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Earthquakes

Earthquakes

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Earthquakes

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  1. Earthquakes Slinky Beaker, Wet Sand, Weight Cardboard Fault models Chewing Gum Wood meter stick pencil

  2. 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

  3. Anatomy of Earthquakes

  4. Earthquakes and faults • Earthquakes are associated with faults • Motion along faults can be explained by plate tectonics

  5. Causes of earthquakes • Sudden release of accumulated strain energy • Creation of new faults by rupturing rocks • Shifting of rocks at preexisting faults

  6. Elastic rebound 1 • Mechanism for Earthquakes • 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

  7. Elastic rebound 2 • 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/ meter stick

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  9. Aftershocks The change in stress that follows a mainshock creates smaller earthquakes called aftershocks The aftershocks “illuminate” the fault that ruptured in the mainshock Red dots show location of aftershocks formed by 3 earthquakes in Missouri and Tennessee in 1811/1812

  10. Relationship Between Stress and Strain Strain can be a change in shape (a deformation) due to an applied stress

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

  12. Relationship Between Stress and Strain under high Temps or Pressure

  13. 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

  14. Dip-Slip Faults

  15. Horizontal Movement Along Strike-Slip Fault

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

  17. 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

  18. Fence offset by the 1906 San Francisco earthquake

  19. 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.

  20. Landscape Shifting, Wallace Creek

  21. Liquefaction Demo: Liquifaction Analogy: Feet at the shore

  22. 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

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

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

  25. Types of seismic waves • Surface waves • Complex motion, great destruction • High amplitude and low velocity • Longest periods (interval between crests) • Termed long, or L waves

  26. Two Types of Surface Waves

  27. 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

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

  29. Locating the source of earthquakes Focus - the place within Earth where earthquake waves originate Epicenter – location on the surface directly above the focus • Epicenter is located using the difference in velocities of P and S waves

  30. Earthquake focus and epicenter

  31. Note how much bigger the surface waves are

  32. Graph to find distance to epicenter

  33. Locating the epicenter of an earthquake • Three seismographs needed to locate an epicenter • 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

  34. Locating Earthquake Epicenter

  35. Locating the epicenter of an earthquake • A circle with radius equal to distance to the epicenter is drawn around each station • The point where all three circles intersect is the earthquake epicenter

  36. Epicenter located using three seismographs

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

  38. Locations of earthquakesfrom 1980 to 1990 Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates

  39. Depths of Earthquakes Earthquakes originate at depths ranging from 5 to , rarely, nearly 700 kilometers WHY 700. Deeper, rock deforms plastically Cannot spring back elastically, no earthquake DEMO: Plastic Deformation Definite patterns exist in depth • Shallow focus 20 km faults between MOR • Large Transform (San Andreas) fault 80 km • Deep earthquakes occur in Pacific landward of oceanic trenches 80 km down to about 600 km Associated with subduction zones

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

  41. Earthquake in subduction zones

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

  43. 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

  44. 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

  45. Earthquake destruction • Amount of structural damage depends on • Intensity and duration of vibrations • Nature of the material upon which the structure rests (hard rock good, soft bad) • Design of the structure

  46. 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

  47. 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

  48. 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