1. Earthquakes

2. Reading Smith Ch 6

3. Tectonic Hazards Tectonic: forces of ‘earth-building’ –Volcanoes –Earthquakes Very much connected –plate tectonics –most earthquakes on plate boundaries, fault lines driven by plate movements.

6. Earthquakes Sudden crustal movements Usually shallow 0-700 km deep

7. Earthquakes Slow build-up of strain –driven by mantle convection? Sudden release as crust moves: earthquake Seismic waves radiate outwards

8. Earthquakes Most damaging earthquakes are shallow –>40 km depth

9. San Fernando EQ 1971 Richter M 6.4 Depth: 13 km

10. Tangshan EQ 1976 At shallow depth Killed 250,000 to 750,000

11. Earthquakes Hypocentre: –underground centre of earthquake Epicentre –the point on the surface above the hypocentre

13. Earthquake Magnitude Richter Scale Moment Magnitude Mercalli Scale

14. Richter Scale a.k.a. Local magnitude Energy of seismic waves 100 km from source =logarithm of ground motion in micrometres 100km from epicentre Scale is logarithmic –M7 is 10x motion of M6

15. Richter Scale >M 5.5 for a major disaster Most Eqs are small Most EQ energy released in the Big Ones

16. Moment Magnitude Surface area of fault displaced <M6.5 use Richter >M6.5 use Moment magnitude

17. Kobe EQ 1995 M6.9 Strong shaking on soft soils, landfill Dense urban area nearby –Older homes with heavy clay-tile roofs not earthquake proof

18. Problems with these scales Richter & Moment Magnitude don’t measure shaking and overall impact, just seismic energy Can only be used if ground movement, epicentre, fault area is known Useless for “historic” quakes

19. Modified Mercalli Scale MM scale: intensity of damage Roman numerals MM: I = not felt at all MM: XII= widespread destruction

21. Seismic Waves Vertical stress, short wave period –‘P’ or primary waves –‘S’ or secondary waves –Moment Magnitude measures this Horizontal stress, long wave period –Love Waves –Rayleigh waves –Richter scale /local magnitude measures this

24. P Waves compression-dilation 8km/sec in solid & liquid

25. S Waves Sine waves 4km/sec in solid only can be destructive

26. Love & Rayleigh Waves Horizontal shaking at 90 degrees to wavefront Most damaging to buildings

27. Other issues Ground acceleration Wave frequency

28. Mexico City EQ 1985 High rise buildings collapse especially if in 22-25 storey range Taller and shorter buildings less damaged EQ has just the right resonance to tear certain buildings apart

29. Small buildings Resonant to short wave periods P&S waves knock them down

30. Tall buildings Resonant to long wave periods Love & Rayleigh waves knock them down

31. Effects of Local Topography Soft bouncy sediments amplify wave effects –ash –alluvium –landfill Mexico City EQ 1985, San Salvador EQ 1986: cities on bouncy sediments

32. Secondary Effects Soil failure Slope failure Tsunamis

33. Soil Liquifaction Wet soil can liquify Loses load-bearing properties Richmond BC: –If Richter M6.5 EQ, high-rises may fall like dominoes in Fraser delta sediments

37. Lateral Spread On shallow slope soil moves sideways

38. Ground Oscillation Soil blocks rotate and surface crumples

39. Loss of Bearing Strength Buildings sink into soil

40. Flow failure On steeper slopes, slope collapses Can cause damage over extensive area, long distance

48. Landslides, Rock & Snow Avalanches EQ triggers slope failure

49. Huascaran Mountains, Peru 1970 30m high wave of rock & mud Travelled 70-100 m/sec buried Yungay & Ranrahirca Debris 10 m deep, Killed 18,000 in 4 minutes

50. Tsunami a.k.a. “tidal waves” Large shallow submarine EQ triggers shockwave in sea Difficult to spot out at sea Moves fast

51. Tsunami Wave height & speed increase as wave moves into shallow coastal water Begins with water drawdown Then violent oscillation for hours

56. Human Vulnerability Many EQ zones well-known but densely inhabited Populations often in coastal zone, valley lands –vulnerable to Tsunamis, debris flows –cities built on wobbly sediments

59. Human Vulnerability Increased by poor construction techniques –not designed for horizontal shear –failure to avoid vulnerable sites, soil conditions –failure to avoid vulnerable zones

60. Maharashtra 1993

61. Timing Deathtoll and damage rises if timing is unfortunate –1923 Tokyo Bay EQ struck at lunchtime, starts fires, 130,000 die –1976 Tangshan EQ struck at night, people trapped indoors

62. Trigger Earthquakes by crustal loading –build a large dam –storm surge in cyclone conditions 1923 Tokyo Bay EQ

63. Prediction Easier to predict where an EQ will happen than when –spotting “locked” areas surrounded by zones of movement Forecasting not advanced –West: relies on science Failed to spot Kobe 1995, Northridge 1994 –China: relies on culture (more accurate) Failed to spot Tangshan 1976