Figure 4.3 left: Earthquake activity on the San Andrea fault in California (from General Geology (5th ed.), by Foster, Merrill Publishing Co., 1988). right: Elastic rebound mechanism (from The Changing Earth - Introduction to Geology (2nd ed.), by Mears, Jr., D. Van Nostrand Co., 1977).

Figure 4.4 An impression of the generation of seismic waves, and a definition of some seismological terms. The period range for the best observation of body waves is 0.5 s to 2.0 s, and for surface waves is 10 s to 60s (from Understanding the Earth (2nd ed.), by Gass, Smith, & Wilson, The M.I.T. Press, 1972).

• Seismographs
  
  
  
  Figure 4.5 left: Elements of a simple seismograph. In this case, recording is done by a pinpoint light beam on photographic paper, a system used on many modern instruments (from The Changing Earth - Introduction to Geology (2nd ed.), by Mears, Jr., D. Van Nostrand Co., 1977). right: Chinese Seismograph.
  
  
• Seismogram
  
  
  
  Figure 4.6 A seismograph record showing the three waves (from General Geology (5th ed.), by Foster, Merrill Publishing Co., 1988).
  
  
  

• Types of Seismic Waves
  • Body Waves
    1. P ==> pressure, primary

    2. S ==> shear, secondary

       SV (vertical)

       SH (horizontal)

  • Surface Waves
    1. Love (L) -- horizontal

    2. Rayleigh (R) -- vertical

• Velocity Properties of Seismic Waves
  • Arrival-times of waves recorded on a seismogram are interpreted in terms of the elastic velocities of the rocks between the earthquake and the seismograph

  • Like density, elastic velocity is a physical property which can be quite diagnostic of various types of materials

  • air = 367 m/sec (= 1,100 ft/sec): density = 0.0 gm/cm³

  • water = 1,500 m/sec: density = 1.0 gm/cm³

  • hard rock = 6,300 m/sec: density = 3.0 gm/cm3³

  • In general,
    • Vs = 0 Liquid or Gas

    • Vp (Liquid) < Vp (Hard Rock)

    • Vs= 0.6Vp ==> the time interval between the arrival of P- and S- waves increases with distance from the earthquake

    • Vr = 0.9 Vs

  • Different rocks commonly have different seismic velocities - so that their distribution about an earthquake can be inferred from analysis of the arrival times of the seismic waves at various locations or stations about the earthquake.

• Travel-Time Curves
  
  
  
  Figure 4.7 Time-distance graph for earthquake waves. P is a push-pull wave that has traversed the core. The line for P is split beyond 10,000 miles by the effects of the solid inner core (modified from Physical Geology (3rd ed.), by Leet & Judson, Prentice-Hall Inc., 1965).
  
  
  
  
  Figure 4.8 Effect of the core on seismic waves (from The Changing Earth - Introduction to Geology (2nd ed.), by Mears, Jr., D. Van Nostrand Co., 1977).
  
  

  ---

  Vs(LC) = 0
  Vp(LC) < Vp(M) & Vp(SC)

  ---

  
  
  
  Figure 4.9 P and S wave structure of the Earth.
  
  
  
  
  Figure 4.10 Two earthquake records showing the effect of the earth's core on wave propagation. H on each record is the time of the earthquake's occurrence (from Physical Geology (3rd ed.), by Leet & Judson, Prentice-Hall Inc., 1965).
  
  
  
  
  
  Figure 4.11 Structure of the mantle and core (from Physical Geology (3rd ed.), by Leet & Judson, Prentice-Hall Inc., 1965).
  
  
  
• Locating Epicenters
  
  
  
  Figure 4.12 Time-distance graph for earthquake waves. P is a push-pull wave that has traversed the core. The line for P is split beyond 10,000 miles by the effects of the solid inner core (modified from Physical Geology (3rd ed.), by Leet & Judson, Prentice-Hall Inc., 1965).
  
  
  
  
  
  Figure 4.13 A, B, and C are seismic stations. The P-S times at each station indicate that the earthquake occurred somewhere on the circle drawn around each station. The point where all three circles intersect is the epicenter (from General Geology (5th ed.), by Foster, Merrill Publishing Co., 1988).