1. Earthquakes and Earth’s Interior
Chapter 9
Earthquakes and Earth’s Interior

2. Introduction
Earthquake – the sudden release of energy, usually along a fault, that produces shaking or trembling of the ground
Most occur at plate boundaries
Fig. 9.1 b, p. 201

3. Introduction
Earthquakes are very destructive and cause many deaths and injuries every year.
Knowing what to do before, during, and after an earthquake could save your life or prevent serious injury.
Fig. 9.15, p. 213

4. Introduction
Some Significant Earthquakes
Table 9.1, p. 200

5. Explains how energy is released during an earthquake
Elastic Rebound Theory
Explains how energy is released during an earthquake
Rocks deform or bend
Rocks rupture when pressure accumulates in rocks on either side of a fault and build to a level which exceeds the rocks' strength.
Fig. 9.1a, p. 201

6. Explains how energy is released during an earthquake
Elastic Rebound Theory
Explains how energy is released during an earthquake
Finally, rocks rebound and return to their original shape when the accumulated pressure is released.
Fig. 9.1a, p. 201

7. Rocks rebound to original undeformed shape
Fence
Original position
Fault
Deformation
Rocks rebound to original undeformed shape
Rupture and release of energy
Stepped Art
Fig. 9-1a, p. 201

8. Seismology Seismology - study of earthquakes
The record of an earthquake, a seismogram, is made on a seismograph.
Fig. 9.2 a-b, p. 202

9. Seismology The Focus and Epicenter of an Earthquake
The point where an earthquake's energy is released is known as the focus.
The epicenter is that point on the surface vertically above the focus.
Fig. 9.3, p. 203

10. Seismology
Relationships exist between the type of plate boundary and focal depth. The foci of Benioff zones mark the location of earthquakes on a subducting plate.
Fig. 9.5, p. 204

11. Where Do Earthquakes Occur, and How Often?
About 80% of all earthquakes occur in the circum-Pacific belt.
15% within the Mediterranean-Asiatic belt
5% occur largely along oceanic spreading ridges or within plate interiors.
Fig. 9.4, p. 204

12. Where Do Earthquakes Occur, and How Often?
More than 900,000 earthquakes occur per year, with more than 31,000 of those strong enough to be felt.
Fig. 9.4, p. 204

13. Where Do Earthquakes Occur, and How Often?
Fig. 9.6, p. 205

14. Seismic Waves
Seismic waves cause most of the damage and shaking people feel during an earthquake.
Two kinds of seismic waves:
Body waves travel through Earth.
Surface waves travel along, or just below, the surface.

15. Seismic Waves Two Types of Body Waves:
P-waves are compressional waves and travel faster than S-waves.
S-waves are shear waves that cannot travel through liquids.
Fig. 9.7, p. 206

16. Direction of wave movement
Undisturbed material
Primary wave (P-wave)
Compression
Expansion
Undisturbed
material
Direction of wave movement
Focus
Surface
Secondary wave (S-wave)
Wavelength
Stepped Art
Fig. 9-7, p. 206

17. Seismic Waves Surface waves
Surface waves are divisible into two types, Rayleigh and Love waves
Fig. 9.8, p. 207

18. Rayleigh wave (R-wave)
Undisturbed material
Rayleigh wave (R-wave)
Rayleigh wave
Love wave
Love wave (L-wave)
Stepped Art
Fig. 9-8, p. 207

19. Locating an Earthquake
First, measure the times of the first arrival of the P and S waves on the seismogram
Fig. 9.9a, p. 208

20. Locating an Earthquake
Then, plot on a time- distance graph the difference in the arrival times of the P- and S-waves. This is used to determine the distance from the seismograph to the focus.
Fig. 9.9b, p. 208

21. Locating an Earthquake
Finally, plot the distances from three seismograph stations on a map
Three seismograph stations are required.
They will intersect at the epicenter of the earthquake.
Fig. 9.10, p. 209

22. Measuring the Strength of an Earthquake
Intensity and magnitude are the two common measures of an earthquake’s strength.
Intensity is a qualitative measurement
Magnitude is a quantitative measurement

23. Measuring the Strength of an Earthquake
Intensity
An earthquake's intensity is expressed with the Modified Mercalli Intensity Scale, which uses a Roman Numeral scale of I to XII. Intensity is a measure of the kind of damage which occurs.
Table 9.2, p. 209

24. Table 9.2, p. 209

25. The Destructive Effects of Earthquakes
Relationship between Intensity and Geology of the 1906 San Francisco Earthquake
Factors that determine an earthquake’s intensity include:
distance from the epicenter
focal depth
population density

26. The Destructive Effects of Earthquakes
geology of the area
type of building construction
the duration of ground shaking
Fig. 9.11, p. 210

27. Fig. 9.11, p. 210

28. Measuring the Strength of an Earthquake
Magnitude - The magnitude of an earthquake is a measure of the amount of energy which is released.
Richter magnitude
Determined by measuring the amplitude of the largest seismic wave recorded on a seismogram
The height of the largest amplitude is converted to a numeric magnitude value using a conventional base- 10 logarithmic scale
Figure 9.12, p. 211

29. Measuring the Strength of an Earthquake
Magnitude - The magnitude of an earthquake is a measure of the amount of energy which is released.
Richter magnitude
Each whole-number increase in magnitude is a 10-fold increase in wave amplitude
This corresponds to an approximately 30-fold increase in energy released
Figure 9.12, p. 211

30. Measuring the Strength of an Earthquake
Magnitude
Seismologists now commonly use the Seismic- moment Magnitude Scale, a modification of the Richter Magnitude Scale.
Seismic-moment Magnitude Scale considers rock strength, the area of the fault where the earthquake occurred, and the amount of movement of the rocks along the fault.
The Seismic-moment Magnitude Scale more effectively measures the amount of energy released by very large earthquakes.

31. What Are the Destructive Effects of Earthquakes?
Ground Shaking
The most destructive of all earthquake hazards is ground shaking.
Shaking lasts longer, and is more vigorous, in loose and wet sediment than solid rock.
Fig. 9.13, p. 212

32. What Are the Destructive Effects of Earthquakes?
Liquefaction occurs when water-saturated clays become fluid during ground shaking.
Replace with a new original
Fig. 9.14, p. 212

33. What Are the Destructive Effects of Earthquakes?
Fire occurs and spreads when natural gas and water lines break.
In some cases, post- quake fires kill more people than the earthquake.
Geo-Insight 3. and 5. , p

34. What Are the Destructive Effects of Earthquakes?
Tsunami: Killer Waves – Earthquakes on the seafloor can generate deathly waves, such as the ones that hit the Japanese coast in 2011.
Fig. 9.16a,b, p. 216

35. What Are the Destructive Effects of Earthquakes?
Ground Failure – Earthquakes trigger landslides and rock slides that are responsible for many deaths and much damage.
Fig. 9.17, p. 218

36. San Andreas Fault
Ground failure can result in building and road collapse.
Fig. 9.6b, p. 205; Geo-Insight 4., p. 215

37. Earthquake Prediction
Seismic risk maps help geologists in determining the likelihood and potential severity of future earthquakes based on the intensity of past earthquakes.
Fig. 9.18, p. 218

38. Earthquake Prediction
Earthquake Precursors – short-term and long-term changes within the Earth prior to an earthquake that assist in prediction.
Seismic gaps – locked portions of the fault where pressure is building
Surface elevation changes and tilting of land surface
Ground water table fluctuations
Local changes in Earth's magnetic field
Fig. 9.19, p. 219

39. Earthquake Prediction
Earthquake Prediction Programs
The United States, Russia, China, and Japan have earthquake prediction research programs.
Research involves laboratory and field studies of rock behavior before, during, and after large earthquakes, as well as monitoring major active faults.
Related studies, unfortunately, indicate that most people would probably not heed a short-term earthquake warning.

40. Earthquake Prediction
It's best to be prepared
Table 9.3, p. 220

41. Table 9.3, p. 220

42. Earthquake Control
Because of fault complexities and tremendous energy involved, it seems unlikely that humans will ever be able to prevent earthquakes.
Subsurface injection of liquid wastes apparently generated small earthquakes in Denver, Colorado in the 1960s.
Fig. 9.20, p. 221

43. Earthquake Control
It might be possible to release small amounts of the energy stored in rocks and thus avoid a large and damaging earthquake.
One controversial means of earthquake control might involve injecting fluids along a locked segment of an active fault.
Fig. 9.20, p. 221

44. Earthquake Control
If all goes well, the pressure would be slowly released as small earthquakes.
However, not understanding all of the fault conditions could generate a severe earthquake!
Fig. 9.20, p. 221

45. What is Earth’s Interior Like?
The concentric layers of Earth, from its surface to interior, are:
Oceanic and Continental crusts
Rocky mantle
Iron-rich core
Liquid outer core
Solid inner core
Fig. 9.22, p. 223

46. What is Earth’s Interior Like?
Geologist study the bending or refraction and reflection of P- and S-waves to help understand Earth's interior.
Fig. 9.23, p. 224

47. What is Earth’s Interior Like?
P- and S-waves indicate boundaries between layers of different densities called discontinuities.
Fig. 9.24, p. 224

48. The Core
The P- and S-waves both refract and reflect as they cross discontinuities.
Sudden decrease in P-wave velocity occurs at the core-mantle boundary.
P-waves are strongly diffracted in the liquid outer core so few of them reach the surface at o angle from an earthquake focus.
This results in a P-wave shadow zone.
Fig. 9.25a, p. 225

49. The Core S-waves will not pass through liquids.
The liquid outer core completely blocks S-waves and creates an S-wave shadow zone at angles of greater than 103o from the focus.
Fig. 9.25b, p. 225

50. The Core Density and Composition of the Core
The density and composition of the inner and outer cores are determined by the behavior of P- waves and S-waves.
The solid inner core is thought to be iron with about 10-20% nickel.
The outer core is iron with perhaps 12% sulfur, silicon, oxygen, nickel, and potassium.
Fig. 9.24,a, p. 224

51. Earth’s Mantle
The boundary between the crust and mantle is known as the Mohorovičić Discontinuity, or "Moho."
The Moho was discovered by the refraction of P- and S-waves.
Fig. 9.26, p. 226

52. Earth's Mantle The Mantle’s Structure, Density and Composition
Several discontinuities exist within the mantle
The velocity of P- and S- waves decrease markedly from 100 to 250km depth, which corresponds to the asthenosphere.
Fig. 9.27, p. 226

53. Earth's Mantle
The Mantle’s Structure, Density and Composition - Several discontinuities exist within the mantle.
The asthenosphere is an important zone in the mantle because this is where magma is generated.
Decreased elasticity accounts for decreased seismic wave velocity in the low-velocity asthenosphere. This decreased elasticity allows the asthenosphere to flow plastically.
Fig. 9.27, p. 226

54. Earth's Mantle
The Mantle’s Structure, Density and Composition
Peridotite is thought to represent the main composition in the mantle.
Experiments indicate that peridotite has the physical properties and density to account for seismic wave velocities in the mantle.
Figure 4.12, p. 95

55. Earth's Mantle The Mantle’s Structure, Density and Composition
Peridotite makes up the lower parts of ophiolite sequences that represent oceanic crust and upper mantle.
Peridotite is also found as inclusions in kimberlite pipes that came from depths of 100 to 300 km.
Figure 4.12, p. 95

56. Seismic Tomography
Tomography - a technique for developing better models of the Earth’s interior
Similar to a CAT-scan for producing 3-D images, tomography uses seismic waves to map out changes in velocity within the mantle.

57. Earth's Internal Heat
Geothermal gradient – measures the increase in temperature with depth in the Earth. Most of Earth's internal heat is generated by radioactive decay in the mantle, in particular uranium, thorium and 40K.
The upper-most crust has an average geothermal gradient of: 25° C/km.
The gradient is less in the mantle and core, probably about: 1° C/km.
The center of the inner core at a depth of 6,360 km has a temperature estimated at: 6,500° C.

58. Earth's Crust Continental crust is mostly granitic and low in density.
It has an average density of 2.7 gm/cm3 and a P-wave velocity of about 6.75 km/sec at the base of the crust.
It averages about 35 kilometers thick, being much thicker beneath the shields and mountain ranges of the continents.

59. Earth's Crust
Oceanic crust is mostly gabbro in its lower parts, overlain by basalt.
It has an average density of 3.0 gm/cm3 and a P-wave velocity of about 7 km/sec.
It ranges from 5-10 kilometers thick, being thinnest at the spreading ridges.

60. End of Chapter 9