the arrival of the P Wave and
  the S Wave to the seismograph station is
  converted to a distance.
• A circle with a radius that equals the distance
  is drawn around the station.
• Two stations can narrow down the location to two
  places where the two circles intersect.
• Locating the focus the lag-time of the L wave
  will determine the depth of the focus.

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Locating the Epicenter
Picture http//www.geo.mtu.edu/UPSeis/reading.htm
l
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Earthquakes Dangers

• Most injuries and death are caused by falling
  objects and most property damage results from
  fires that start.
• Tsunami seismic wave sometimes generated when an
  earthquake originates on the ocean floor.

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Tsumani
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Earthquake Dangers

• Seiche rhythmic sloshing of small bodies of
  water.
• A seiche is a sloshing of a closed body of water
  from earthquake shaking.
• Swimming pools often have seiches during
  earthquakes.

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Earthquake Dangers

• Liquefaction unconsolidated materials that are
  water saturated may turn to a fluid causing some
  underground objects such as storage tanks to
  float to the surface.
• Landslides