Chapter 5 Earthquake Notes
What are Earthquakes?
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The shaking or trembling caused by the sudden release of energy
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Usually associated with faulting or breaking of rocks
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Continuing adjustment of position results in aftershocks
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Near plate boundaries, there are many breaks in the rock called faults
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Over time, as the plates gradually get pushed along, more and more pressure builds up at the faults.
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Elastic rebound occurs when a fault can’t take any more pressure and snaps.
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We experience this motion as an earthquake.
Seismology
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The study of earthquakes
Seismologist
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scientist who studies earthquakes and seismic waves
Fault
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a break in the crust along which plates shift and slip past each other
Deformation
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rocks changing shape due to stress from tectonic motion
• Plastic deformation - Stress causing a fold (ex: metal bending)
• Elastic deformation - Stress causing a fold then a break or earthquake (ex: rubber band breaking OR stick
breaking)
• Elastic deformations are the ones that cause earthquakes!
Where Do Earthquakes Occur and How Often?
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~80% of all earthquakes occur near the Pacific Ocean (Ring of Fire)
most of these result from convergent activity
more than 150,000 quakes strong enough to be felt are recorded each year
What causes an earthquake?
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The build-up of stress along a fault line causing elastic deformation & then an earthquake (elastic rebound)
Elastic rebound
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stress builds up to the point where the rock breaks and snaps back
Explains how energy is stored in rocks
Rocks bend until the strength of the rock is exceeded
Rupture occurs and the rocks quickly rebound to an un-deformed shape
Energy is released in waves that radiate outward from the fault
Differentiate between an Earthquake’s focus and its epicenter
Seismic Waves
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Earthquake vibrations that travel through the Earth
Different types of seismic waves travel through the Earth’s layers at different speeds
When an earthquake occurs, seismic waves are shot out in all directions:
• Some travel across the Earth’s surface (surface waves)
• Some travel through the Earth (body waves)
P- Waves
Full name
Where they travel
Relative speed
Type of motion
S-Waves
L-Waves
Locating the epicenter:
Step 1: a _____________ is drawn around a seismograph station. The _____________ of the circle equals the
distance from the seismograph to the epicenter (distance is taken from the time-distance graph)
Step 2: When a ______________ circle is drawn around another seismograph station, the circle
__________________ the first circle in two spots. One of these spots is the earthquake’s epicenter.
Step 3: When a circle is drawn around a ________________ seismograph station, all three circles _________________
in one spot- the earthquake’s epicenter. In this case, the ______________________ was in San Fransico.
1. What does the red line in the graph represent?
2. What does the purple line represent?
3. Which location is the farthest away from the
earthquake’s epicenter?
4.
What is the S-P time interval (ΔT) for location C?
5. Which location is the closest to the earthquake’s
epicenter?
6. How far is station A from the epicenter?