Earth Systems 3209

1. Earth Systems 3209 Reference: Chapters 4, 15, 16, 19; Appendix A & B Unit: 4The Forces Within Earth

2. Unit 4: Topic 4.2 Causes and Locations of Earthquakes Focus on . . . • describing the causes of an earthquake. • discussing the relationship between earthquakes and the elastic rebound theory. • identifying the environments where earthquakes occur and relating them to locations along plate boundaries .

3. Cause of Earthquakes • What is an Earthquake? • An earthquake is the vibration of Earth produced by the sudden, rapid release of energy. • Seismology • When an earthquake occurs, the elastic energy is released and sends out vibrations that travelthroughout the Earth. These vibrations are called seismic waves. The study of how seismicwaves behave within Earth is called seismology. Text Reference: Pages 441-443

4. Earth’sSurface Epicenter Fault Plane Focus and Epicenter • Earthquake Focus and Epicenter? • Focus: • The exact location within Earthwere seismic waves originate and are generated bysudden release of stored elasticenergy. Most often located on a pre-existing fault. • Epicenter • The point onthe surface of Earth directly abovethe focus. Focus

5. Cause of Earthquakes • Earthquakes occur when energy stored in elastically strained rocks is suddenly released. • Thisrelease of energy causes intense ground shaking in the area near the source of the earthquake (Focus) and sends waves of elastic energy, called seismic waves, in all directions throughoutEarth. • Earthquakes canbe generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. • Earthquakesare definitely a geologic hazard for people living in earthquake regions, but the seismicwaves generated by earthquakes are invaluable for studying the interior of the Earth.

6. Cause of Earthquakes • How does an Earthquakes Originate? • Movement in areas along the fault plane stops (fault sticks). • Elastic energy is stored in the rock as the rock becomes deformed and bends, much like a bent stick. • When the elastic strain built up along the fault exceeds the elastic limit, the rock will break or slip at its weakest point which we call the the focus. • This slippage along the fault allows the rock to “snap back” and the vibrations sends out waves of energy in all directions called seismic waves, or earthquake waves. • The springing back of the rock is called “elastic rebound”.

7. Original position of rocks before any strain Elastic Rebound Theory • The mechanism that cause earthquakes was not understood until H. F. Reid proposed his idea of “Elastic rebound.” • Reid suggested that most natural earthquakes are caused by suddenslippage along a fault zone.

8. Fault A feature across the fault bends as the rocks on both sides of the fault pushes in opposite directions and elastic strain builds up Elastic Rebound Theory • The elasticrebound theory suggests that if movement alonga fault gets stuck, elastic strainenergy builds up deforming rocks oneither side of the fault.

9. Fault The fault ruptures releasing elastic energy Energy Released Elastic Rebound Theory • When the rocks along the fault can no longer hold the strain slippage occurs at the weakest point along a fault (focus) which causes rock to start moving on both sides of the fault. • Energy is released in all directions causing anearthquake.

10. Fault Elastic strain on rocks released Elastic Rebound Theory • When the elastic strain is released the rocks on both sides of the fault will “snap-back” to their unstrained positions.

11. Location of Earthquakes • Earthquake Belts: • About 95% of the energy released by earthquakes originate in a few narrow zones that wind around the globe. • The greatest concentration of earthquakes are located along the outer edge of the Pacific Ocean in an area known as the “Pacific Ring of Fire.”

12. Location of Earthquakes • Earthquake Depths: • Seismic records show that earthquakes originate at depths ranging from 5km to 700km and can be classified according to depth of occurrence. • 90% of all earthquakes occur at depths less than 100km. • Depths are classified as follows: • Shallow Focus (less than 70 km) • Intermediate Focus (70km to 300km) • Deep Focus (greater than 300km) Text Reference Pages 447-451

13. Location of Earthquakes • Earthquake Depths: • Earthquakes can occur at any of the three plate boundaries. • Divergent Plate Boundary: • Earthquakes generated alongoceanic ridge systems alwayshave a shallow focus and arenot very strong. • Example: Mid Atlantic Ridge.

14. Location of Earthquakes • Earthquake Depths: • Earthquakes can occur at any of the three plate boundaries. • Transform Plate Boundary: • Earthquakes generated alongtransform plate boundaries, haveshallow and very powerful earthquakes. • Example: San Andreas Fault.

15. Location of Earthquakes • Earthquake Depths: • Earthquakes can occur at any of the three plate boundaries. • Convergent Plate Boundary: • Earthquakes generated along convergent plateboundaries range in depths and are very powerful. • Focus depth increases with distance away formthe trench ranging from shallow to deep. Thisearthquake zone is called the Wadati-Benioff Zone. • Example: Japan Trench.

16. Example 1: The most common cause of earthquakes is; the collapse of limestone caverns on the borders of continents. the increasing stress between tectonic plates. energy released by volcanic eruptions. the gravitational pull of the moon and sun on the continental landmasses.

17. Example 2: At which of the following locations would you least likely find earthquakes? (A) transform boundary divergent boundary subduction zone (D) nuclear testing site

18. Your Turn . . . Take the time and complete the following questions . . .(Solutions to follow) Question: Earthquakes commonly occur at plate boundaries, with reference to elastic rebound, what causes an earthquake?

19. Solutions . . . Question: Earthquakes commonly occur at plate boundaries, with reference to elastic rebound, what causes an earthquake? Answer: There is a build up of stress (energy) within the tectonic plates, where it reachesthe elastic limit. The plates then rupture (snap) and the stored energy is released.Plates then move (snap) back to unstressed positions releasing the stored energyas seismic waves. This is what we call an earthquake.

20. Summary . . . Overview of Points covered: • Elastic rebound can explain the cause of earthquakes • Earthquakes are limited to narrow areas around the globe. Example: Pacific Ring of Fire • Earthquakes exist at all plate boundaries. • Earthquake depths vary from shallow, intermediate, to deep.