1. Chapter 3
Earthquakes

2. Learning Objectives
Understand how scientists measure and compare earthquakes
Be familiar with earthquake processes such as faulting, tectonic creep, and the formation of seismic waves
Know which global regions are most at risk for earthquakes and why they are at risk
Know and understand the effects of earthquakes such as shaking, ground rupture, and liquefaction

3. Learning Objectives, cont.
Identify how earthquakes are linked to other natural hazards such as landslide, fires, and tsunami
Know the important natural service functions of earthquakes
Know how human beings interact with and affect the earthquake hazard
Understand how we can minimize seismic risk, and recognize adjustments we can make to protect ourselves

4. Introduction to Earthquakes
There are many earthquakes in any given day
They are compared based on:
Magnitude, the amount of energy released
Intensity, the effects on people and structures

5. Table 3.1

6. Earthquake Magnitude They are mapped according to epicenter
Focus is directly below the epicenter
Measured by moment magnitude
Determined from area of rupture along, amount of slippage, and the rigidity of the rocks
Richter scale was previously used
Figure 3.2

7. Earthquake Magnitude, cont.
Both scales are logarithmic
Based on powers of 10
Ground displacement for a magnitude 3 earthquake is 10 times that for a magnitude 2
Ground motion is measured by seismograph
Related to magnitude, depth, and geologic setting
Table 3.2

8. Table 3.3

9. Earthquake Intensity Measured by Modified Mercalli Scale
Qualitative scale (I-XII) based on damage to structures and people’s perceptions
Modified Mercalli Intensity Maps show where the damage is most severe

10. Table 3.4

11. Shake Maps
Shake Maps use seismograph data to show areas of intense shaking
Figure 3.3

12. Earthquake Processes Earthquakes are distributed along faults
Places where rocks are broken and displaced
All plate boundaries are faults
Movement along faults are slip rates
Measured in mm/yr or m/1000yr
Sudden rupture of rock produces seismic waves
Release of stored energy

13. Figure 3.5

14. Fault Types—Strike-Slip
Crust moves in horizontal direction
Figure 3.6a

15. Fault Types—Dip-Slip Vertical movement
Include two walls defined by miners as:
Footwall where miners put their feet
Hanging-wall where they put their lanterns

16. Fault Types—Dip-Slip, cont.
Normal fault
Hanging wall moves down relative to footwall
Reverse fault
Hanging wall moves up relative to footwall
If angle is 45° or less it is a thrust fault
Blind faults do not extend to the surface

17. Figure 3.6b
Figure 3.6c

18. Figure 3.7

19. Fault Activity Active fault Potentially active faults Inactive
Moved during the past 10,000 years of the Holocene Epoch
Potentially active faults
Moved during the Pleistocene, but not the Holocene Epoch
Inactive
Not moved during the past 2 million years
Paleoseismicity of the fault

20. Table 3.5

21. Tectonic Creep and Slow Earthquakes
Tectonic creep occurs when movement is gradual such that earthquakes are not felt
Can produce slow earthquakes
Also called fault creep
Can slowly damage roads, sidewalks and building foundations
Can last from days to months
Magnitudes can be between M 6 and M 7

22. Seismic Waves—Body Waves
Caused by a release of energy from rupture of a fault
Travel through the body of the Earth
P waves, primary or compressional waves
Move fast with a push/pull motion
Can move through solid, liquid and gas
It is possible to hear them
S waves, secondary or shear waves
Move slower with an up/down motion
Can travel only through solids

23. Figure 3.9a, b

24. Seismic Waves—Surface Waves
Move along Earth’s surface
Travel more slowly than body waves
Move both vertically and horizontally with a rolling motion
Are responsible for most of the damage near epicenter
Love wave—horizontal ground shaking

25. Figure 3.9c

26. Earthquake Shaking Shaking experience depends on: Earthquake magnitude
Location in relation to epicenter and direction of rupture
Local soil and rock conditions

27. Distance to Epicenter
Both types of body waves are emitted from epicenter of quake
Seismographs record arrivals of waves to station site
Seismogram is the record of the waves
P and S waves travel at different rates and arrive at station at different times
Distance to epicenter can be found by comparing travel times of the waves

28. Figure 3.10c

29. Figure 3.10d

30. Location of Epicenter
At least three stations are needed to find exact epicenter
Distances from epicenter to each station are used to draw circles representing possible locations
The place where all three circles intersect is the epicenter
Process is called triangulation

31. Figure 3.11

32. Depth of Focus Depth of earthquake influences the amount of shaking
Focus is the place within the Earth where the earthquake starts
Deeper earthquakes cause less shaking at the surface
Loss of energy is called attenuation

33. Figure 3.12

34. Direction of Rupture
Direction that the rupture moves along the fault influences the shaking
Path of greatest rupture can intensify shaking
Directivity

35. Supershear
Occurs when the propagation of rupture is faster than the velocity of shear waves or surface waves produced by the rupture
Can produce shock waves that produce strong ground motion along the fault
May significantly increase the damage from a large earthquake

36. Figure 3.13

37. Local Geologic Conditions
Nature of the ground materials affects the earthquake energy
Different materials respond differently to an earthquake
Depends on their degree of consolidation
Seismic wave move faster through consolidated bedrock
Move slower through unconsolidated sediment
Move slowest through unconsolidated materials with high water content
Material amplification
Energy is transferred to the vertical motion of the surface waves

38. Figure 3.14

39. The Earthquake Cycle
There is a drop in elastic strain after an earthquake and a reaccumulation of strain before the next event
Strain is a deformation
Elastic strain is deformation that is not permanent
Stage 1: Period of inactivity along a segment of fault
Stage 2: Period of small earthquakes where the stress begins to release causing strain
Stage 3: Foreshocks occur prior to a major release of stress
Doesn’t always occur

40. The Earthquake Cycle, cont.
Stage 4: Mainshock and aftershocks where the fault releases all pent up stress releases the major quake
Cycle is hypothetical and periods are variable
Stages have been identified for many large earthquakes

41. Figure 3.19

42. Figure 3.20

43. Plate Boundary Earthquakes
Subduction Zones
Site of the largest earthquakes
Megathrust earthquakes
Example: Cascade Mountains
Convergence between a continental and oceanic plate
Example: Aleutian Islands
Convergence between two oceanic plates
Transform Fault Boundaries
Example: San Andreas Fault in California, Loma Prieta earthquake
Boundary between North American and Pacific plates

44. Intraplate Earthquakes
Earthquakes that occur within plates
New Madrid seismic zone
Located near St. Louis, MO
Historic earthquakes similar in magnitude to West Coast quakes
Earthquakes are often smaller than plate boundary quakes
Can be large and cause considerable damage due to lack of preparedness and because they can travel greater distances through stronger continental rocks

45. Effects of Earthquakes
Ground rupture
Displacement along the fault causes cracks in surface
Fault scarp
Shaking
Causes damage to buildings, bridges, dams, tunnels, pipelines, etc.
Measured as Ground Acceleration
Buildings may be damaged due to resonance
Matching of vibrational frequencies between ground and building

46. Effects of Earthquakes, cont. 1
Liquefaction
A near-surface layer of water-saturated sand changes rapidly from a solid to a liquid
Causes buildings to “float” in earth
Common in M 5.5 earthquakes in younger sediments
After shaking stops, ground re-compacts and becomes solid
Elevation changes
Regional uplift and subsidence
Can cause substantial damage on coasts and along streams

47. Effects of Earthquakes, cont. 2
Landslides
Earthquakes are the most common triggers in mountainous areas
Can cause a great loss of human life
Can also block rivers creating “earthquake lakes”
Fires
Displacements cause power and gas lines to break and ignite
Hard to put out because water lines are often broken
Disease
caused by a loss of sanitation and housing, contaminated water supplies, disruption of public health services, and the disturbance of the natural environment

48. Natural Service Functions
Water, oil, and natural gas may be rerouted due to faults
New mineral resources may be exposed
Scenic landscapes may form
Future earthquakes may be reduced due to release of energy

49. Human Interaction with Earthquakes
Loading Earth’s crust, as in building a dam and reservoir
The weight from water reservoirs may create new faults or lubricate old ones
Injecting liquid waste deep into the ground through disposal wells
Liquid waste disposals deep in the earth can create pressure on faults
Creating underground nuclear explosions
Nuclear explosions can cause the release of stress along existing faults

50. Minimizing the Earthquake Hazard
Focus of minimization is on forecasting and warning
National Earthquake Hazard Reduction Program Goals
Develop an understanding of the earthquake source
Determine earthquake potential
Predict effects of earthquakes
Apply research results

51. Estimating Seismic Risk
Hazard maps show earthquake risk
Probability of a particular event or the amount of shaking
Figure 3.29

52. Short-Term Prediction
Pattern and frequency of earthquakes
Foreshocks
Deformation of ground surface
Changes in land elevation
Seismic gaps along faults
Areas that have not seen recent quakes
Geophysical and Geochemical changes
Changes in Earth’s magnetic field, groundwater levels.

53. Earthquake Warnings Plan for issuing a prediction or warning
Earthquake warning systems
Figure 3.31

54. Community Preparations for Earthquake Hazard
Critical facilities must be located in earthquake safe locations
Requires detailed maps of ground response
Buildings must be designed to withstand vibrations
Retrofitting old buildings may be necessary
People must be prepared through education
Insurance must be made available

55. Personal Reactions and Preparation
Do a home inspection to make sure that your home is structurally sound
Secure large objects
Make a personal plan of how to react to a quake
Leave buildings AFTER shaking stops
Turn off main gas line
Move to an open area

56. End
Earthquakes
Chapter 3