1. The Violent Earth Faults, seismology, and the Bay Area

2. Earthquakes and Plate Tectonics
Review last week’s topics
Faults & earthquakes
Magnitude of an earthquake
Earthquake location
Bay Area faults
Virtual Earthquake exercise

3. Plate boundary characteristics Convergent plate boundaries Two plates pushing together
Convergent (oceanic crust-continental crust)
• Deep ocean trenches next to high, narrow mountain belts
• Numerous shallow, intermediate, and deep earthquakes
• Lots of volcanic activity,
Forms volcanic chains (some island chains like Japan)
• Think SUBDUCTION ZONES!
Old, cold oceanic crust sinks when it hits a continent
• Example: the Andes Mountains
Convergent (continental crust-continental crust)
• Broad, high mountain belt (e.g., the Himalayas)
• Diffuse earthquakes
• Little to no volcanism (no subduction)
• Example: the Himalayas

4. Plate boundary characteristics Divergent plate boundaries Two plates pulling apart
Mid-ocean ridges/spreading centers and continental rifts
• Small, shallow earthquakes
• Significant volcanism but not large volcanoes —
forms new oceanic crust
• Crust is very thin and spreading apart
• Youngest oceanic crust found on mid-ocean ridges
• Form topographic highs in the ocean floor (i.e., mid-ocean ridges)
Example of a mid-ocean ridge:
Mid-Atlantic ridge and Iceland
Example of a continental rift:
East African rift zone

5. Plate boundary characteristics Transform plate boundaries Two plates sliding past one another
Strike-slip faults and fracture zones
• Numerous relatively shallow earthquakes in a narrow zone
earthquakes can be small or quite large
• No volcanism!
• No big topographic changes across a transform boundary
(i.e., no trenches, no ridges or mountains)
• Fracture zones are transform faults in the oceanic crust —
they are closely related to mid-ocean ridges
Example of a transform plate boundary:
The San Andreas fault

7. Seismology Map – Earthquake Locations and Depths

8. Volcanology Map – Recent Volcanic Activity
This map shows the locations of recent volcanic or thermal features on the Earth. In this dataset, recent refers to the past 10,000 years. The dots represent volcanoes, geysers, hot springs, and similar features.

9. Geochronology Map – Seafloor Age
This map shows the age of the oceanic crust under most of the world’s oceans. This map really highlights divergent plate boundaries, also known as mid-ocean ridges or spreading centers. The scale bar on the right shows how the colors correspond to age of the seafloor in millions of years. Red signifies the youngest crust. Blue signifies the oldest known oceanic crust.

10. Geography Map – Topography and Bathymetry
This map, and perhaps the richest of the data maps, shows the topography and bathymetry of the Earth. This is the elevation of the land surface and the depth of the oceans. The map uses color to indicate varying elevation and depth and simulates sun shading to add a sense of 3-dimensionality to the map. The scale bar on the right shows how colors on the map correspond to elevation in meters.

11. What is an earthquake?
Fault
Stress builds
Rocks “break”
along the fault
The energy released in the earth when the stress is relieved along a fault is felt as an earthquake

12. To understand earthquakes, we need to learn about faults...

13. Fault A fault is a fracture or crack in rocks -
weak zones along which rocks can move

19. The Basin & Range Province

20. Continental Rifts & Normal Faults
Normal faults produce
Basin & Range structure

22. FAULTS
Normal Fault
Hanging wall moves downward, relative to the footwall.
Extensional forces.
Divergent plate boundaries.

24. Mid-ocean ridges &
Spreading centers

28. FAULTS Normal Fault Reverse Fault
Hanging wall moves downward, relative to the footwall.
Extensional forces.
Divergent plate boundaries.
Reverse Fault
Hanging wall moves upward, relative to the footwall.
Compressional forces.
Convergent plate boundaries.

31. Low-angle reverse fault is
called a thrust fault

36. Pacific plate
North American plate

38. FAULTS Normal Fault Reverse Fault Strike-Slip Fault
Hanging wall moves downward, relative to the footwall.
Extensional forces.
Divergent plate boundaries.
Reverse Fault
Hanging wall moves upward, relative to the footwall.
Compressional forces.
Convergent plate boundaries.
Strike-Slip Fault
Two blocks slide laterally past one another.
Shearing forces.
Transform plate boundaries.

40. Left- vs. right-lateral strike-slip faults
The San Andreas
is a right-lateral
strike-slip fault

43. Earthquake rupture area vs. magnitude

44. 4 types of seismic waves P-wave (primary) S-wave (secondary) Love wave
Compression and dilatation
S-wave (secondary)
Up-down and side to
side motion
Love wave
Side to side motion
Rayleigh wave
Rolls like ocean wave

45. A simple seismograph

46. What do seismographs show?

47. Magnitude of an earthquake
Richter scale Earthquake magnitude is related to the amplitude of the
S-wave on a seismograph

50. What does the Richter scale mean in terms of what we feel?
Richter Magnitudes Earthquake Effects
Less than Generally not felt, but recorded.
Often felt, but rarely causes damage.
Under At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
Can be destructive in areas up to about kilometers across where people live.
Major earthquake. Can cause serious damage over larger areas.
8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.
Loma Prieta was mag. 7.1
In 1960, Chile had a 9.5 magnitude quake;
In 1964, Alaska experienced a 9.2 magnitude quake

51. How much energy is released in an earthquake?

53. Three factors primarily determine what you feel in an earthquake:
1. Magnitude
You feel more intense shaking from a big earthquake than from a small one; big earthquakes also release their energy over a larger area and for a longer period of time.
2. Distance from the fault
Earthquake waves die off as they travel through the earth so the shaking becomes less intense farther from the fault
3. Local soil conditions
Certain soils greatly amplify the shaking in an earthquake. Seismic waves travel at different speeds in different types of rocks. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard rock at the same distance from the same earthquake. The looser and thicker the soil is, the greater the amplification will be, (e.g, Loma Prieta earthquake damage area of Oakland and Marina (SF) were 100 km (60 mi) and most of the Bay Area escaped serious damage).
(4. Building type) The tops of high-rise buildings “feel” smaller earthquakes more

54. Soil type vs. shaking intensity
Bay mud, etc.
Mud, silt, clay soils
Unconsolidated sediment
Weathered sandstone
Weathered hard rock

55. Earthquake waves travel out in all directions from the hypocenter

56. Earthquake location - Triangulation
Use seismic wave arrival times from multiple seismometer stations to triangulate epicenter

57. Triangulation

58. Properties of the two most important seismic waves
P-waves
S-waves
• Travel fastest (arrives first)
• Smaller than S-waves
• Travel through all materials
(including liquid and air)
• Compressional wave (push-pull)
• Travel slower (arrives after P-wave)
• Larger amplitude than P-wave
• Does not travel through liquid,
only solids
• Transverse wave (up-down/left-right)

59. Seismic waves….
Reflect ….and refract

62. The Mercalli scale
I. Not felt except by a very few under especially favorable conditions.
II. Felt only by a few persons at rest, especially on upper floors of buildings.
III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck.
IV. Felt indoors by many, outdoors by few. At night, some awakened. Dishes, windows, doors disturbed. Like heavy truck striking building. Standing motor cars rocked noticeably.
V. Felt by almost everyone; many awakened. Some dishes, windows broken. Unstable objects overturned.
VI. Felt by all, many frightened. Some heavy furniture moved; fallen plaster. Damage slight.
VII. Damage light in buildings of good design/construction; slight to moderate in well-built structures; much damage in poorly built/badly designed structures; some chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in substantial buildings, partial collapse. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
IX. Damage considerable in specially designed structures. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.
X. Some well-built wood structures destroyed; masonry structures destroyed. Rails bent.
XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.
XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

63. Loma Prieta Mercalli intensity

65. The 1906 rupture vs. the 1989 Loma Prieta rupture
The 1906 quake was
magnitude on the
Richter scale
The Loma Prieta quake
was a 7.1 Richter
magnitude

66. Actual slippage during the 1906 earthquake

67. 1906 quake damage
Santa Rosa city hall
Tomales Bay

69. Plate boundary
not a discrete line,
but rather a zone
the width of the
Bay Area

70. Major Bay Area faults San Andreas Hayward/Rodgers Creek Calaveras
San Gregorio

71. Bay Area seismicity
Plate boundary not a discrete line, but rather a zone

72. North American Plate
Pacific Plate

75. Pacific Plate
North American Plate

77. San Andreas Fault

79. Locked and creeping fault segments

80. 1989 Loma Prieta earthquake
Seismicity along the San Andreas
Locked segment
Creeping segment
Seismicity from the
1989 Loma Prieta earthquake

81. Hayward fault - offset stream

82. Hayward fault - offset stream
Right-lateral
strike-slip

83. Offset fence

84. Offset fence

85. Hayward fault

86. Cal stadium
The trace of the Hayward fault runs through goalposts

87. Cal stadium
The trace of the Hayward fault runs through goalposts

88. Offset of Cal stadium

89. Hollister - The Calaveras fault

90. Hollister - Calaveras fault

91. Virtual Earthquake Web exercise Pick up a work sheet and
go to the Virtual Earthquake web site
listed at the top of the page

92. P- and S-Wave arrival times

94. Triangulation