Ch 6 Earthquakes Sec 6-1 Forces in Earth’s Crust

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					          Ch 6 Earthquakes
    Sec 6-1 Forces in Earth’s Crust

• Stress – forces that act on rock
  – Tension – force that pulls on the Earth’s
    crust, stretching rock in opposite directions
    making it thinner in the middle. (apart)
  – Compression – force that squeezes the
    Earth’s crust until it folds or breaks.
  – Shearing – pushes a mass of rock in 2
    opposite directions.
                 Types of Faults
Faults are cracks in the earth’s crust where
   movement occurs. Found at plate boundaries
1. Normal Fault – Tension stress pulling on the rocks
   where the hanging wall slides down.
     (↔) Rio Grande rift valley New Mexico
2. Reverse Fault – Compression stress pushes the
   rocks together where the hanging wall slides up.
     (→←) Northern Rocky Mountains
3. Strike-Slip – Shearing stress causes the plates to
   move past each other in opposite directions.
     (→ ←) San Andreas Fault California (164 – 165)

Hanging Wall is above the fault and Footwall is below
Anticline – an upward fold in rock.
 Our Appalachian Mountains were formed
 this way.
Syncline – a downward fold in rock (valley)

Fault-Block Mountain – formed by faults
 moving and chunks of rock lifting up
 above the crust
Rift-Valley – formed by faults moving and
 allowing a chunk of rock to slide beneath
 the crust (page 166)
             Sec 6-2 Earthquakes
Earthquake – sudden shaking of the Earth’s surface.
• Focus –under the surface where the earthquake starts
  moving. (pg 170 – 171)
• Epicenter – point on the surface directly above the focus
  where most damage is done.

Seismic Waves – shockwaves produced by an earthquake
• Primary Waves - (P waves) Fastest of all three waves,
  travel through solids, liquids, and gases, pushes and pulls.
  Originates at the focus.
• Secondary Waves - (S waves) Slower than P waves,
  travel with a side to side motion, don’t go through liquids -
  molten rock. Also originate at the focus.
• Surface Waves - (L waves) Slowest moving, cause most
  of the damage of the Earthquakes. Move along the
  surface, like ocean water. Originate at the epicenter.
Richter Scale – earthquake scale based on the size of
   seismic waves and power. Most commonly used.
• Magnitude for Richter scale starts at 0 and goes up
   to 10. It is an exponential scale, meaning every
   number increases the power by 10. Any Earthquake
   under a 3 on the scale is not felt by humans.
• Great Alaskan Quake was a 9.2 (largest in U.S.)
Moment Magnitude Scale – estimates the total energy
   released by an earthquake, usually works in
   conjunction with the Richter scale. Each increase of
   one number on the scale is 32 times more energy.
Mercalli Scale – measures earthquakes according to
   damage. Goes from 1 -12.
   Monitoring Earthquakes Sec 6-3
Seismograph – instrument for measuring seismic waves. (page
  179) The machine is made of a pen on a pendulum and a drum.
• Seismogram – paper record of a seismic event. The bigger the
  jagged lines the bigger the quake.
Motion Detectors: (pg 181)
• Tiltmeter – measures tilting or raising of the ground with a level
  like instrument.
• Creep Meter – measures horizontal movement over a fault with
  a wire.
• Laser-Ranging Device – measures horizontal movement over a
  fault with a laser.
• GPS Satellites – measures horizontal movement and changes
  in elevation using satellites.
Predicting Earthquakes is not practical. By mapping faults and
  studying the past we can get a idea on where they may occur
       Earthquake Safety Sec 6-4
Geologists can determine earthquake risk by location
  where faults are active and where past earthquakes
  have occurred.
The west coast of the US has a high risk for
  earthquakes because of these conditions.
Damages in a quake are due to shaking, liquefaction,
  aftershocks, and tsunamis.
Today’s buildings are designed stronger and more
  flexible to prevent damages.
  Base-isolated building is built on a shock like
  system, and some even have rollers. They may
  have flexible pipes so they give instead of break
  when an earthquake hits. (pg 193)

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