1. Earthquakes

2. Earthquake
The shaking that results from the movement of rock beneath Earth’s surface
There are about _____ earthquakes per day, worldwide.
8,000

3. Where do Earthquakes Occur?
Earthquakes occur at tectonic plate boundaries, and along fracture zones (fault lines).
Red lines = plate boundaries Orange lines = fracture zones

4. The movement between plates and along faults is not smooth
The movement between plates and along faults is not smooth. The plates move in jerks, giving rise to earthquakes. The locations of earthquakes throughout the world mark the major tectonic boundaries.
The red dots indicate areas where earthquakes have occurred.

5. The movement of Earth’s plates creates powerful forces that ___ or ___ the rock in the crust.
squeeze
pull
Stress
Stress: A force (push or pull) that acts on rock to change its shape or volume

6. Compression
Stress that squeezes rock until it folds or breaks

7. Tension
Tension: Stress that stretches rock so that it becomes thinner in the middle

8. Shearing
Shearing: Stress that pushes a mass of a rock in opposite, horizontal directions

9. Deformation
Deformation: A change in the volume or shape of Earth’s crust (which causes it to bend, stretch, break, tilt, fold or slide)
Most changes in the crust occur so slowly that they cannot be observed directly

10. Review: How do stress forces affect rock?
The three kinds of forces that affect rock are:
Shearing
The rocks break and slip apart
Tension
The rock stretches and becomes thin in the middle
Compression
The rock squeezes until it folds or breaks
These stresses work over millions of years to change the shape and volume of rock

11. Faults
A break in the Earth’s crust, no more than 10 miles deep, where slabs of rock slip past each other
Faults occur when enough stress builds up in rock to make it move.

12. Why do faults form and where do they occur?
Faults usually occur along plate boundaries or at fracture zones, where the forces of plate motion compress, pull, or shear the crust so much that the crust breaks (deforms)
Rocks on both sides of the fault can move up or down, or sideways

13. Strike-Slip Faults
A type of fault where rocks on either side move past each other sideways with little up or down motion.
Shearing causes these types of faults

14. Normal Faults (sometimes called Thrust Faults)
A type of fault where the hanging wall slides downward
Tension forces cause normal faults

15. Hanging Wall & Footwall
Hanging wall: The block of rock that forms the upper half of a fault
Footwall: The block of rock that forms the lower half of a fault

16. Reverse Faults A type of fault where the hanging wall slides up
Compression forces cause reverse faults

17. Review: What are the three types of fault
Review: What are the three types of fault? What force of deformation produces each type?
Strike-slip faults
Produced by shearing
Normal faults
Produced by tension
Reverse faults
Produced by compression

18. Friction exists because…
What is friction?
A force that opposes the motion of one surface as it moves across another surface
Friction exists because…
surfaces are not perfectly smooth.

19. Describe what occurs when the friction along a fault line is low.
The rocks on both sides of the fault slide by each other without much sticking

20. Describe what occurs when the friction along a fault line is moderate.
The sides of the fault jam together
From time to time they jerk free
Small earthquakes occur

21. Describe what occurs when the friction along a fault line is high.
Both sides of the fault lock together and do not move
The stress increases until it is strong enough to overcome the force of friction
Larger and/or more frequent earthquakes will occur

22. The San Andreas fault in California is a transform boundary that contains ___ stress.
high

23. Fault-Block Mountain
A mountain that forms where a normal (thrust) fault uplifts a block of rock

24. How does the process of a fault-block mountain begin?
Where two plates move away from each other, tension forces create many normal faults
When two of these normal faults form parallel to each other, a block of rock is left lying between them
As the hanging wall of each normal fault slips downward, the block in between moves upward
When a block of rock lying between two normal faults slides downward, a valley forms

25. Focus
The point beneath Earth’s surface where rock breaks under stress and causes an earthquake

26. Epicenter
The point on Earth’s surface directly above an earthquake’s focus

27. Seismic Waves

28. Seismic Wave
A vibration that travels through Earth, carrying the energy released during an earthquake

29. At what point do seismic waves first reach the surface?
The epicenter

30. What determines how much the ground shakes during an earthquake?
How close a location is to the epicenter
The types of rock and soil surrounding the epicenter determines how much the ground shakes

31. There are three types of seismic waves:
Primary Waves (P waves)
Secondary Waves (S waves)
Surface Waves
An earthquake releases two types of waves: P waves and S waves
When the waves reach Earth’s surface at the epicenter, surface waves develop.

32. Primary Waves (P Waves)
A type of seismic wave that compresses and expands the ground
The first wave to arrive at epicenter

33. Secondary Waves (S Waves)
A type of seismic wave that moves the ground up and down or side to side
Second wave to arrive at epicenter

34. How are P waves different from S Waves?
P waves compress and expand the ground
P waves can travel through solids, liquids and gases
S waves move the ground up and down or side to side
S waves can only travel through solids

35. Surface Waves
A type of seismic wave that forms when P waves and S waves reach Earth’s surface
Surface waves are the most destructive of the 3 types of waves, because they move both sideways and up-and-down.

36. Why do you think surface waves produce more severe ground movements than P waves and S waves?
Surface waves travel through loose soil, sand, gravel, mud, and small rocks (not solid rock)
These looser substances are more likely to shift and slide

37. How does the energy of an earthquake travel through Earth?
Seismic waves carry the energy of an earthquake from the focus, through Earth’s interior, to the epicenter, and across the surface

38. Detecting Seismic Waves

39. Seismograph or Seismometer
A device that records ground movements caused by seismic waves as they move through Earth
iPhone seismometer-- yes, there’s an app for that!
The old-fashioned kind

40. Describe how a mechanical seismograph records ground movement.
A heavy weight attaches to a frame by spring or wire
A pen connected to the weight rests its point on a rotating drum
During an earthquake the seismic waves cause the drum to shake while the pen stays in place
The pen records lines on the paper around the drum

41. Measuring Earthquakes

42. Magnitude
The measurement of an earthquake’s strength based on seismic waves and movement along faults
Intensity
the strength of ground movement in a given place.

43. Mercalli Scale
A scale that rates earthquakes according to their intensity and how much damage they cause
Developed in the early twentieth century

44. The Mercalli scale has ______ steps and describes how an earthquake affects ________, ________, and the ________. 12
People
Buildings
Land surface

45. How would you rate the damage to the Foligno city hall on the Mercalli scale?
The damage would probably rate VII - VIII

46. Richter Scale
A scale that rates the amplitude (height) of seismic waves measured by a seismograph
It is a logarithmic scale, which means each level has 10 times the magnitude of the level below it.

47. What is a strength of the Richter scale when measuring earthquakes
What is a strength of the Richter scale when measuring earthquakes? What is a weakness?
Strength: Provides accurate measurements for small, nearby earthquakes
Weakness: Does not measure larger and more distant earthquakes well

48. How are the Mercalli scale and the Richter scale similar
How are the Mercalli scale and the Richter scale similar? How are they different?
Both measure the strength of an earthquake
The Mercalli scale measures the strength in terms of the amount of damage caused and the amount of shaking that people experienced
The Richter scale measures the size of an earthquake’s seismic waves

49. Moment Magnitude Scale
A scale that rates earthquakes by estimating the total energy released by an earthquake

50. Why is the moment magnitude scale used today by geologists to measure earthquakes?
The moment magnitude scale determines the total energy released by an earthquake
This scale uses a electronic seismograph that can measure earthquakes that are big or small, and near or far
Geologist examine movement along the fault and the strength of broken rock
These two measurements give a more accurate measurement of an earthquake

51. On which scale would an earthquake’s strength vary from one place to another? Explain.
The Mercalli scale, because the amount of shaking that people would feel and the damage to objects would be greater in a place closer to the earthquake’s epicenter

52. Review: What are three scales for measuring earthquakes
Review: What are three scales for measuring earthquakes? Explain what each one measures.
Mercalli Scale
Measures earthquakes on how much damage they cause
Richter Scale
Measures seismic waves using a seismograph
Measures small and nearby earthquakes
Moment Magnitude Scale
Looks at the total energy released
Measures large and distant earthquakes
Helps scientists predict how much fault movement there was

53. Locating the Epicenter

54. How do scientists calculate how far a location is from the epicenter of an earthquake?
Scientists calculate the difference between arrival times of the P waves and S waves
The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

55. Figure 17: Use the map scale to determine the distances from Savannah and Houston to the epicenter. Which one is closer?
Houston
800 Km
Savannah
900 km

56. Seismicity of the Caribbean plate

57. Haiti earthquake