Earthquakes

1
Earthquakes

• What, Where, How, and Why
• 7th grade Science

2
Earthquakes

• What is an Earthquake?
• An earthquake is a shaking of the ground caused
  by the sudden breaking and movement of large
  sections (tectonic plates) of the earth's rocky
  outermost crust.
• The edges of the tectonic plates are marked by
  faults (or fractures). Most earthquakes occur
  along the fault lines when the plates slide past
  each other or collide against each other.

3
What is the difference between the focus and the
epicenter?

• The focus of an earthquake is the place where an
  earthquake occurred along the fault underground,
  while the epicenter is the place directly above
  it where the seismic waves were felt.

4
Normal Faults

• A normal fault is one in which the hanging wall
  falls down relative to the foot wall due to
  tensional stress . The Teton Mountains in North
  America are a result of normal faulting. A graben
  forms when a block of rock falls between two
  faults. Huge, relatively flat bottomed valleys,
  like Death Valley in California , are created in
  this way.

• Divergent plate boundaries
• Tensional forces
• Plates are moving apart one plate/part FALLS DOWN

5
Reverse Faults

• Convergent Plate Boundaries
• Compressional forces
• Plates are moving towards each other one
  plate/part is shifted UPWARD

6
Reverse Faults

• A reverse fault  is one in which the hanging wall
  moves up relative to the foot wall due to
  compression If the hanging wall is pushed up and
  then over the foot wall at a low angle it is
  called a thrust fault. A horst is formed when a
  block of rock is pushed up between two faults.
  Large plateau surfaces form in this fashion. The
  Basin and Range Province of North America is
  noted for its faulted mountains and flat-bottomed
  valleys.

7
Strike-slip faults

• Strike slip or transform  faults are those that
  primarily exhibit horizontal movement The San
  Andreas fault is a well-known strike slip fault
  caused by the Pacific Plate sliding past the
  North American Plate.

8
Seismic waves

• Seismic waves are waves of force that travel
  through the Earth or other elastic body, for
  example as the result of an earthquake,
  explosion, or some other process that imparts
  forces to the body.

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Major types of seismic waves

• Body waves
• Primary (p) waves
• longitudinal or compressional waves
• Secondary (s)waves
• transverse or shear waves
• Surface waves
• analogous to water waves and travel just under
  the Earth's surface

10
Primary (p) waves

• In solids, these waves generally travel almost
  twice as fast as S waves and can travel through
  any type of material (solid or liquid).
• P waves travel at the speed of sound. Typical
  speeds are 330 m/s in air, 1450 m/s in water and
  about 5000 m/s in granite.
• LESS destructive than S waves and surface waves.
• Felt FIRST

11
Secondary (s) waves

• With an S wave, the ground moves alternately to
  one side and then the other.
• S waves can travel only through solids, as fluids
  (liquids and gases) do not support shear
  stresses.
• Their speed is about 60 of that of P waves.
• Felt SECOND.

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Surface waves

• They travel more slowly than body waves.
• Because of their low frequency, long duration,
  and large amplitude, they can be the most
  destructive type of seismic wave.
• There are two types of surface waves Rayleigh
  waves and Love waves
• Felt as the p and s waves travel through the
  crust and the layers below.

13
Which wave is felt first, if the x-axis
represents TIME?
14
How are earthquakes measured?

• Seismograph (data collected from a seismograph
  was on the last slide) Earthquakes generate
  seismic waves which can be detected with a
  sensitive instrument called a seismograph.
  Advances in seismograph technology have increased
  our understanding of both earthquakes and the
  Earth itself.
• Perhaps the earliest seismograph was invented in
  China A.D. 136 by a man named Choko.

15
Magnitude

• A Magnitude scale is used to express the seismic
  energy released by each earthquake.

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What is the Richter scale used for?

• Used to determine Earthquake Severity based on
  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.