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What is an earthquake?

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

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What is an earthquake?

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  1. 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

  2. Anatomy of Earthquakes Earthquakes are associated with faults

  3. 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

  4. 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

  5. Relationship Between Stress and Strain Demo: Rubber Band Strain can be a change in shape (a deformation) due to an applied stress

  6. Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress Demo: Pencil

  7. Relationship Between Stress and Strain under High Temps or Pressure Demo: gum

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

  9. Vertical Movement along Dip-Slip Faults Divergent Convergent

  10. Horizontal Movement Along Strike-Slip Fault

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

  12. 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

  13. 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.

  14. Landscape Shifting, Wallace Creek San Andreas Fault, a Transform Margin

  15. Liquefaction Demo: Liquifaction

  16. Seismology Seismometers - instruments that record seismic waves • Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

  17. A seismograph designed to record vertical ground motion The heavy mass doesn’t move much The drum moves

  18. Lateral Movement Detector In reality, copper wire coils move around magnets, generating current which is recorded.

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

  20. 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 • Moves at right angles to their direction of travel • Travels only through solids

  21. P and S waves Demo: P and S waves Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L

  22. Earthquake focus and epicenter

  23. Note how much bigger the surface waves are

  24. Graph to find distance to epicenter

  25. Locating Earthquake Epicenter

  26. Epicenter located using three seismographs

  27. 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

  28. Earthquake Depth and Plate Tectonic Setting Subduction Zones discovered by Benioff

  29. Earthquake in subduction zones

  30. Earthquakes at Divergent Boundaries - Iceland Crust pulling apart – normal faults

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

  38. Tsunami 1960, Hilo Hawaii

  39. Tsunami Model, Alaska Quake

  40. 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

  41. Long TermPredictions Seismic Gaps

  42. Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

  43. Seismic Gap along Himalayas 2005

  44. Short-Term Earthquake Prediction Dilatancy of Highly Stressed Rocks

  45. 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)

  46. Investigating Earth’s Interior

  47. Surface Components magnified !

  48. Seismic-wave velocities are faster in the upper mantle Velocity increases w depth, waves bend back to surface. Waves that travel via mantle arrive sooner at far destinations Mohorovičić discontinuity

  49. Wave Velocities Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast

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