Assignments:

1. Smith and Punn Chapter 11
2. Earth Revealed: movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.Sign in and view #9 Earthquakes

Terms: stress, strain, elastic, plastic, brittle, earthquake, compression, tension, shear, seismic waves (p-wave, s-wave, love wave, rayleigh wave), liquefaction, epicenter, focus (hypocenter), richter magnitude, seismic moment magnitude, intensity, tsunami

Introductory concepts:

Deformation

Rocks deform when stressed (when force is applied)

Deformation = Strain (change in shape or volume)

• Behaviors
  • elastic (recoverable): no direct evidence
  • plastic (flow): produces folds in rocks
  • brittle (rupture): produces joints and faults
• Factors governing deformation
  • Heat
  • Pressure
  • Rate at which stress is applied
  • Strength of material

Generation of earthquakes

Earthquakes occur along faults: major breaks in the crust along which movement occurs and where stress tends to accumulate

Elastic rebound theory: Accumulated strain energy is suddenly released upon rupture

Energy travels as seismic waves

• Body waves: These energy waves radiate from the epicenter through the body of the earth (animation from the PBS Savage Earth Earthquake site)
  • S-wave (shear, secondary): travels only through solids
  • P-wave (push-pull, primary): travels through liquids and solids
• Surface waves
  • Rayleigh wave--orbital wave responsible for vertical acceleration
  • Love wave--causes lateral shaking

(View wave animations from the PBS Savage Earth Earthquake site)

Locating and mapping earthquakes

• Focus/Hypocenter: The point of initial rupture where the waves are generated. The hypocenter may be shallow (0-70 km), intermediate (70-350 km), or deep (350-670 km)
• Epicenter: Point on the earth's surface directly over the focus.
• Wave motion is recorded on a seismogram at at seismograph station

Locating an earthquake epicenter from seismic data

1. Location requires data from at least 3 seismic station
2. Determine the difference in S- and P-wave arrival times (lag time) for each station
3. Convert each time to distance (D) using a time-distance curve for S and P waves
4. Around each station draw a circle(circle radius=D computed for that station)
5. Point of intersection marks location of epicenter

Distribution of earthquakes: Where does faulting take place?

Plate Boundaries

1. subduction zones: Characterized by shallow (0-70 km) through deep focus (350-670 km) earthquakes
2. continental collision zones: Characterized by shallow through intermediate focus (70-350 km) earthquakes
3. Transform boundaries: Mostly shallow focus
4. spreading centers and rift zones: Mostly shallow focus

Visit Understanding Plate Motion / Seismic Monitor

intraplate earthquakes (much less common, but significant)

• hotspot volcanic activity and/or incipient rifting: shallow focus
• release of stress along ancient fault systems
• isostatic rebound or expansion caused by erosion or glacial unloading
  

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• Examples of intraplate earthquakes in the U.S.
  • New Madrid Seismic zone: Uncovering Hidden Hazards in the Mississippi Valley / Earthquake Hazard in the Heart of the Homeland (USGS Fact Sheet 131-02 ) / New Madrid Earthquakes 1811-1812
  • 1886 Charleston, SC /Photo documentation
  • Massachusetts Seismicity /Cape Ann Earthquake 1775

Measuring Earthquakes

• magnitude and intensity
  • Richter Magnitude scale: Magnitude (0-9.9 open ended scale)
    • measured from amplitude of largest surface wave recorded on a seismogram
    • for each point increase in the scale:
      • amplitude increase 10x
      • Energy released increases 30x
      • example: Alaska: RM=8.4
  • Seismic-Moment magnitude scale
    • Calculated using the paramenters that determine magnitude:
    • seismic moment = amount of slip*rupture length*rupture depth*rock strength
    • example: Alaska 1964: SM=9.2
  • Modified Mercalli scale: Intensity (I-XII)--The amount of shaking experienced
    • factors that govern the Intensity of an earthquake:
      • Magnitude
      • Distance from epicenter
      • Depth to focus
      • Ground conditions (bedrock or deep soil)
      • Duration
      • ground saturation: results in liquefaction
      The severity of an earthquake: USGS fact sheet

Loss of life is determined by:

1. Intensity
2. Building construction: design and composition
3. Timing
4. Population density
5. Secondary hazard
   • Tsunami (seismic sea waves)
     • National Earthquake information Center (NEIS)
     • Tsunamis and Earthquakes (USGS)
     • University of Washington Tsunami web site

       Tsunami animations: Sea of Japan 1993, 1960 Chilean tsunami

   • Destruction of urban infrastructure (water mains, gas and oil lines, transportation, etc.) results in:
     • Disease (Natural disasters and epidemics/Disease risks-BBC)
     • Fire
   • Landslides (e.g Turnagain Heights landslide in Anchorage 1964)
   • Flooding caused by
     • stream drainage derangement
     • failure of dams and levees (LA Dam)
     • liquefaction (Squeezing Water From Rock Laura Schmidt - Earth Observatory)

Global effects of earthquakes Global economic disruption: In addition to the local loss of life and the disruption of the local infrastructure earthquake in cities and high technology industrial regions can cause economic damage on a global scale. Consider that Los Angeles, Tokyo, and Taiwan, which are important economic and manufacturing centers, will probably face a major earthquake within the next 100 years.

Earthquake prediction

Earthquake prediction still largely an inexact science. Some observations used in prediction are:

• seismic gaps (define areas of high stress buildup)
• ground swelling
• foreshocks
• change in water levels in wells
• increase in radon gas in wells
• strange animal behavior
  

Additional Internet sites to explore:

• Index to Earthquake sites
  Glossary of Earthquake Terms
• USGS Plate Tectonics Site
• The San Andreas Fault
• Earthquakes: causes, distribution, measurements, and prediction
• Central US earthquake Center
• US seismicity and earthquake history
  • US earthquakes of Magnitude 7 or greater
    • The 15 largest earthquakes in the US
• Geologic Structure images (Earth Science Australia)