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  1. 1. What is an earthquake? <ul><li>Shaking or vibration of the ground </li></ul><ul><li>rocks undergoing deformation break suddenly along a fault </li></ul>1906 San Francisco earthquake
  2. 2. Oblique view of the San Andreas fault and San Francisco
  3. 3. Where are earthquakes found? <ul><li>The Earth’s surface is composed of a number of mobile “ tectonic plates ” which are in constant motion </li></ul><ul><li>Most earthquakes are found at plate margins </li></ul>
  4. 4. Plate tectonics <ul><li>The constant movement of the plates is referred to as plate tectonics </li></ul><ul><li>There are three main types of plate boundaries: </li></ul><ul><ul><li>divergent </li></ul></ul><ul><ul><li>convergent </li></ul></ul><ul><ul><li>transform </li></ul></ul>
  5. 5. Divergent margins <ul><li>Here two tectonic plates are in the process of being created </li></ul><ul><li>Magma is injected into a crack, then cools and becomes new crust </li></ul>
  6. 6. An example of a wide, mature divergent margin <ul><li>The middle of the Atlantic Ocean is a divergent margin which is being torn, or rifted, apart…the two plates are separating continuously at a rate of several cm/yr </li></ul>
  7. 7. An immature divergent plate margin <ul><li>The Red Sea represents a young rift which is just beginning to separate Arabia from Africa… </li></ul><ul><li>Here, too, volcanism is evident, as a result of rifting </li></ul>
  8. 8. Volcanism in the Afar triangle <ul><li>‘ Erta ‘Ale, a volcano slightly west of the Red Sea, represents the splitting apart and thinning of the African continent </li></ul>
  9. 9. Convergent margins I <ul><li>Instead of two plates being created, they are being consumed… </li></ul><ul><li>Here an oceanic plate slides beneath a continental plate, since the former is denser </li></ul><ul><li>geologists refer to this process as subduction </li></ul><ul><li>Large, destructive earthquakes occur here </li></ul>
  10. 10. Convergent margins II <ul><li>If two continental plates collide, they do not subduct, because they are too buoyant </li></ul><ul><li>Instead, intense compression with crustal shortening and thickening occur </li></ul><ul><li>Large, destructive earthquakes also are generated in this situation </li></ul>
  11. 11. Transform margins <ul><li>The third type of plate margin is called a transform boundary </li></ul><ul><li>Here, plates are neither created nor destroyed… </li></ul><ul><li>they simply slide by one another </li></ul>
  12. 12. So here’s the big picture of what we’re living on
  13. 13. Where are the world’s earthquakes in terms of plate tectonics? <ul><li>The great majority of earthquakes are located at plate margins </li></ul><ul><li>This where magmatism, friction, faulting, etc., are most intense </li></ul><ul><li>Earthquakes in plate interiors are comparatively rare </li></ul>
  14. 15. The Pacific Rim of Fire <ul><li>This notorious zone is characterized by subduction zones </li></ul><ul><li>Earthquakes and volcanoes here are particularly violent </li></ul><ul><li>friction from subduction produces large destructive quakes </li></ul>
  15. 16. North American seismic hazards
  16. 17. Canadian seismic hazards
  17. 18. Seismic hazard in eastern Canada
  18. 19. Faults associated with earthquakes <ul><li>Faults are planes of weakness along which the Earth has been broken </li></ul><ul><li>Movements on a fault can be either slow (ductile deformation) or fast (brittle fracture) </li></ul><ul><li>When a fault behaves in a brittle manner and breaks, earthquakes are generated </li></ul>
  19. 20. Three types of dominantly vertical faults <ul><li>A normal fault is the result of tensional forces (e.g., rifting) </li></ul><ul><li>Reverse and thrust faults are the result of horizontal compression </li></ul>
  20. 21. Faults whose movement is dominantly horizontal <ul><li>These faults are termed strike-slip faults </li></ul><ul><li>They are a small-scale version of transform plate tectonic margins </li></ul><ul><li>They are termed left-lateral (sinistral) or right-lateral (dextral) according to their movement </li></ul>
  21. 22. Earthquake generation along a fault <ul><li>The earthquake focus is its point of origin along a fault plane </li></ul><ul><li>Its epicenter is the vertical projection of the focus to the surface </li></ul>
  22. 23. Elastic rebound theory <ul><li>Before fault rupture, rock deforms </li></ul><ul><li>after rupture, rocks return to their original shape… </li></ul><ul><li>...maybe 1 </li></ul><ul><li>1 Pallett Creek shows similar slip amounts after different periods of time; possibly not resetting to zero? See Sieh and Levay, 1998, p. 90 </li></ul>
  23. 24. Richter magnitudes <ul><li>The Richter magnitude measures the maximum amplitude of ground shaking </li></ul><ul><li>It is a logarithmic scale </li></ul><ul><li>1 Richter unit difference is x 10 for ground motion and x 33 for energy </li></ul><ul><li>Globally, small earthquakes are more frequent than large: </li></ul><ul><li>~800,000/yr for events of magnitude 2.0-3.4 </li></ul><ul><li>while an event of magnitude 8 occurs once every 5-10 years </li></ul>
  24. 25. Richter magnitudes
  25. 26. Destructiveness of an earthquake <ul><li>Earthquake magnitude </li></ul><ul><li>Distance to epicenter </li></ul><ul><li>Depth </li></ul><ul><li>Strength of building </li></ul><ul><li>Nature of soil or bedrock on which foundations are built </li></ul><ul><li>Other local conditions </li></ul>
  26. 27. A challenge <ul><li>You yourself can calculate Richter magnitudes and epicenters from seismogram data. Go to: </li></ul><ul><li> </li></ul><ul><li>Not only will you understand the science behind earthquake determinations, there are also material rewards ... </li></ul>
  27. 28. Diplomas !
  28. 29. The San Andreas fault <ul><li>Along much of the west coast, the plate boundary is a transform margin </li></ul>
  29. 30. San Andreas fault <ul><li>Although some people think San Francisco is “falling” into the Pacific Ocean, part of the city is actually already part of the Pacific plate </li></ul><ul><li>The San Andreas is a right-lateral strike-slip or transform fault </li></ul>
  30. 31. San Andreas fault
  31. 32. Right-lateral motion Photos from Shelton, 1966
  32. 33. Right-lateral motion Photo, diagram from Sieh and LeVay, 1998
  33. 34. Some history <ul><li>The strike-slip nature of the San Andreas was not widely appreciated for up to 50 years after the 1906 San Francisco earthquake </li></ul><ul><li>Yet rocks on either side of the fault are different </li></ul><ul><li>The older the rocks, the greater the displacement </li></ul><ul><li>Eocene-age rocks (37-58 Ma) show offsets up to 300 km </li></ul>
  34. 35. San Francisco, 18 April 1906 <ul><li>Magnitude 7.8, epicenter near San Francisco </li></ul><ul><li>$ 400 million US in damage </li></ul><ul><li>this is 1906 dollars ; equivalent to hundreds of billions of dollars today </li></ul><ul><li>~700 people reported killed </li></ul><ul><li>this is probably a 3-4 times underestimate ; thus 2,000-3,000 dead, mostly in San Francisco </li></ul>
  35. 36. 1906 - location and seismic trace Seismic trace of 1906 quake from a seismic station 15,000 miles away in Gottingen, Germany
  36. 37. 1906 - comparative magnitude <ul><li>This event is northern California’s most powerful event in recorded history </li></ul>
  37. 38. 1906 - extent and slip The northernmost 430 km of the San Andreas ruptured, with horizontal slippage up to 8-9 meters
  38. 39. 1906 - slip <ul><li>This photograph shows a fence near Bolinas offset 2.5 meters </li></ul>
  39. 40. 1906 - intensity and shaking <ul><li>Maximum Mercalli values were VII to IX, which represent severe damage </li></ul><ul><li>Shaking lasted 45-60 seconds (for Loma Prieta 1989 and Northridge 1994, shaking lasted 5-10 s) </li></ul><ul><li>Shaking intensity correlated with geology, e.g., bedrock vs. landfill </li></ul>
  40. 41. 1906 - earthquake damage in San Francisco
  41. 42. 1906 - earthquake damage in San Francisco
  42. 43. 1906 - earthquake damage in San Francisco
  43. 44. 1906 - some lessons learned <ul><li>Big quakes can be followed by decades of seismic quiet </li></ul><ul><li>Quakes the size of the 1906 event appear to occur every several hundred (200?) years </li></ul>
  44. 45. 1906 - some lessons learned (ctd.) <ul><li>In the short term, San Francisco and environs are most at risk from an event of magnitude 6-7 </li></ul>
  45. 46. 1906 - some lessons not learned A topographic map of San Francisco from 1950... … and a 1980 version of the same map
  46. 47. Future quakes in the San Francisco Bay area Note the high probability of an earthquake of M > 6.6 occurring before 2030 in this area
  47. 48. Cascadia <ul><li>In the Pacific Northwest, the tectonic regime is subduction-related , rather than transform as we have seen in California </li></ul>
  48. 49. Cascadia Here, there is evidence for very large earthquakes over the last several thousand years…the most recent is 300 years ago
  49. 50. Quebec <ul><li>The St. Lawrence region has high levels of seismicity for a zone in the interior of a tectonic plate </li></ul><ul><li>This seismicity may be related to old, aborted rifts about 200 Ma ago </li></ul>Map from Lamontagne (1999)
  50. 51. Quebec - Montreal region <ul><li>Ottawa River axis </li></ul><ul><li>more active Montreal-Maniwoki axis </li></ul><ul><li>M 5.8, 1732, Montreal </li></ul><ul><li>M 6.2, 1935, Temiscamingue </li></ul><ul><li>M 5.6, 1944, Cornwall-Massena, NY </li></ul>
  51. 52. Quebec - Charlevoix region <ul><li>Events: 1638; M7 1663; M6 1791; M6.5 1870; M6.2 1925 ($ 2 million in damage at the time) </li></ul><ul><li>fracturing and high pore fluid pressures </li></ul><ul><li>old rift faults serving as conduits for pressurized crustal fluids, which trigger quakes </li></ul>
  52. 53. Charlevoix Charlevoix also has evidence for a meteorite impact crater, which served to fragment and fracture rocks (from Lamontagne, 1999)
  53. 54. Effects of earthquakes: aftershocks <ul><li>Aftershocks normally occur after a major earthquake </li></ul><ul><li>There may be many thousands of aftershock events over the space of months or even years </li></ul><ul><li>Although their magnitudes generally decrease with time, aftershocks have potential to cause significant damage to already weakened materials (e.g., rocks, soils, buildings, power and gas lines) </li></ul>
  54. 55. Effects: liquefaction <ul><li>Wet, unsolidated soils and sediments are highly vulnerable </li></ul><ul><li>Under shaking, the ground simply flows </li></ul><ul><li>Landfills, harbours, and the like are at risk </li></ul>Liquefaction hazard in the San Francisco Bay area
  55. 56. Effects: landslides <ul><li>The ground vibrations and severe shaking associated with an earthquake can induce landslides in mountainous areas </li></ul><ul><li>This example in the Santa Susana Mtns. was caused by the 1994 Northridge event near Los Angeles </li></ul>
  56. 57. Effects: tsunamis <ul><li>Tsunamis are ocean waves caused by displacements from earthquakes, landslides, etc. </li></ul><ul><li>They can be devastating at great distances from the epicenter </li></ul>Tsunami damage in Hilo, Hawaii, as a result of the 22 May 1960 Chile earthquake
  57. 58. Effects: building destruction <ul><li>Buildings are damaged or destroyed by ground vibrations and shaking </li></ul><ul><li>The magnitude and duration of shaking are important factors in the extent of damage </li></ul><ul><li>Liquefaction and aftershocks increase the damage </li></ul>Building damage near the epicenter of the 1989 Loma Prieta earthquake
  58. 59. Effects on building materials <ul><li>Masonry is not capable of withstanding significant bending stresses </li></ul><ul><li>Wood is more resistant because it is more yielding </li></ul><ul><li>But wood is vulnerable to fires ... </li></ul>
  59. 60. Effects: fires <ul><li>The ground shaking will rupture power and gas lines… </li></ul><ul><li>… and damage to water mains prevents or hinders fire fighting efforts </li></ul><ul><li>the photo shows a broken gas line from the 1994 Northridge earthquake </li></ul>
  60. 61. Devastating fires in San Francisco after the 1906 earthquake
  61. 62. Effects: personal loss <ul><li>We are examining earthquakes from a scientific perspective… </li></ul><ul><li>… but we must not forget the human element and the pathos conveyed by this photograph from the 1994 Northridge earthquake </li></ul>
  62. 63. Mitigating earthquakes <ul><li>Seismic hazard maps and risk maps help to properly site and construct buildings </li></ul>
  63. 64. Where to build your dream or trophy house - and where not to build <ul><li>Avoid unstable soils and unconsolidated materials... </li></ul><ul><li>avoid mountainous terrain prone to landslides… </li></ul><ul><li>and above all, avoid active faults ! </li></ul>
  64. 65. Appropriate building codes which can withstand earthquake damage <ul><li>Bedrock foundations best </li></ul><ul><li>Avoid asymmetrical buildings </li></ul><ul><li>Bolt house firmly to foundations </li></ul><ul><li>Appliances firmly bolted down </li></ul><ul><li>Gas lines flexible </li></ul><ul><li>Cupboards, shelving attached to walls </li></ul><ul><li>Heavy objects at low levels; anchor heavy furniture </li></ul><ul><li>Beds away from windows to avoid broken glass </li></ul>
  65. 66. Warning and prediction <ul><li>Precursory seismicity </li></ul><ul><li>Precursory deformation </li></ul><ul><li>Changes in physical properties of rocks near a fault </li></ul><ul><li>Changes in water levels, soil gases </li></ul><ul><li>Unusual behaviour of animals </li></ul>
  66. 67. Earthquake prediction <ul><li>Important concepts: </li></ul><ul><li>earthquake recurrence interval…seismic gap </li></ul><ul><li>role of paleoseismology </li></ul><ul><li>Yet our predictive ability is rudimentary , so we use probabilities </li></ul><ul><li>e.g., 86% probability that a destructive quake of M>7 will hit southern California in the next 30 years (1994 estimate) </li></ul>
  67. 68. Earthquakes - reading <ul><li>U.S. Geological Survey, 1999. Major quake likely to strike between 2000 and 2030. U.S. Geological Survey Fact Sheet 152-99, 4 pp. ( ) </li></ul><ul><li>Pelman, D., 2000. Tiny movements ease fault risk in East Bay; pressure builds up less in northern Hayward segment. San Francisco Chronicle, 18 August 2000. ( ) </li></ul><ul><li>Eastern Canadian seismicity: </li></ul><ul><ul><li> </li></ul></ul>
  68. 69. Earthquakes - web <ul><li>Canadian seismicity: </li></ul><ul><ul><li> </li></ul></ul><ul><li>US seismicity: </li></ul><ul><ul><li> </li></ul></ul><ul><li>San Francisco Bay area: </li></ul><ul><ul><li> </li></ul></ul>