Earthquakes

1. Earthquakes

2. 10.1 How & Where Earthquakes Occur

3. Earthquakes The shaking of Earth’s crust caused by the release of energy. • Over 3 million occur every year

4. Earthquakes The shaking of Earth’s crust caused by the release of energy. • Over 3 million occur every year • Most are too small to notice

5. Earthquakes The shaking of Earth’s crust caused by the release of energy. • Over 3 million occur every year • Most are too small to notice • Large earthquakes are extremely destructive and dangerous

6. Causes of Earthquakes Volcanic Eruption

7. Causes of Earthquakes Volcanic Eruption Collapse of a cavern

8. Causes of Earthquakes Volcanic Eruption Collapse of a cavern Impact of a meteor

9. Causes of Earthquakes Volcanic Eruption Collapse of a cavern Impact of a meteor Built up strain along fault lines (most major earthquakes)

10. Faults • A break in the lithosphere along which movement has occurred

11. Elastic Rebound Theory • Friction keeps the plates from moving all of the time

12. Elastic Rebound Theory • Friction keeps the plates from moving all of the time • Creates Strain

13. Elastic Rebound Theory • Friction keeps the plates from moving all of the time • Creates Strain • Plates begin to change shape

14. Elastic Rebound Theory • Friction keeps the plates from moving all of the time • Creates Strain • Plates begin to change shape • Eventually strain becomes too great and the plates shift = earthquake

15. Elastic Rebound Theory • Friction keeps the plates from moving all of the time • Creates Strain • Plates begin to change shape • Eventually strain becomes too great and the plates shift = earthquake • Plates snap into shape but at a new location

16. Focus • The point at which the first movement occurs during and earthquake • Where rock begins to move or break • Located many kilometers beneath the surface

17. Epicenter • The point on earth’s surface directly above the focus

18. Depth of Earthquake Type of plate boundary determines the depth of an earthquake origin • Transform Boundary = very shallow • Divergent Boundary= ~ 30 km down • Subduction Boundary = 700 km down Typically the shallower an earthquake, the more damage to structures it causes

19. Seismic Waves

20. Seismic Waves Energy that is released during an Earthquake travels in waves 2 Types • Body Waves: travel below the surface

21. Seismic Waves Energy that is released during an Earthquake travels in waves 2 Types • Body Waves: travel below the surface • Surface Waves: travel along Earth’s surface

22. Body Waves “P” Waves = primary or compression waves • Squeeze and stretch rock material • Can travel through any material: solid rock, magma, water, air • Faster • https://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html

23. Body Waves “S” Waves = secondary or shear wave • Move materials right and left • Can travel through solid materials but not through liquids or gas • Slower

24. Surface Waves When “P” and “S” waves reach the Earth’s surface, they produce “Surface Waves” Love Waves • Cause materials to move side to side, perpendicular to waves direction of travel

25. Surface Waves Rayleigh Waves • Slower than love waves • Moves in an elliptical (circular) pattern

26. 10.2 Locating & Measuring Earthquakes

27. Seismograph • An instrument used to detect and record waves • Are used to locate epicenter and measure the magnitude of Earthquakes • Over 10,000 stations around the world

28. Seismographs Different waves require 3 different seismographs • Records up and down motion • Records side to side – north and south • Records side to side – east and west

29. Seismographs Design • Heavy weight with a pen is attached to base • Base is anchored in bedrock • A record sheet (seismogram) is placed on a drum attached to the base • Drum is turned slowly by a clock • Earthquakes cause the drum to move and the pen to make zigzag marks

30. Interpreting a Seismogram 1st major zigzag is the “P” wave (fastest) 2nd major zigzag is the “S” wave Followed by surface waves

31. Interpreting a Seismogram As the P and S waves travel through the ground, the slower S wave lag farther and farther behind So, the larger the gap (lag time) between the 2 waves, the further away the epicenter is from the seismograph station

32. Travel Time Graph • Shows the relationship between P-wave and S-wave arrival times and the distance from the epicenter

33. Locating the Epicenter • If you have the distance traveled for 3 seismograph stations you can determine the location of the epicenter • Draw circles according to distance traveled • The spot where all 3 circles overlap is the epicenter

34. Earthquake Magnitude Magnitude = • the amount of energy released in an earthquake Richter Scale increases logarithmically

35. 10.4 Earth’s Interior

36. Earth’s Interior By studying earthquakes, scientists have learned about the structure of Earth’s interior. How? Observing and understanding the properties of S and P waves

37. Observations about P and S Waves • Velocities increase when the waves travel through more dense materials, and decrease when they travel through less dense materials. • S waves cannot travel through liquids

38. Velocity Example • At a depth of 2900 km, P waves slowed down and S waves stop • Suggests that the material is liquid (outer core) • At a depth of 5200 km, P waves velocity increases again • Suggests that this material is solid (inner core)

39. The Shadow Zone • A wide belt around the side of Earth opposite the focus of the Earthquake • Seismograph stations do not receive a signal if they are in the shadow zone

40. Why? • P waves pass through the mantle are refracted (bent) into arcs back to the surface • S waves cannot pass through liquid

41. The Moho • AndrijaMohorovicic – Croatian seismologist observed a change in velocities of S and P waves between the crust and the mantle • Determined that there is a boundary where the denser rock of the mantle meets the less dense rock of the crust • Located about 32 km under continents and 5-10 km under the ocean

42. The Transition Zone • A region in the middle of the mantle where changes in the P and S wave velocities occur • Separates the upper and lower mantle

43. 10.3 Earthquake Hazards

44. Ground Shaking and Foundation Failure • Earthquake waves cause the ground to move up and down and side to side • Side to Side motions cause more buildings to collapse

45. Liquefaction • Severe ground shaking can cause soil under buildings to settle or liquefy

46. Other Foundation Failures • San Francisco 1989– buildings constructed on landfill collapsed after the area liquefied • Mexico City 1985– buildings constructed on lake sediments collapsed

47. Aftershocks • Large earthquakes can be followed by a series of smaller ones • Can be as many as 1000 per day • Can cause damage to building and other structures, especially ones already weakened by the ground shaking

48. Fire • Earthquakes can cause gas lines and water lines to break • San Francisco 1906 earthquake fire destroyed over 3000 homes and burned 11 sq km of the city

49. Tsunamis • Underwater Earthquakes and landslides can cause huge waves • Tsunamis can travel very quickly over large expanses of water • Scientists try to predict tsunamis by monitoring wave activity

50. Preventing Earthquake Damage • Earthquake-prone areas have specific building codes • Engineers and scientists are continually trying to develop buildings that can withstand earthquakes