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Earthquakes
Earthquakes and the Earth’s Structure
• Focus is the point within Earth where the earthquake starts.
• Epicenter is the location on the surface directly above the focus.
An earthquake is the vibration of Earth produced by the rapid release of energy
Focus and Epicenter
• Faults are fractures in Earth where movement has occurred.
Faults
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What causes an earthquake?
• Caused by sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip
• Caused by volcanic or magmatic activity, • Caused by other sudden stress changes in the
earth.
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Focus, Epicenter, and Fault
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Cause of Earthquakes
Earthquakes
Elastic Rebound Hypothesis• Most earthquakes are produced by the rapid
release of elastic energy stored in rock that has been subjected to great forces.
• When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.
1) Elastic deformation – rock stretches then reaches a breaking point, releasing energy, causes earthquakes.
2) Plastic deformation – does not cause earthquakes
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Elastic Rebound Hypothesis
Elastic deformation – rock stretches then reaches a breaking point, releasing energy
Plastic deformation – does not cause earthquakes
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SeismographSeismographs—Instruments that records the vibrations of the earth during an Earthquake.
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How Seismographs Work
the pendulum remains fixed as the ground moves beneath it
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Seismogram
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Earthquake Terms
• Seismographs—Instruments that records the vibrations of the earth during an Earthquake.
• Seismologists—People who study earthquakes.
• Richter Scale— Scale used to measure the strength of an earthquake.
• Magnitude—The strength of an earthquake.
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Seismic Waves
Body Waves
• P waves
• Identified as P (primary) waves or S (secondary) waves
- Have the greatest velocity of all earthquake waves 8 km/sec
- Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel
- Travel through solids, liquids, and gases
Surface Waves
• S waves
- Slower velocity than P waves 4.5 km/sec
- Shake particles at right angles to the direction that they travel
- Travel only through solids. Cannot travel through liquids
- L or Love waves- Rayleigh waves
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Primary Waves (P Waves)
• A type of seismic wave that compresses and expands the ground
• The first wave to arrive after an earthquake 8 km/sec
http://daphne.meccahosting.com/~a0000e89/insideearth2.htm
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Secondary Waves (S Waves)
• A type of seismic wave that moves the ground up and down or side to side. Second wave to arrive after earthquake 4.5 km/sec
http://daphne.meccahosting.com/~a0000e89/insideearth2.htm
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Comparing Body Seismic Waves
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Surface Seismic Waves• Move along the Earth’s surface• Love or L waves and Rayleigh waves• Produces motion in the upper crust
– Motion can be up and down– Motion can be around– Motion can be back and forth
• Travel more slowly than S and P waves, • More destructive
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Seismic Waves Effects
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Seismic Waves Paths Through the Earth•P and S waves travel through the interior of the earth
•Density differences in the layers of the earth cause P and S waves to be refracted
•P and S waves travel faster through denser material
•P waves travel faster through solids than through liquids
•S waves cannot travel through liquids
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Refraction of Waves
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Waves Travel The Fastest Through Densest Material
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1. Assume the Earth is uniform.• We know it isn't, but it's a useful place to
start.
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2. Actual seismic signals don't match the predictions
• If we match the arrival times of nearby signals, distant signals arrive too soon
• If we match the arrival times of distant signals, nearby signals arrive too late.
• Signals are interrupted beyond about 105 degrees
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3. We conclude:
• Distant signals travel through deeper parts of the Earth, therefore ..
• Seismic waves travel faster through deeper parts of the Earth, and ..
• They travel curving paths (refract)
• Also, there is an obstacle in the center (the core).
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P Wave and S Wave Shadow Zones
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P Wave and S Wave Shadow Zones
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In Reality the travel paths of P and S waves are more complicated!