1. Section 6.1 Objectives 1. Discuss the elastic rebound theory.
2. Explain why earthquakes generally occur at plate boundaries.

2. Earthquakes & Plate Tectonics
Earthquakes (EQs): vibrations of the Earth’s crust
Occur when rocks under stress suddenly shift

3. Elastic Rebound Theory: the theory geologists use to explain many EQs
Rocks on each side of fault are moving
If fault is locked, stress increases
At certain point rocks fracture & rebound

4. Focus: the area along a fault where slippage 1st occurs.
Depths vary

5. Epicenter: the point on the Earth’s surface directly above the focus.

6. Aftershocks: series of smaller tremors.
Caused by released energy (seismic waves)
Increases stress in other rocks along the fault

7. Focus depths vary Shallow focus - w/in 70 km of surface
Tend to cause the most damage

8. Focus depths vary
Intermediate focus - 70 to 300 km

9. Focus depths vary
Deep focus (occur in subduction zone)
300 to 650 km

10. Focus depths vary
About 90% of Continental EQs are shallow focus

11. 3 Major EQ Zones Links btwn EQs & Plate Tectonics
Plate movements = stress buildup

12. Pacific Ring of Fire: ring of tectonic/volcanic activity surrounding the pacific.

13. Mid-ocean ridges: spreading motion = stress

14. Eurasian-Melanesian mountain belt: Eurasian plate colliding w/African & Indian plates = stress

15. Fault Zones: are groups of interconnected faults.
Form at plate boundaries b/c of intense stress
e.g. San Andreas fault

16. Not all EQs occur at plate boundaries!
e.g. New Madrid, Missouri, 1812
Ancient fault? (600 million yrs ago)