EARTHQUAKES

1. EARTHQUAKES

2. Rocks move along faults… • A fault is a fracture or break in the Earth’s lithosphere where blocks of rock move past each other. • Along some parts of the fault, the rock on either side may slide along slowly and constantly. • Along other parts of the fault, the rocks may stick or lock together. • Rocks bend as stress is put on them. • As the stress increases, the rocks finally break free.

3. Stress in Lithosphere = Earthquake • A sudden release of stress in the lithosphere causes an earthquake. • An earthquake is a shaking of the ground caused by sudden movement of large blocks of rock along the fault. • Most faults are located along plate boundaries, so most earthquakes occur in these areas.

4. About 80% of the earthquakes occur in the belt around the edges of the Pacific Ocean.

5. What is the best known fault in this belt in the United States? • San Andreas Fault in California. • It forms part of the boundary between the North American Plate and the Pacific Plate. • Unlike many other faults, parts of the San Andreas Fault can be seen on the surface of the ground.

7. Strength of an earthquake • The strength of an earthquake depends in part on… • How much stress builds up before the rocks move • The distance the rocks move along the fault

8. 3 Types of Faults • Normal Faults • Reverse or Thrust Faults • Strike-Slip Faults • The blocks of rock along different types of faults move in different directions, depending on the kinds of stress they are under. • More than one type of fault may be present along the same plate boundary.

9. Normal Faults • Earthquakes along normal faults are common near boundaries where tectonic plates are moving apart. (Divergent Boundaries) Example: Great Rift Valley of Africa Pull Pull Direction rocks move

10. Reverse or Thrust Faults • Stress that presses rock together causes reverse faults. (Convergent Boundaries) • The Himalaya Mountains, which rise in the area where the Indian Plate is pushing into the Eurasian Plate, have may earthquakes along reverse faults. Direction rocks move Push

11. Strike-Slip Faults • These faults can occur where plates scrape past each other. (Transform Boundaries) • The San Andreas Fault is a strike-slip fault.

12. Earthquakes Release Energy • Earthquake energy travels outward in all directions – up, down, and to the sides. • The energy travels as seismic waves, which are vibrations caused by earthquakes. • All earthquakes start beneath Earth’s surface. • The focus of an earthquake is the point underground where the rock first begins to move. • Seismic waves travel outward from the earthquakes focus. • The epicenter is the point on the Earth’s surface directly above the focus.

14. Earthquakes produce 3 types of seismic waves… • Primary Waves • Secondary Waves • Surface Waves • Each type moves through material differently. • Scientists learn about Earth’s layers by studying the paths and speeds of seismic waves traveling through Earth.

15. Primary Waves, P Waves • These are the fastest waves that are the first to reach any particular location after an earthquake occurs. • Primary waves can travel through solids, liquids, and gases. • Primary waves = P waves

16. Secondary Waves, S Waves • Secondary waves travel through Earth’s interior at the same time as primary waves, but at about half the speed. • Therefore, they are the second waves to arrive at any particular location after an earthquake. • Secondary waves can travel through rock, but unlike primary waves they cannot travel through liquids or gases. • Secondary waves = S waves

17. Surface Waves • Surface waves move along Earth’s surface, not through its interior. • They make the ground roll up and down or shake from side to side. • Surface waves cause the largest ground movements and the most damage. • They travel more slowly than the other types of seismic waves.

19. Seismic waves can be measured… • There are seismic stations all over the world where ground movements are measured. • A seismograph is an instrument that constantly records ground movement. • The recording produced by a seismograph is called a seismogram. • By studying seismograms, scientists can determine the locations and strengths of earthquakes.

20. Seismograph and Seismogram • The height of the wiggles on the seismogram indicates the amount of ground movement produced by seismic waves at the seismograph’s location.

21. Interesting Facts… • Most injuries and deaths due to earthquakes are not directly caused by the movement of the ground. • They are caused by collapsing buildings and other structures and by fires. • After an earthquake, fires may start due to broken natural-gas lines, broken electrical power lines, or overturned stoves.

22. How do we measure an earthquakes magnitude? • The Richter Scale : An earthquake’s magnitude is based on how fast the ground moves at the seismic station. • The Modified Mercalli Intensity Scale: The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction.

24. http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swfhttp://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

25. Review Questions… • What is a fault? • How does an earthquake occur in the lithosphere? • What are the 3 types of faults? How are they different? • What are seismic waves? • What are the 3 types of seismic waves? How are they the same and how are they different? • How are seismic waves measured? • How do we measure an earthquakes magnitude?