Earthquakes

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Chapter 8

• Earthquakes

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Section 1 What are Earthquakes?

• Objectives
• Explain where earthquakes take place.
• Explain what causes earthquakes.
• Identify three different types of faults that
  occur at plate boundaries.
• Describe how energy from earthquakes travels
  through the Earth.
• Terms to Learn
• seismology P waves deformation
• S waves elastic rebound
• seismic waves

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• Seismologists scientists who study earthquakes

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Where do Earthquakes Occur?

• Most earthquakes take place near the edges of
  tectonic plates.
• Two plates can push toward or pull away from each
  other. They can also slip slowly past each other.
  
• Fault area where rocks break and move in
  opposite directions
• Applied forces cause rocks to bend stretch,
  when elastic limit is passed, the rocks break
• Earthquakes occur along faults because of this
  sliding.

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What Causes Earthquakes

• Deformation is the change in the shape of rock in
  response to stress.
• Plastic deformation, does not lead to
  earthquakes.
• Resembles what a piece of molded clay does when
  you put stress on it
• Elastic deformation, leads to earthquakes.
• Rock can stretch farther without breaking than
  steel can, but rock will break at some point.
• Elastic Rebound the sudden return of elastically
  deformed rock to its undeformed shape
• like the return of the broken rubber-band pieces
  to their unstretched shape.
• During elastic rebound, rock releases energy.
  Some of this energy travels as seismic waves that
  cause earthquakes.

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Earthquake Zones

• Earthquakes vibrations in Earth due to the
  breaking of rocks along a fault
• Most earthquakes occur along tectonic plate
  boundaries
• Can happen both near Earths surface or far below
  it.
• Earthquake zones are places where a large number
  of faults are located.
• Earthquakes can also happen along faults in the
  middle of tectonic plates.
• Most earthquakes are the result of plates moving
  over, under, and around each other

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How do Earthquake Waves Travel

• Seismic waves energy waves generated by an
  earthquake
• Focus point in Earths interior where earthquake
  occurs
• Body Waves Seismic waves that travel through the
  Earths interior
• two types of body waves P waves and S waves

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• Epicenter point on surface above the focus

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How do Earthquake Waves Travel

• P Wave (Pressure Wave)
• Fastest seismic waves,
• Also called primary waves, because they are
  always the first waves of an earthquake to be
  detected.
• Waves that move through Earth by causing
  particles in rocks to move back and forth in the
  same direction the wave is moving

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How do Earthquake Waves Travel

• Secondary Wave (Shear Waves)
• Cause particles in rocks to move at right angles
  to the direction of the wave
• Stretch the rock sideways
• Cannot travel through parts of the Earth that are
  completely liquid
• Slower than P waves
• S waves are also known as secondary waves.
• always arrive later

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• Surface Wave give particles a circular motion
• Move along the Earths surface
• Produce motion mostly in the upper few kilometers
  of Earths crust.
• Two types of surface waves.
• Surface waves are different from body waves in
• travel more slowly and are more destructive.

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• Primary gt secondary gt surface
• Farther apart the waves, farther away the
  epicenter
• At certain depths, the speed and path of seismic
  waves change (make boundaries)

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Section 2 Earthquake Measurement

• Objectives
• Explain how earthquakes are detected.
• Describe how to locate an earthquakes epicenter.
• Explain how the strength of an earthquake is
  measured.
• Explain how the intensity of an earthquake is
  measured.
• Terms to Learn
• seismograph epicenter
• seismogram focus

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Locating Earthquakes

• Seismograph instrument that records primary,
  secondary, and surface waves from all over the
  world
• Seismogram a tracing of earthquake motion that is
  created by a seismograph
• Epicenter the point on Earths surface directly
  above an earthquakes starting point, or focus
• Focus the point along a fault at which the first
  motion of an earthquake occurs

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S-P Time Method

• Multi Step process
• First step collect several seismograms of the
  same earthquake from different locations.
• Second Step, seismograms are placed on a
  time-distance graph
• Then the seismogram tracing of the first P wave
  is lined up with the P-wave time distance curve,
  and the tracing of the first S wave is lined up
  with the S-wave curve
• Process is repeated for the 2 other Seismograms

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Measuring Earthquake Strength and Intensity

• Mercalli Scale
• Developed in the 12th Century
• Rates earthquakes according to their intensity
• Intensity values are usually higher near an
  earthquakes epicenter.
• Has 12 steps that look at the affects on people,
  buildings, and the surface
• Richter Scale
• Developed in the 1930s by Seismologist Charles
  Richter
• Provides accurate measurements for small, nearby
  earthquakes
• Moment Magnitude Scale
• Used to rate earthquakes of all sizes, near or
  far
• Help to infer how much movement occurred
• 5.0 earthquakes and below cause little damage

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Measuring Earthquake Strength and Intensity

• Magnitude measure of the energy released by an
  Earthquake (Richter scale)
• Increase of 1 is 30 times the energy

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Earthquakes and Society

• Objectives
• Explain how earthquake-hazard level is
  determined.
• Compare methods of earthquake forecasting.
• Describe five ways to safeguard buildings against
  earthquakes.
• Outline earthquake safety procedures.
• Terms to Learn
• gap hypothesis seismic gap

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• Earthquake hazard is a measurement of how likely
  an area is to have damaging earthquakes in the
  future
• An areas earthquake-hazard level is determined
  by past and present seismic activity.

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• Gap Hypothesis a hypothesis that is based on the
  idea that a major earthquake is more likely to
  occur along the part of an active fault where no
  earthquakes have occurred for a certain period of
  time
• Seismic Gap an area along a fault where
  relatively few earthquakes have occurred recently
  but where strong earthquakes have occurred in the
  past

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Earthquakes and Buildings

• Retrofitting Process of making older structures
  more earthquake resistant

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Pg 237 Figure 4
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Are You Prepared for an Earthquake?

• Safety
• 1) study the history of a region
• 2) make your house earthquake safe
• 3) keep away from windows powerlines
• 4) outdoor may be better than indoor

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• Most loss of life in an earthquake occurs when
  people are trapped in a crumbling structure
• Seismic safe resistant to vibrations that occur
  during an earthquake

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Making Buildings Safer

• Choice of location
• Steep slopes pose the danger of landslides
• Filled in land can also shake violently
• Built farther from a fault
• Construction Methods
• Brick and wood buildings need to be reinforced to
  strengthen them so they do not fall apart
• Bridges and other tall buildings should be
  anchored to hard ground to prevent them from the
  affects of liquefaction
• Base-isolated buildings rest on shock-absorbing
  rubber pads or springs

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Making Buildings Safer

• Protecting Yourself During and Earthquake
• Drop, cover, and hold is the best way to protect
  yourself
• This means crouch under a desk and hold onto it
  so it doesnt jiggle away during the shaking.
• The next best option is to crouch against an
  interior wall and over your head and neck with
  your hands

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How Earthquakes Cause Damage

• Local Soil conditions
• Solid rock shakes less then loose packed soil
• Liquefaction
• Occurs when an earthquakes violent shaking
  suddenly turns loose, soft soil into liquid mud
• Can also trigger landslides
• Aftershocks
• An earthquake that occurs after a larger
  earthquake in the same area
• May occur hours, days, or even months later

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• Tsunamis ocean waves generated by earthquakes
• Water displace by an earthquake
• Can be 30 meters high
• Caused when plate movement causes the ocean floor
  to rise slightly and push water out its way