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How it happens..........

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  1. 1. EARTHQUAKE ! By Deepak
  2. 2. Topics <ul><li>What is an earthquake? </li></ul><ul><li>What makes the earth quake? </li></ul><ul><li>Measuring Earthquakes. </li></ul><ul><li>Can we predict earthquakes? </li></ul><ul><li>Should you worry about earthquakes? </li></ul><ul><li>How can you minimize your risk? </li></ul><ul><li>TSUNAMI ! </li></ul>
  3. 3. What is an Earthquake? <ul><li>An earthquake is a shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other. </li></ul>
  4. 4. <ul><li>Tectonic Activity </li></ul><ul><li>Volcanic Activity </li></ul><ul><li>Meteoroids </li></ul><ul><li>Trucks </li></ul>What makes the earth quake ?
  5. 5. Tectonic Earthquakes <ul><li>where do earthquakes happen? </li></ul>
  6. 6. Earthquake map
  7. 7. Tectonics and Earthquakes <ul><li>Along what boundaries do DEEP earthquakes occur? </li></ul><ul><li>Along what boundaries do ONLY small earthquakes occur? </li></ul><ul><li>Along what boundaries does strike-slip motion occur? </li></ul>
  8. 8. FOCAL MECHANISMS <ul><li>The DIRECTION the ground moves in at seismometers tells us the direction of motion AT THE FOCUS. </li></ul>
  9. 10. Map views of bottom half of focal spheres
  10. 11. What does the focal mechanism for a strike-slip earthquake look like?
  11. 12. What does the focal mechanism for a strike-slip earthquake look like?
  12. 13. FOCAL MECHANISMS AND TECTONICS What is the relative motion? How many plates are shown?
  13. 14. If earthquakes occur along these plate boundaries, what will the focal Mechanisms look like?
  15. 18. MEASURING EARTHQUAKES <ul><li>Seismometers </li></ul><ul><li>Locating earthquakes </li></ul><ul><li>Discovering the interior of the earth </li></ul>
  16. 19. Chinese Seismoscope - first earthquake sensor
  17. 21. Seismograms from many stations for one earthquake
  18. 22. Earthquake Location if P and S-wave data are available
  19. 23. What if you only have P-wave arrivals at three seismometers… Can you locate the quake? Station: Arrival Time A 10:22:04 B 10:22:04 C 10:23:34
  20. 25. Interior of the Earth Almost everything we know about the deep interior of the earth comes from the study of seismic waves
  21. 28. Seismic velocities and density changes in the earth
  22. 29. Predicting Earthquakes <ul><li>types of prediction </li></ul><ul><ul><li>long-term </li></ul></ul><ul><ul><li>intermediate term </li></ul></ul><ul><ul><li>short term </li></ul></ul><ul><li>earthquake model </li></ul><ul><li>science, politics, and the law </li></ul>
  23. 30. How can you tell a VALID prediction from a scam? <ul><li>Three factors are needed: </li></ul><ul><ul><li>HOW LARGE will the quake be? </li></ul></ul><ul><ul><li>WHERE will it occur </li></ul></ul><ul><ul><li>WHEN will it happen </li></ul></ul><ul><li>Predicting very small earthquakes is easy - they occur all the time. </li></ul><ul><li>Predicting LARGE quakes may be impossible. </li></ul>
  24. 31. LONG-TERM Predictions (years to decades) are based on tectonic indicators - fault activity, distance to active faults, seismic gaps etc INTERMEDIATE-TERM: (Year to days) based on increased activity in an area SHORT-TERM: (Day to hours) very difficult, but very valuable Predictions should include STATISTICS, such as: “ There is an 80% probability that a magnitude 7.5 or larger earthquake will occur in the San Francisco area within the next two weeks.”
  26. 33. We need to know how earthquakes are generated at faults. <ul><li>Tectonic plate motions </li></ul><ul><li>Elastic Rebound </li></ul><ul><li>Properties of materials - elasticity,strength </li></ul><ul><li>Strength of rocks vary with pressure, temperature and composition </li></ul>
  27. 34. Elastic Rebound Theory
  28. 35. A Simple Earthquake Model: <ul><li>What if the STRAIN across the fault continues to grow until a fixed maximum value is reached, at which time the earthquake occurs? AND - </li></ul><ul><li>What if the STRAIN always drops to the same minimum value ? </li></ul>
  29. 36. <ul><li>With a fixed maximum and minimum strain, earthquakes will all have the same size and will occur at predictable times. </li></ul>
  30. 37. What if only the Maximum Strain is fixed? In this case, the TIME of the next quake will be predictable, but the SIZE will not be. Small quake LARGE quake
  31. 38. What if only the Minimum Strain is fixed? In this case the SIZE of the next quake can be predicted - if it happens tomorrow, but WHEN it occurs is not constrained.
  32. 39. Let’s Try an experiment to see if either of these assumptions hold with a SIMPLE earthquake model… Turning the crank represents the passage of time, and the spring stretches (representing strain across the fault) until the weight moves - representing an earthquake.
  33. 40. What information do we need? We need to measure the length “A” to get our measure of how much time has passed, and We need to measure the length “B” to see how large the earthquakes are. “B” only changes during an earthquake. A-B is a measure of strain, and A is a measure of time, so, from these two measurements, we can make plots of strain vs. time and check our hypotheses.
  34. 41. If we plot “A” along the x axis and “B” along the y axis we get plots like: If we plot “A” along the x axis and “A-B” along the y axis we get plots like:
  35. 42. What did we learn? <ul><li>Did our hypothesis fit the data? </li></ul><ul><li>Could we predict the time and size of the earthquakes? </li></ul><ul><li>What about predicting earthquakes along a REAL fault? </li></ul>
  36. 43. Should you worry about earthquakes? <ul><li>Learn the earthquake history in your area </li></ul><ul><li>Are you likely to be hurt? </li></ul><ul><li>Are you likely to loose property? </li></ul>
  37. 45. Damage is caused by things falling on you from above
  38. 47. Adobe is NOT a good building material in earthquake-prone areas.
  39. 48. LOMA PRIETA, I 80
  40. 49. <ul><li>Sometimes things drop out from under you… </li></ul>
  41. 50. Alaska, 1964: Often the GROUND fails, not the building. Build on HARD ROCK!
  42. 52. KOBE
  43. 53. San Francisco Area Earthquakes
  44. 54. The California 1906 quake <ul><li>history </li></ul><ul><li>the earthquake </li></ul><ul><li>the fire </li></ul><ul><li>could it happen again? </li></ul>
  45. 55. Before the earthquake and fire
  46. 58. Buildings built on soft fill often collapsed.
  47. 59. Much damage in 1989 earthquake occurred in soft fill Where the 1906 trash had been dumped into the bay.
  48. 60. Seismogram wiggle size Vs. earthquake magnitude
  49. 61. How can you minimize your risk? <ul><li>avoid earthquake-prone regions </li></ul><ul><li>LEARN the earthquake history and fault locations in your area </li></ul><ul><li>don’t live below something that can fall on you, like a steep hill </li></ul><ul><li>don’t live below a dam </li></ul><ul><li>Don’t build on soft ground </li></ul><ul><li>store emergency supplies </li></ul>
  50. 62. TSUNAMI ! <ul><li>How can you minimize your risk? </li></ul><ul><li>What are the differences between surfing waves and tsunami? </li></ul><ul><li>What would you do if you were on the beach and the siren blows? </li></ul><ul><li>What would you do if you were on the beach and the water starts to slowly recede? </li></ul>
  51. 64. Thank you