What is an earthquake?



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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
  • Slinky, Rubber Band SEISMOGRAM
  • Beaker, Wet Sand, Weight
  • Cardboard Fault models
  • Chewing Gum
  • Wood meter stick or plastic ruler
  • pencil

Anatomy of Earthquakes

  • Earthquakes are associated with faults
  • ) ) ) ) ) ) )
  • ( ( ( ( ( ( (
  • Earthquakes are caused by sudden release of accumulated strain energy along Faults
        • Rocks on sides of fault are deformed by tectonic forces
        • Rocks bend and store elastic energy
        • Frictional resistance holding the rocks together is overcome by tectonic forces
  • Hands Demo

      • Earthquake mechanism
        • Slip starts at the weakest point (the focus)
        • Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)
        • The motion moves neighboring rocks
        • And so on.
        • DEMO – elastic rebound w/ ruler

Relationship Between Stress and Strain

  • Strain can be a change in shape (a deformation) due to an applied stress
  • Demo: Rubber Band

Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress

  • Demo: Pencil

Relationship Between Stress and Strain under High Temps or Pressure

  • Demo: gum

Strike and Dip

  • Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
  • Strike is long line, dip is short line
  • Note the angle of dip given 45o

Vertical Movement along Dip-Slip Faults

  • Divergent
  • Convergent

Horizontal Movement Along Strike-Slip Fault

  • Normal Fault Quake - Nevada
  • Reverse Fault Quake - Japan
  • Strike Slip Fault Quake - California
  • DEMO – Types of faults

Fence offset by the 1906 San Francisco earthquake

  • San Andreas is the most studied transform fault system in the world
  • discrete segments 100 to 200 kilometers long
      • slip every 100-200 years producing large earthquakes
      • Some portions exhibit slow, gradual displacement known as fault creep
  • Fires caused by 1906 San Francisco Earthquake
  • Gas mains break, fires shaken out of furnaces. Water mains break, cannot fight fires. Debris in streets, Fire department cannot reach fires.

Landscape Shifting, Wallace Creek

  • San Andreas Fault, a Transform Margin

Liquefaction

  • Demo: Liquifaction

Seismology

  • Seismometers - instruments that record seismic waves

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.
      • Seismic Waves 1: Surface waves
        • Complex motion, great destruction
        • High amplitude and low velocity
        • Longest periods (interval between crests)
        • Termed long, or L waves

Types of seismic waves (continued)

  • Types of seismic waves (continued)
      • Body waves
        • Travel through Earth’s interior
        • Two types based on mode of travel
        • Primary (P) waves
          • Push-pull motion
          • Travel thru solids, liquids & gases
        • Secondary (S) waves
  • Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
  • P and S waves
  • Demo: P and S waves

Earthquake focus and epicenter

Graph to find distance to epicenter

Locating Earthquake Epicenter

Epicenter located using three seismographs

95% of energy released by earthquakes originates in narrow zones that wind around the Earth These zones mark of edges of tectonic plates

  • Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates

Earthquake Depth and Plate Tectonic Setting

  • Subduction Zones discovered by Benioff

Earthquake in subduction zones

Earthquakes at Divergent Boundaries - Iceland

  • Crust pulling apart – normal faults

Measuring the size of earthquakes

  • Two measurements describe the size of an earthquake
      • Intensity – a measure of earthquake shaking at a given location based on amount of damage
      • Magnitude – estimates the amount of energy released by the earthquake

Intensity scales

  • Intensity scales
      • Modified Mercalli Intensity Scale was developed using California buildings as its standard
      • Drawback is that destruction may not be true measure of earthquakes actual severity

Magnitude scales

  • 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

Tsunamis, or seismic sea waves

  • Tsunamis, or seismic sea waves
      • Destructive waves called “tidal waves”
      • Result from “push” of underwater fault or undersea landslide
      • In open ocean height is > 1 meter
      • In shallow coast water wave can be > 30 meters
      • Very destructive

Formation of a tsunami

  • Tsunamis are actually huge, extending from the fault on the sea floor up to the surface, but they don’t stick up more than a meter or so in the deep ocean. However, when they reach shallow water they must rear up and slow down. Discussion: Kinetic vs. potential energy
  • Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

Tsunami 1960, Hilo Hawaii

Tsunami Model, Alaska Quake

Earthquake prediction

  • Long-range forecasts
      • Calculates probability of a certain magnitude earthquake occurring over a given time period
  • Short-range predictions
      • Ongoing research, presently not much success
  • Long Term Predictions
  • Seismic Gaps
  • Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

Seismic Gap along Himalayas

  • 2005

Dilatancy of Highly Stressed Rocks

  • Short-Term Earthquake Prediction

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 properties
  • Attenuation of Shear Waves in fluids
  • Direction changes (Refraction)
  • Investigating Earth’s Interior

Surface Components magnified

  • !

Seismic-wave velocities are faster in the upper mantle

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

Wave Velocities

  • Upper Mantle Fast
  • Asthenosphere
  • Slow
  • Lower Mantle Fast

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.
  • http://en.wikipedia.org/wiki/Richard_Dixon_Oldham

The P-Wave Shadow Zone

  • Behavior of waves through center reveal Earth’s Interior
  • 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
  • http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html


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