Ch 6 Earthquakes Sec 6-1 Forces in Earth

1
Ch 6 EarthquakesSec 6-1 Forces in Earths Crust

• Stress forces that act on rock
• Tension force that pulls on the Earths crust,
  stretching rock in opposite directions making it
  thinner in the middle. (apart)
• Compression force that squeezes the Earths
  crust until it folds or breaks. (together)
• Shearing pushes a mass of rock in 2 opposite
  directions.

2
Types of Faults

• Faults are cracks in the earths crust where
  movement occurs. Found at plate boundaries
• Normal Fault Tension stress pulling on the
  rocks where the hanging wall slides down. (?)
  Rio Grande rift valley New Mexico
• Reverse Fault Compression stress pushes the
  rocks together where the hanging wall slides up.
  (??) Northern Rocky Mountains
• 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

3

• 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)

4
Sec 6-2 Earthquakes

• Earthquake sudden shaking of the Earths
  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, dont 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.

5

• 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.

6
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

7
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.
• Todays 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)