Earthquakes Waves

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EarthquakesWaves Seismograms

• Lecture prepared by Mr. B

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Topics

• Elastic rebound theory
• Seismic waves
• Seismograph and seismogram
• Finding distance to an earthquake
• Locating an earthquake
• Distribution of earthquakes
• Depth of earthquakes
• Earthquake intensity
• Earthquake magnitude

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Earth In Cross Section
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Under the Mantle, its like a Lava Lamp
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Turkey, 1999
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Can Earthquakes be Predicted?

• Earthquake Precursors
• changes in elevation or tilting of land surface
• fluctuations in groundwater levels
• magnetic field
• electrical resistance of the ground
• Release of gases

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Can Earthquakes be Predicted?

• Earthquake Prediction Programs
• include laboratory and field studies of rocks
  before, during, and after earthquakes
• monitor activity along major faults
• produce risk assessments

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Elastic Rebound Theory
Rocks bend under stress while storing elastic
energy. When the strain in the rocks exceeds
their strength, breaking will occur along the
fault. Stored elastic energy is released as the
earthquake. Rockssnap back, or rebound to their
original condition.
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Spread of the Seismic Waves
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Types of Earthquake (Seismic) Waves

• Body Waves
• P-Waves (primary waves)
• S-Waves (secondary waves)
• Surface Waves
• - L Love Waves
• - R Raleigh Waves (surface, vertical)

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P-Waves
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P Wave Animation
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S-Waves
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S Wave Animation
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Body Waves P and S waves

• Body waves
• P or primary waves
• fastest waves
• travel through solids, liquids, or gases
• compressional wave, material movement is in the
  same direction as wave movement
• S or secondary waves
• slower than P waves
• travel through solids only
• shear waves - move material perpendicular to wave
  movement

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Surface Waves R and L waves

• Surface Waves
• Travel just below or along the grounds surface
• Slower than body waves rolling and side-to-side
  movement
• Especially damaging to buildings

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The Poor House ?
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Seismograph (Horizontal)
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Seismograph (Vertical)
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Distribution of Quakes
A direct result of Continental Drift
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Earthquake Risk
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The Continental Plates
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Profile of Subduction Zone
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Tsunami
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Tsunami
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Seismogram
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How is an Earthquakes Epicenter Located?

• Seismic wave behavior
• P waves arrive first, then S waves, then L and R
• Average speeds for all these waves is known
• After an earthquake, the difference in arrival
  times at a seismograph station can be used to
  calculate the distance from the seismograph to
  the epicenter.

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How is an Earthquakes Epicenter Located?

• Time-distance graph showing the average travel
  times for P- and S-waves. The farther away a
  seismograph is from the focus of an earthquake,
  the longer the interval between the arrivals of
  the P- and S- waves

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Locating the Earthquake
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How is an Earthquakes Epicenter Located?

• Three seismograph stations are needed to locate
  the epicenter of an earthquake
• A circle where the radius equals the distance to
  the epicenter is drawn
• The intersection of the circles locates the
  epicenter

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How are the Size and Strength of an Earthquake
Measured?

• Intensity
• subjective measure of the kind of damage done and
  peoples reactions to it
• isoseismal lines identify areas of equal intensity

• Modified Mercalli Intensity Map
• 1994 Northridge, CA earthquake, magnitude 6.7

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Mercalli Scale of Earthquake Intensity

• Advantages
• No high-tech instruments are required.
• Disadvantages
• Damage depends on geologic materials and type of
  structures in area
• Damage varies with distance from epicenter
• Subjective - different people may view damage and
  effects very differently

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The Goofy Mercalli Scale
I. People do not feel any Earth movement.
II. A few people might notice movement if they
are at rest and/or on the upper floors of tall
buildings. III. Many people indoors feel
movement. Hanging objects swing back and forth.
People outdoors might not realize that an
earthquake is occurring
IV. Most people indoors feel movement. Hanging
objects swing. Dishes, windows, and doors rattle.
The earthquake feels like a heavy truck hitting
the walls. A few people outdoors may feel
movement. Parked cars rock.
XI. Most buildings collapse. Some bridges are
destroyed. Large cracks appear in the ground.
Underground pipelines are destroyed. Railroad
tracks are badly bent. XII. Almost everything is
destroyed. Objects are thrown into the air. The
ground moves in waves or ripples. Large amounts
of rock may move.
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How are the Size and Strength of an Earthquake
Measured?

• Magnitude
• Richter scale measures total amount of energy
  released by an earthquake independent of
  intensity
• Amplitude of the largest wave produced by an
  event is corrected for distance and assigned a
  value on an open-ended logarithmic scale

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Richter Scale Math

• Increase by 1 whole number means a 10X increase
  in the Magnitude of the quake
• For every increase on the Richter Scale, the
  amount of energy released increases 30X

Compare a 5.0 to a 7.0 quake 7.0 has 10 X 10
100 times greater magnitude 7.0 has 30 X 30 900
times more energy! Compared to a 1.0 quake, a 7.0
has 10 X 10 X 10 X 10 X 10 X 10 1 000 000
greater strength And 30 X 30 X 30 X 30 X 30
X 30 729 000 000 more energy
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Earthquake Magnitude aand Worldwide Occurrence
Magnitude Number/Year Less than
2.0 600,000 2.0-2.9 300,000 3.0-3.9
49,000 4.0-4.9 6,200 5.0-5.9
800 6.0-6.9 266 7.0-7.9 18 Greater
than 8,0 1
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Terminology Used in the Study of Earthquakes

• Earthquake intensity
• Earthquake magnitude
• Richter Scale
• Mercalli Scale
• Elastic rebound
• Fault
• Focus

• P-wave
• Seismic wave
• Seismogram
• Seismograph
• S-wave
• Tsunami