Earthquakes - Chapter 10 by GU9jBy8C


Plate Tectonics
         What is an earthquake?
   An earthquake is the vibration of Earth
    produced by the rapid release of energy
        Energy radiates in all directions from its
         source, the focus
        Energy moves like waves
        Seismographs record the event
Anatomy of Earthquakes
       Causes of earthquakes

   Sudden release of accumulated strain energy
   Creation of new faults by rupturing rocks
   Shifting of rocks at preexisting faults
              Elastic Rebound Theory
   Rocks on sides of fault are deformed by tectonic
   Rocks bend and store elastic energy
   Frictional resistance holding the rocks together is
    overcome by tectonic forces
   Slip starts at the weakest point (the focus)
   Earthquakes occur as the deformed rock
    “springs back” to its original shape (elastic
   The motion moves neighboring rocks
(((( (((   ))))   )))
The 3 Basic Types of Plate
         (a review…)
Divergent Boundary
Convergent Boundary
Transform Fault Boundary
Horizontal Movement Along
      Strike-Slip Fault
     San Andreas: An active
        earthquake zone
 San Andreas is the most studied fault system in
  the world
 Displacement occurs along discrete segments 100
  to 200 kilometers long
       Most segments slip every 100-200 years
        producing large earthquakes
       Some portions exhibit slow, gradual
        displacement known as fault creep
Fence offset by the 1906 San
   Francisco earthquake
Earthquake focus and
Seismometers - instruments that
 record seismic waves
   Records the movement of
    Earth in relation to a
    stationary mass on a rotating
    drum or magnetic tape
 A seismograph designed to
record vertical ground motion
             The heavy mass doesn’t move much
                     The drum moves
          Lateral Movement Detector

In reality, copper wire coils move around magnets, generating current which is recorded.
Two Types of Surface Waves
   P and S waves

    Demo: P and S waves

Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
Earthquakes are located by
 finding the difference in
velocities of P and S waves
Note how much bigger the surface waves are
Graph to find distance to epicenter
Locating the epicenter of an earthquake

  Three seismographs needed to locate an

  Each station determines the time interval
   between the arrival of the first P wave and
   the first S wave at their location

  A travel-time graph then determines each
   station’s distance to the epicenter
Locating Earthquake Epicenter
Locating the epicenter of an earthquake

 A circle with radius equal to distance to
  the epicenter is drawn around each

 The point where all three circles
  intersect is the earthquake epicenter
Epicenter located using three seismographs
        Measuring the size of
 Two measurements describe the size of an

    Intensity – a measure of earthquake shaking
     at a given location based on amount of

    Magnitude – estimates the amount of energy
     released by the earthquake
          Intensity scales

 Modified Mercalli Intensity Scale was
  developed using California buildings as its

 Drawback is that destruction may not be
  true measure of earthquakes actual severity
        Earthquake destruction
 Amount    of structural damage depends
      Intensity and duration of vibrations
      Nature of the material upon which the
       structure rests (hard rock good, soft
      Design of the structure
Magnitude scales

     Richter magnitude - concept introduced by
      Charles Richter in 1935
     Richter scale
       –Based on amplitude of largest seismic
        wave recorded
       –LOG10 SCALE
        Each unit of Richter magnitude
        corresponds to 10X increase in wave
        amplitude and 32X increase in Energy
     Magnitude scales

 Moment magnitude was developed because
  Richter magnitude does not closely estimate
  the size of very large earthquakes

  –Derived from the amount of displacement
   that occurs along a fault and the area of
   the fault that slips
Investigating Earth’s Interior
   Seismology helps us understand Earth’s
    Interior Structure. We use:
   Speed changes in different materials
    due changes rigidity, density, elasticity
   Reflections from layers with different
   Attenuation of Shear Waves in fluids
   Direction changes (Refraction)
Three Major Components of Earth

Seismic-wave velocities are faster in the upper mantle

                                               Mohorovičić discontinuity

                Velocity increases w depth, waves bend back to surface.
   Waves that travel via mantle arrive sooner at far destinations
Wave Velocities

                  Upper Mantle Fast


                  Lower Mantle Fast
Mineralogy of Earth’s Layers

                The P-Wave Shadow Zone

                                                                                  P-waves through the liquid
                                                                                  outer core bend, leaving a
                                                                                  low intensity shadow zone
                                                                                  103 to 143 degrees away
                                                                                  from the source, here
                                                                                  shown as the north pole

                                                                                  HOWEVER, P-waves
                                                                                  traveling straight through
                                                                                  the center continue, and
                                                                                  because speeds in the
                                                                                  solid inner core are faster,
                                                                                  they arrive sooner than
                                                                                  expected if the core was
                                                                                  all liquid.

                                                                             Inge Lehmann

                Behavior of waves through center reveal Earth’s Interior
        The S-Wave Shadow Zone

                                                    Since Shear (S) waves
                                                    cannot travel through
                                                    liquids, the liquid
                                                    outer core casts a
                                                    larger shadow for S
                                                    waves covering
                                                    everything past 103
                                                    degrees away from
                                                    the source.

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