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Earthquakes

Earthquakes. Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency and regularity Prediction? Mitigation and preparedness. Causes: accumulated strain leads to fault rupture - the elastic rebound model.

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Earthquakes

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  1. Earthquakes • Causes - tectonics and faults • Magnitude - energy and intensity • Earthquake geography • Seismic hazards - shaking, etc. • Recurrence - frequency and regularity • Prediction? • Mitigation and preparedness

  2. Causes:accumulated strainleads to fault rupture- the elastic rebound model

  3. North American tectonic regimes (much simplified)

  4. Styles of faulting

  5. Causes: fault movement releases energy as seismic waves radiating from rupture Seismic waves

  6. Seismic wave forms S wave P wave L wave (Rayleigh wave) L wave (Love wave)

  7. Earthquake magnitude:scales based on seismograms • ML=local (e.g. Richter scale) - based on amplitude of waves with 1s period within 600 km of epicentre. • Mb=body-wave (similar to above) • Ms=surface wave (wave periods of 20s measured anywhere on globe • Mo=seismic moment • Mw= moment magnitude

  8. The Richter scale Steps:1. Measure the interval (in seconds) between the arrival of the first P and S waves.2. Measure the amplitude of the largest S waves.3. Use nomogram to estimate distance from earthquake (S-P interval) and magnitude (join points on S-P interval scale and S amplitude scale).4. Use seismograms from at least three geographic locations to locate epicentre by triangulation.

  9. 2 The Richter scale nomogram 1 Steps 3 Nomogram

  10. Z Locating the epicentre:X, Y and Z are seismograph stations 280 km Y 220 km X epicentre 150 km

  11. Earthquake magnitude:scales based on rupture dimensions (equivalent to energy released ) • Mo= seismic moment. = m * A * d, where m is the shear modulus of rock; A is the rupture area, and d is displacement • Mw= moment magnitude. = 2/3 * log Mo - 10.7 N.B. moment scales do not saturate

  12. Saturation of non-moment scales

  13. Earthquake magnitude:scales based on shaking intensity e.g. Mercalli, Rossi-Forel, San Francisco scalesMMI (=Modified Mercalli Index)I Not felt…..VI Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Pictures fall off walls. Furniture moved, trees shaken visibly. ….XII Damage nearly total. Objects thrown into air. Sichuan earthquake, May 12, 2008

  14. Earthquake geography Source: GSHAP, Switzerland

  15. Seismic hazard: North & Central America

  16. Seismic hazards • Locating faults • Estimating recurrence: history and geology • Measuring relative motions and crustal deformation • Learning from analogies • Assessing probabilities

  17. Locating faults: Seattle Fault (LIDAR image)

  18. Prediction:where will the next earthquake in the Bay Area occur? Berkeley San Andreas Oakland Hayward San Francisco San Jose Santa Cruz

  19. The Hayward fault runs through UC Berkeley campus(US $1 billion seismic upgrade program) Lawrence Livermore UC Berkeley

  20. Recurrence - historical records San Francisco City Hall, 1906

  21. Recurrence:geological evidencee.g. Pallett Creek, CA(after Sieh, et al. 1989)

  22. Prediction:current crustal deformation

  23. Prediction: crustal velocity (mm/yr)from repeated GPS measurements at permanent stations Why are all stations moving to NW?

  24. Learning from analogues(Turkey - California)

  25. N.B. A probability of 70% over 30 years is equivalent to a daily probability of 1 : 15 000 The Bay Area:earthquake probabilities(AD2000-2030)

  26. Probabilities, yes!but prediction, no! • 1996 - Earthquake prediction group of Japanese Seismological Survey voluntarily disbands (after Kobe) • 2000 - British researcher argues that prediction of main shock impossible at present; immediate goal should be prediction of aftershock location and magnitude

  27. Individual seismic hazards • Shaking = accelerated ground motion • Liquefaction = failure of waterlogged sandy substrates • Landslides, dam failures, etc. • Tsunamis = seismic sea waves • Fire, etc.

  28. Predictions of shaking intensity onSan Andreas fault (long segment) in the Bay Area Rupture

  29. Shaking and liquefaction: the importance of surficial geology

  30. Building collapse as a result of soil liquefaction, Niigata, Japan, 1964

  31. Liquefaction and the urban fire hazard:San Francisco, 1906 2-6 m of lateral displacement in old marsh soils -> 300 breaks in water lines City lost 90% of water supply; fires raged out of control Photos: Archives, Museum of San Francisco

  32. Ground motion, structural damage and basin morphology: Mexico City, 1985 Damage heavy light heavy body\surface surface/body ridge basin basin random periodic periodic

  33. Bedrock topography underlying Fraser delta

  34. Earthquakes don’t kill; buildings do! Building harmonics Buildings at high risk • URM = unreinforced masonry; • open lower storeys; • poor ties to foundations and between storeys; • lack of cross-bracing; • poor quality materials. Collapsed school building, Ying Xiu, Sichuan, China (May 12, 2008); >10,000 children died in this earthquake

  35. The response of mud-brick buildings to ground shaking Pre-earthquake The 2 000-yr old citadel in Bam, Iran Post-earthquake (Dec. 2003)

  36. Muzaffarabad, Pakistan (October 8, 2005 M 7.7; depth 10km) “Much of the building is done by people putting up their own houses. But they cannot afford proper materials and do not use skilled labour. There are many small kilns producing bricks but because of demand these are not fired for the 28 days needed to make them strong.” Mohsen Aboutorabi, Professor of Architecture, (BBC News, 2003/12/30, discussing the Bam earthquake in which ~40,000 died)

  37. << << wall collapse, Pakistan, 2005 Complete collapse of multi-storey apartment, Pakistan, 2005 >>>> <<<< pancaking of ‘soft-storey’ buildings near Algiers (May, 2003);

  38. Bridge collapse Loma Prieta earthquake, CA (1989)

  39. Preparedness (examples) Buildings - site selection, design to code, retrofit, upgrade codes; Strengthen bridges, dams, pipelines; Earthquake drills - houses, schools, search & rescue; Emergency planning - survival kits, evacuation routes, fire prevention, utility failures, communication alternatives, education

  40. Preparedness: Modifying the building code in the western US 1969 1976 1988 1996 UBC = Uniform Building Code

  41. Public education?

  42. Post-earthquake adjustments Compare: • abandonment of Antigua Guatemala (mid-C18th) vs. • reconstruction of Lisbon (post-1755), San Francisco (post-1906), Kobe (post-1995).

  43. Cascadia: megaearthquakes at the plate boundary Mw = 9.2?

  44. 9.2 (1964) 9.3 (2005)

  45. Earthquake sequences, Nankai Trough and Cascadia or here? S U W W? Y

  46. The scientistsKenji Satake Alan Nelson Brian Atwater

  47. Y U W S Buried marsh soils as evidence for interplate earthquakes at Cascadia

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