Chapter 3

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 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