Earthquakes

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Earthquakes
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Whats an Earthquake?

• Earthquakes are movements of the ground that are
  caused by a sudden release of energy when along a
  fault move.
• Occur when rocks under stress shift suddenly
  along a fault.
• Stress is a force that can change the size
  shape of rocks.

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Elastic Rebound Theory

• This theory states --- sudden return of
  elastically deformed rock to its undeformed
  shape.
• If the fault is locked, stress increases.
• When stress reaches passed a certain point, the
  rocks fracture, separate at weakest point, and
  spring back or rebound, to original shape.
• As they fracture and slip the rocks along the
  fault release energy in the form of an earthquake.

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Anatomy of an Earthquake

• Hypocenter (Focus) The area along the fault
  where slippage first occurs.
• Epicenter The point on the Earths surface
  directly above the focus.

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Anatomy of an Earthquake

• A seismic wave is an elastic wave generated by an
  impulse such as an earthquake or an explosion.
  Seismic waves may travel either along or near the
  earth's surface
• 2 main types of seismic waves
• Body waves a seismic wave that moves through
  the interior of the earth.
• Surface waves a seismic wave that travels near
  the earth's surface.

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Types of Body Waves

1. A P wave, or compressional wave, is a seismic
   body wave that shakes the ground back and forth
   in the same direction and the opposite direction
   as the direction the wave is moving.
2. An S wave, or shear wave, is a seismic body wave
   that shakes the ground back and forth
   perpendicular to the direction the wave is
   moving.

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2 Types of Surface Waves

• Surface waves or Love waves or L waves cause
  rock to move side to side Perpendicular to
  direction waves are traveling.
• Rayleigh waves -- cause the ground to move with
  an elliptical, rolling motion.

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Major Earthquake Zones

• Most earthquakes occur along or near edges of the
  earths lithospheric plates
• Stress is greatest along these moving plate
  boundaries.
• Earth has 3 major earthquake zones
• Convergent Oceanic environments move toward
  each other and collide as the plates move, the
  overriding plate scrapes across the top of the
  subducting plate.
• Divergent Oceanic environments Make up the
  mid-ocean ridges, plates are moving away from
  each other. Spreading causes earthquakes.
• Continental environments 2 continental plates
  converge, diverge, or move horizontally in
  opposite directions.

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Fault zones --- Form at plate boundaries because
of intense stress that results when plates
separate, collide, subduct, or slide past each
other.
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Recording Earthquakes

• Seismograph records detects vibrations in the
  ground.
• Consists of 3 separate devices
• One device records the vertical motion of the
  ground.
• The other 2 devices record horizontal motion in
  the east-west and north-south.
• The seismograph records motion by tracing
  wave-shaped lines on paper and translating the
  motion into electronic signals (known as a
  seismogram).

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Measuring an Earthquake

• Richter scale measures the ground motion from
  an earthquake to find its strength.
• Moment magnitude is a measurement of a quakes
  strength based on size of area the fault moves,
  the average distance that the fault blocks move,
  and the rigidity of the rocks in the fault zone.
• Mercalli scale expresses the intensity of an
  earthquake or the amount of damage it causes.
  Expressed by a roman numeral and a description.