What are Earthquakes?

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What are Earthquakes?

• Where do most earthquakes occur?
• By looking at maps showing past seismic activity,
  one can see that earthquakes occur mostly at the
  boundaries of tectonic plates, where the plates
  move with respect to each other.

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What are Earthquakes? continued

• Earthquakes are vibrations resulting from rocks
  sliding past each other at a fault.
• Seismic waves are waves of energy released during
  an earthquake.
• focus the location within Earth along a fault at
  which the first motion of an earthquake occurs
• epicenter the point on Earths surface directly
  above an earthquakes starting point, or focus

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Focus and Epicenter
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What are Earthquakes? continued

• Energy from earthquakes is transferred by waves.
• The energy released from an earthquake is
  measured as shock waves.
• Earthquakes generate three types of waves
• Longitudinal waves, also known as P waves
• Transverse waves, also known as S waves
• Surface waves

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What are Earthquakes? continued

• Waves move through Earth and along its surface.
• Both P waves and S waves spread out from the
  focus in all directions through the earth.
• Surface waves move only on Earths surface.
• surface wave a seismic wave that travels along
  the surface of a medium and has a stronger effect
  near the surface of the medium than it has in the
  interior

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Measuring Earthquakes

• How do scientists learn about earthquakes and the
  Earths interior?
• Because energy from earthquakes is transferred by
  waves, scientists can measure the waves to learn
  about earthquakes and about the interior of Earth
  through which the waves travel.

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Measuring Earthquakes, continued

• Seismologists detect and measure earthquakes.
• Seismologists use sensitive equipment called
  seismographs to record data about earthquakes.
• Records of seismic activity are called
  seismograms.
• seismology the study of earthquakes including
  their origin, propagation, energy, and prediction

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Measuring Earthquakes, continued

• Three seismograph stations are necessary to
  locate the epicenter of an earthquake.
• There are more than 1000 seismograph stations
  across the world.
• Because P waves travel faster, the difference
  between the arrival of P waves and the arrival of
  S waves allows scientists to calculate how far
  away the focus is.

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Measuring Earthquakes, continued

• Geologists use seismographs to investigate
  Earths interior.
• The way P and S waves travel through Earths
  interior helps scientists make a model of Earth
  with layers of different densities.
• Scientists have used this information to develop
  a model of Earths interior structure.

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Measuring Earthquakes, continued

• The Richter scale is used to measure earthquakes.
• Although the Richter scale was used popularly for
  much of the 20th century, scientists today more
  often use other scales, such as the
  moment-magnitude scale.
• Richter scale a scale that expresses the
  magnitude of earthquakes

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Volcanoes

• What is a volcano?
• A volcano is any opening in Earths crust through
  which magma has reached Earths surface.
• crust an opening in the surface of Earth through
  which volcanic material passes

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Volcanoes
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Volcanoes, continued

• Most volcanoes occur at convergent plate
  boundaries.
• 75 of the active volcanoes on Earth are located
  in an area known as the Ring of Fire.
• The Ring of Fire is located along the edges of
  the Pacific ocean, where oceanic tectonic plates
  are colliding with continental plates.

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Ring of Fire
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Volcanoes, continued

• Underwater volcanoes occur at divergent plate
  boundaries.
• As plates move apart at divergent boundaries,
  magma rises to fill the gap.
• This magma creates the volcanic mountains that
  form ocean ridges.
• Iceland is a volcanic island on the Mid-Atlantic
  ridge that is growing outward in opposite
  directions.

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Volcanoes, continued

• Volcanoes occur at hotspots.
• Some volcanoes occur in the middle of plates.
• Mantle plumes are mushroom shaped trails of hot
  rock that rise from deep inside the mantle, melt
  as they rise, and erupt from volcanoes at hot
  spots at the surface.
• The plumes remain in the same place as the
  tectonic plate moves, creating a trail of
  volcanoes.
• The Hawaiian Islands are an example of this type
  of volcanic activity.