1. Earthquakes (Chapter 8)
Earth Science
Earthquakes
(Chapter 8)

2. Earthquakes General features
Vibration of Earth produced by the rapid release of energy.
Focus – the place within Earth where earthquake starts.
Epicenter - Point on the surface, directly above the focus
Caused by slippage along a break in the lithosphere, called a fault.
Fault: Fracture in earth where movement has occurred.

3. Earthquake focus and epicenter

4. Earthquakes Types of Faults (draw and label each fault type on right)
Normal Fault: Fault in which the rock above the fault plane has moved down relative to the rock below.
Reverse Fault: Fault in which the material above the fault plane moves up relative to the material below.

5. Earthquakes Types of Faults (draw faults on right)
3. Thrust Fault: Reverse fault with a dip less than 45 degrees.
4. Strike-slip fault: A fault along which the movement is horizontal and parallel to the trend of the fault.

6. The Cause of Earthquakes
Deformation of Rocks
Forces within Earth slowly deform the rock that makes up Earth’s crust causing the rock to change shape, or bend on both sides of a fault.
When the rocks bend they store energy, much like a rubber band when stretched.

7. The Cause of Earthquakes
Elastic Rebound sequence of events…
Stored energy builds as the rock is deformed.
Suddenly the rocks slip, or break at their weakest point (the focus) and the stored energy is released all at once.
The deformed rock ‘springs back’ to its original shape. This is called elastic rebound.

8. Elastic Rebound

9. Earthquakes General features
Earthquakes are often preceded by foreshocks and followed by aftershocks.

10. Earthquakes Earthquake waves
Study of earthquake waves is called seismology
Earthquake recording instrument (seismograph)
Records movement of Earth
Record of Earth’s movement is called a seismogram

11. Seismograph

12. Earthquakes Earthquake waves Types of earthquake waves P - waves
Push–pull (compressional) motion
Travel through solids, and liquids
Greatest velocity of all earthquake waves

13. P - waves

14. Earthquakes Earthquake waves Types of earthquake waves S - waves
Up-down motion
Travel through solids, but NOT liquids.
S - waves travel slower than P-waves

15. S - waves

16. Earthquakes Earthquake waves Types of earthquake waves
Surface waves (occurs when an S-wave reaches the surface)
Travel slower than P-waves and S-waves
Move up and down as well as side to side (similar to an ocean wave)
Most destructive seismic wave

17. A seismogram records wave amplitude vs. time

18. Earthquakes Earthquake intensity and magnitude Intensity Magnitude
A measure of the degree of earthquake shaking at a given locale based on the amount of damage
Most often measured by the Modified Mercalli Intensity Scale
Magnitude
Concept introduced by Charles Richter in 1935

19. Earthquakes Earthquake intensity and magnitude Magnitude (M)
Often measured using the Richter Scale
Based on the amplitude of the largest seismic wave
A tenfold increase in wave height equals an increase of 1 on the Richter scale.
Does not estimate adequately the size of very large earthquakes.
Scientists no longer use it routinely.

20. Earthquakes Earthquake intensity and magnitude Magnitude
Moment magnitude scale
Measures very large earthquakes
Derived from the amount of displacement that occurs along a fault zone
More precise than the Richter Scale

21. Magnitude 5 or greater earthquakes over 10 years

22. Earthquakes Locating an earthquake
Located using the difference in the arrival times between P and S wave recordings, which are related to distance (Travel-time graph)

23. Travel-time graph
How long does it take a P wave and and S wave to travel 1000 km?
P-wave =
S-wave =
Which seismic wave is faster?

24. Earthquakes Locating an earthquake Epicenter
At least three station recordings are needed to locate an epicenter
Circle equal to the epicenter distance is drawn around each station
Point where three circles intersect is the epicenter

25. The epicenter is located using three or more seismic stations

26. Earthquakes Causes of Earthquake Damage Earthquake-related hazards
Seismic shaking
Liquefaction
Landslides
Mudflows
Tsunamis

27. Earthquakes Earthquake destruction Destruction results from …
Seismic shaking (the ground vibrating)
May jolt and twist structures
Unreinforced brick or concrete structures may collapse
Liquefaction of the ground
Saturated soil material turns fluid
Underground objects may float to surface

28. Damage caused by the 1964 earthquake in Alaska

29. Damage from the 1964 Anchorage, Alaska, earthquake

30. Earthquakes Earthquake destruction Destruction resulting from…
Landslides and Mudflows
Movement of loose rock and soil down slope
If the water content of the soil is high an earthquake can start a mudflow

31. Landslide

32. Mudflow

33. Earthquakes Earthquake destruction Destruction resulting from…
Tsunamis
Ocean wave formed when the ocean floor suddenly shifts.
Move fast (100+ km/hr) across the open ocean
A Tsunami slows down and increases in size as it approaches shore

34. Formation of a tsunami

35. Tsunami travel times to Honolulu

36. Earthquakes Assessing Earthquake risk
Earthquakes are more frequent along the boundaries of Earth’s Tectonic plates.
No reliable method of predicting an Earthquake occurrence in the near future.