2/15/2017
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What are Earthquakes?
• Large vibrations that move through rock or other earth materials.
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• How does this happen?
– Elastic Rebound: When rocks strain and then break, the broken pieces snap back
• Rocks deform slowly over long periods of time
• Potential energy builds up in them
• Energy is released suddenly when rocks break and move
• The movement causes vibrations that move through Earth
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• Fault: the surface of a break in rock
– normal fault: caused by tension forces
• rock above moves down compared to the rock below the fault
– Reverse fault: caused by compression forces
• rock above the fault moves upward compared to the rock below it
– Strike-slip fault: caused by shear forces
• Rock on either side of the fault moves past one another in opposite directions
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Where do Earthquakes Start?
• The starting point of an earthquake below ground is called a focus, or hypocenter.
• The area directly above the hypocenter on land is called the epicenter.
• Earthquakes are strongest at the epicenter and become gradually weaker farther
away!
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Seismic Waves
• Start at focus and travel away in ALL directions
– P waves: cause rock to move back and forth in the same directions that the waves
are moving
– S waves: cause rock to vibrate at right angles
– Surface waves: STRONGEST, SLOWEST, LARGEST, MOST DESTRUCTIVE waves
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How is the Size of Earthquakes Measured?
1. A seismograph is the tool used to measure the strength of an earthquake’s vibrations
– The data is translated into a 1-10 rating on the Richter Scale.
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• Richter Scale: measures an earthquake’s size based on the heights of lines
representing the amount of energy released from seismic waves recorded on a
seismograph.
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How are the Effects Measured?
• The Mercalli Scale measures the earthquake’s effects on a scale of 1-12 (in Roman
Numerals).
– This is determined by scientists surveying the damage and then rating it on the
scale. This scale is VERY subjective!
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Earthquake Damage
• Most earthquake damage is caused by surface waves
•
• Tsunamis: when an earthquake occurs on the seafloor, the sudden movement pushes
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How are the Effects Measured?
• The Mercalli Scale measures the earthquake’s effects on a scale of 1-12 (in Roman
Numerals).
– This is determined by scientists surveying the damage and then rating it on the
scale. This scale is VERY subjective!
9
Earthquake Damage
• Most earthquake damage is caused by surface waves
•
• Tsunamis: when an earthquake occurs on the seafloor, the sudden movement pushes
against the water and creates a powerful wave
2/15/2017
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