1. Chapter 6, Lesson 1, Earthquakes and Plate Boundaries

2. Earthquakes and Plate Boundaries Slide 1
*What is the main idea?
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-What is an earthquake?
*Most earthquakes occur at plate boundaries when rocks break and move along faults.
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-An earthquake is the rupture and sudden movement of rocks along a fault.
-A fault ruptures, or breaks, when rocks are strained so much that they can no longer stretch or bend.
-This movement causes the release of complex waves that can shake objects.

3. Earthquakes and Plate Boundaries Slide 2
-What is an earthquake? (continued)
-Most earthquakes occur in Earth’s crust, although some happen at great depths where lithospheric plates subduct.

4. Earthquakes and Plate Boundaries Slide 3
*What is elastic strain energy?
*Some of the heat energy from Earth’s interior is transformed into kinetic energy, or energy of motion, for Earth’s lithospheric plates.
*The plates’ kinetic energy is transferred to rocks near the faults.
*This energy is eventually released as earthquakes.
*Energy stored as a change in shape is called elastic strain.
*When rocks cannot stretch to change shape, faults break and become earthquakes.

5. Earthquakes and Plate Boundaries Slide 4
*What is elastic strain energy? (continued)

6. Earthquakes and Plate Boundaries Slide 5
-How are faults and earthquakes related?
-As rocks slowly move past each other, elastic strain energy builds up along a strike-slip fault.
-Eventually, rocks rupture and slip along a fault sending complex waves radiating out in all directions.
-It is the energy in the waves that causes the shaking during an earthquake.

7. Earthquakes and Plate Boundaries Slide 6
*What is the focus of an earthquake?
*Earthquakes start at the focus which is the location on a fault where rupture and movement begin.
*In general, the closer the focus is to Earth’s surface, the stronger the shaking will be.
*Larger faults can have larger ruptures, which tend to produce larger earthquakes.

8. Earthquakes and Plate Boundaries Slide 7
*What is the focus of an earthquake? (continued)

9. Earthquakes and Plate Boundaries Slide 8
-What are fault zones?
-Instead of a single fault, plate boundaries are usually zones.
-These fault zones are about km wide.
-The San Andreas Fault is an example of a fault zone.
-It is a group of faults that make up the San Andreas Fault.

10. Earthquakes and Plate Boundaries Slide 9
*How are plate boundaries and earthquakes related?
*Lithospheric plates interact at different boundaries and produce earthquakes.
*Earthquake size and depth, and the types of faults on which earthquakes occur depend on the type of plate boundary.
*1. Divergent Plate Boundaries-rocks break under tension stress forming normal faults. Most earthquakes occur in the crust at relatively shallow depths and are relatively small in size.
*2 Convergent Plate Boundaries-rocks break under compression stress, forming reverse faults. The deepest and most devastating earthquakes occur with convergent plate boundaries.

11. Earthquakes and Plate Boundaries Slide 10
*How are plate boundaries and earthquakes related? (continued)
*3. Transform Plate Boundaries-rocks slide horizontally past one another, forming strike-slip faults. These earthquakes, also, occur at shallow depths. They can cause major earthquakes.

12. Earthquakes and Plate Boundaries Slide 11
-What happens with earthquakes away from plate boundaries?
-Most earthquakes occur along plate boundaries, but there are some that occur away from plate boundaries.
-Most of these earthquakes occur in the middle of continents.
-Even though these earthquakes do not occur often, they can be dangerous.
-One idea, from scientists, is that there are old buried faults within continents
-For example, in the winter of 1811, three large earthquakes shook New Madrid, Missouri, far from any plate boundary.

13. Earthquakes and Plate Boundaries Slide 12
*Summary
*Earthquakes occur when elastic strain energy builds up to the point that rocks break and move.
*Boundaries between lithospheric plates are locations where stresses cause rocks to deform, or become strained, as plates move relative to one another.
*At convergent, divergent, and transform plate boundaries, faults are common.
*Some earthquakes also occur along faults located in the middle of plates, far from present-day plate boundaries.

14. Questions???
What is the rupture and sudden movement of rocks along a fault?
Where do most earthquakes occur?
What is the location on a fault where rupture and movement begin?
The deepest and most devastating earthquakes occur at which type of plate boundary?
Where, in the winter of 1811, did three large earthquakes shake in the United States?