1. EARTHQUAKES
When good rock goes bad!

2. An Earthquake begins at the…
Focus: The point where the energy is released after elastic limit is reached.
Epicenter: The point on the surface of the Earth directly above the focus.

3. EARTHQUAKES
Shaking of the ground caused by sudden release of energy stored in rocks.

4. A force that acts upon a rock to change its shape or volume
STRESS!
A force that acts upon a rock to change its shape or volume

5. Stress Types
Compression - pushing together
Add compression…

6. Stress Types
Tension - pulling apart
Add tension…

7. Stress Types
Shearing – pushing in opposite directions
Add shearing…

8. Fault Terminology

9. Types of Faults
Normal Fault – results from tensional stress, hanging wall moves down relative to foot wall

11. Types of Faults
Reverse Fault – results from compressional stress, hanging wall moves up relative to foot wall

14. Types of Faults
Strike-Slip Fault – results from shearing stress, rocks on either side of fault slip past each other sideways with little motion up or down

15. Offset produced by 1906 San Francisco quake

17. Focus and Epicenter

19. Types of seismic waves:
Primary Waves (P-Waves)
Secondary Waves (S-Waves)
Surface Waves (Love and Rayleigh)

20. Primary Waves (P-Waves)
The fastest wave, they arrive 1st
Compressional motion in the wave (push-pull)
Vibration is parallel to the direction of wave propagation

21. Primary (P) Waves: Move out from the earthquake focus.
Travel the fastest of the 3 waves.
Travel twice as fast as secondary waves.
Move by causing particles in rocks to move back and forth in the same direction that the wave is traveling.
Example: slinky
Are bent and slowed when
they hit the outer core.
Longitudinal wave Blue-

22. Secondary Waves (S-Waves)
Shear waves (side-side)
Vibration is perpendicular to the direction of wave propagation

23. Surface Waves Slowest and most destructive
Rayleigh Waves: elliptical motion
Love Waves: horizontal motion (perpendicular to travel)

24. Secondary (S) Waves: Move out from the earthquake focus.
Move slower than primary waves.
Move by causing particles in rocks to move at right angles to the direction of wave travel.
Example: rope
Cannot travel through liquids,
so they are stopped by the outer
core.
Transverse Wave Red-

25. Surface Waves: Form when P and S waves reach the surface.
Slowest Waves, Most destructive
Can cause the ground to shake making rock roll and sway from side to side.
Only travel
through crust

27. How are earthquakes detected?
Seismographs

28. Seismograph:
instrument used to record the energy released by an earthquake. Recording time of wave arrival.
Produces paper sheet called a seismogram
A stationary pen traces a record of vibrations

29. Seismograph Stations P waves arrive first S waves arrive second
Surface waves arrive last (slowest)
3 or more seismograph stations are needed to determine the location of the epicenter.
When an epicenter is far from a location, the p wave has more time to put distance between it and the s and surface waves.

30. Reading a Seismogram

31. Calculating lag time (oh no, more math!)
To calculate lag time,
simply subtract arrival
time of S-wave from
arrival time of P-wave.
7:14.2
7:17.4
P-wave
arrival time
S-wave
arrival time
S - P = 7:17.4 – 7:14.2 = 3.2 minutes

32. 3:00
4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
13:00
14:00
15:00
1:00
2:00
16:00
17:00
18:00
19:00
20:00
21:00

34. P wave arrival = Lag time = S wave arrival = L wave arrival = 5:00
6:00
7:00
8:00
9:00
P wave arrival =
S wave arrival =
L wave arrival =
Lag time =

36. Finding the Epicenter

37. Shadow Zone Animation

38. How damage occurs in an earthquake
SHAKING some areas shake more than others
unconsolidated sediments
landfill
wetlands
LIQUIFACTION
water rises to
Surface of sediments

52. August 31, Charleston, SC

53. Charleston, SC August 31, 1886 ~ 6.8 - 7.2 - 7.6 magnitude
over 60 people died
felt from NEW YORK to CUBA; from BERMUDA to MISSISSIPPI RIVER
Wooden houses did better than brick - why?
Damage greatest on ‘made ground’ - why?
Sand/mud volcanoes common; some fissures

54. Fissures

55. Offset Rail Road Tracks

56. Broad Street

57. East Bay Street

58. College of Charleston

59. Destruction was random

60. Approximately 6 minutes
S – P = approx. 6 minutes
Find that lag time difference between the
S & P arrival time from seismic velocity graph, then come straight down to find the distance that station was to the earthquake
Approximately 6 minutes

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