VII. Earthquakes

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VII. Earthquakes

1. Introduction
2. Source of seismic energy
3. Propagation of seismic energy
4. Recording earthquakes
5. Magnitude scales

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San Francisco, 1906

• Building design could not withstand accelerations
• Millions of damage
• Thousands of people killed

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Geology in the News
http//news.nationalgeographic.com/news/2003
San Andreas Fault
California, 2003

• Two die in 6.5 magnitude Earthquake near San Lois
  Obispo California
• Earthquake triggers mudslides

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Earthquake Bam, Iran
Photos from AP

• A Magnitude 6.5 Earthquake hits a stone- and
  mud-house city of 100,000 in Iran December
  26, 2003
• 30,000 Dead
• 30,000 Refugees
• US sends aid and releases sanctions
• Relations improved

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Geological Hazards Related to Earthquakes
Volcanoes
Landslides
Mudslides
Tsunamis
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B Sources of Seismic Energy

• Elastic Rebound
• Buildup of elastic energy during elastic strain
• Sudden release due to slippage along a fault or
  brittle rupture

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Anatomy of an Earthquake

• Fig. 8.35
• Focus Source
• of energy
• Epicenter Location directly above focus at the
  surface (ground motion is greatest)
• Fault Trace Shows intersection of fault and the
  surface of the land
• Fault Scarp Indicates vertical motion of fault

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Propagation of Seismic Energy

• Body Waves travel through the earths interior
  (crust, mantle, core)
• P wave Compression and expansion of rock
• S Wave Shearing motion of particles
• Fig. 8.42

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Propagation of a P-Wave
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Propagation of Seismic Energy

• Body Waves travel through the earths interior
  (crust, mantle, core)
• P wave Compression and expansion of rock
• S Wave Shearing motion of particles
• Surface Waves

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Propagation of an S-Wave
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Propagation of Surface Waves

• Surface Waves travel along the earths surface
• Love Wave Lateral movement of the surface
• Rayleigh Wave Rolling movement of the surface
  (similar to an ocean wave)

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Propagation of a Surface Wave
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Propagation of a Surface Wave
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Recording Earth Motion

• Seismograph
• An instrument that measures the horizontal or
  vertical motion of Earths surface
• Seismograms
• The plot of the motion

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Measuring Velocity of Seismic waves

• Because the P wave travels faster the the S wave
• The S-P interval increases with distance

Time of Earthquake
Time of Earthquake
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Reading a Seismogram
See Fig. 8.44

• Ground motion vs. Time
• Each tick mark is 1 minute
• P-S Time interval indicates distance to epicenter

First S wave Arrival
First P wave Arrival
First Surface wave
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Time-Distance Relationships
See Fig. 8.45

• Use P-S interval to determine distance to focus

Distance from focus (Kilometers)
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Seismic Waves and Velocities

• Body Waves and Surface Waves

• P-waves
• a ?(k ¾m)r
• k Bulk modulus
• m Mod. of rigidity
• r Density
• S-waves
• b ?m/r

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Locating the Epicenter

• Triangulation using 3 seismograph stations
• Depth can be determined with four or more stations

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Finding the Depth of Earthquakes Using 4 or more
seismograph stations
Seismicity of the Pacific Rim 1975-1995
0 33 70
150

• Shallow quakes at mid ocean ridges (lt33km) and
• Oceanic trenches
• Deep quakes over the subduction zone (gt70 km)

300
500
800
Depth (km)
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Earthquakes, Plate Interior

• New MadridFault Zone
• Faults activated by crustal warping
• Bowling Green Fault
• Largest Earthquake on the N. American Continent

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New Madrid Earthquake, 1811

• Modified Mercalli Intensity Scale
• Subjective observations of
• Damage and
• Ground motion
• Is not a quantitative measure

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Earthquake Intensity Scales

• Modified Mercalli Scale
• Subjective observations of
• Damage and
• Ground motion
• Is not a quantitative measure
• Richter Scale
• Indicates Ground Motion Amplitude
• Logarithmic (e.g., 6 is ten times stronger than
  5)
• Does not directly indicate energy or destruction

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Earthquakes around the World
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Assessing Risk
Major Quake Likely to Strike San Francisco Bay
Region Between 2003 and 2032
Geologic Hazards

• Assessing Risks
• Avoiding Risks
• Preventing Damage
• Predicting Impact

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Seismic Risk Analysis

• Quake history (statistics)
• Locations of active faults
• Competency of surficial materials (soil and rock)
• Ocean basin source? Tsunamis

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Solomon Islands Earth Quake and Tsunami 4-1-07
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Profiling Earths Interior

• Velocities of seismic waves vs depth Fig.
  19.19 20

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Imaging Earths Interior

• P and S waves are refracted (bent) within the
  earth
• S waves do not travel through fluids
• Fig. 9.21