Earthquakes occur on faults

1. Earthquakes occur on faults Active Fault

2. Earthquakes Create Seismic Waves

3. (also hypocenter)

4. 3 Types of Seismic Waves • P wave: Primary • S wave: Secondary • Surface waves Fastest Slowest

5. Depiction of Seismic Waves

7. Types of Earthquake Waves • Wave terminology • Period: time for one complete cycle between successive wave peaks to pass • Wavelength: distance between wave crests • Amplitude: amount of positive, negative wave motion • Frequency: number of peaks per second

8. Seismograph or seismometer is the machine, seismogram is the record

10. Northridge earthquake seismograms Keller, 2002

11. How do we measure Earthquakes? • How big is it? • Perceived effects: intensity • Amount of energy released: magnitude

12. #1: Intensity Scale • Mercalli Intensity Scale developed in 1902 • Based on effects • Local small quake is similar to distant large quake

13. An example of felt effects using Mercalli Scale http://www.scec.org/instanet/01news/images/NorthridgeSMap.gif

14. Felt effects using Mercalli Scale Hypothetical M7.8 earthquake

17. #2: Richter Magnitude Scale – ML – Amplitude • Logarithmic scale • Less accurate >M6.5

18. #3: Moment Magnitude Scale • Moment magnitude is measure of total energy expended during earthquake • Moment = (shear strength of rocks) x (surface area of rupture) x (slip distance on fault) • Mw • This is the most common scale for quakes >3.5

19. Comparison of two >M7.0 quakes Haiti Chile 2/27/10 Mw=8.8 Mercalli=VIII Subduction zone Depth=35 km Deaths: 521 • 1/10/10 • Mw=7.0 • Mercalli=IX • Strike-slip fault • Depth=13 km • Deaths: 92,000-230,000

20. Logarithmic Scales Each number is 32X energy Each 2 numbers =1000X Difference between 7.0 and 8.8 is >500X the energy released

22. Another example of M~1/f

23. Ground Shaking causes most damage • Ground shaking depends on • How much fault moved • Where fault moved • Local conditions amplify shaking and increase damage

24. Earthquake Hazards: Material amplification

25. Collapsed Floors Punctured by Load-Bearing Column Severe resonance oscillations of the buildings caused strain at the juncture between columns and ceiling slabs. The vertical columns were punched through the heavy floors that collapsed around them.

26. Generalized geologic map of Mexico city showing ancient lake deposits where greatest damage occurred. Keller, 2002

27. Ground Acceleration and Shaking Time • Acceleration – percentage of gravity (g) • Duration of shaking depends on size of earthquake • Quake >magnitude 6 increases area and total time of shaking • Amount of shaking decreases with distance from earthquake • Severity of shaking also depends on type of material waves travel through • Softer material  more intense shaking

28. Shaking in 1995 Kobe earthquake, Japan Causes collapse of freeway

29. Shaking in 1989 Loma Prieta earthquake, SF Bay Area Causes collapse

30. Secondary Ground Effects • Surface rupture-scarp • Earthquakes often trigger landslides • Can also cause liquefaction • Soils become almost liquid when shaken, solidify when shaking stops • Significant damage to structures atop liquefied sediments • Fires

32. Surface rupture-scarp

33. 1906 earthquake surface rupture. 8’ fence offset above http://mnw.eas.slu.edu/Earthquake_Center/1906EQ/1906thumb.html And http://quake.wr.usgs.gov/info/1906/images/fenceoffset_big.html

35. Australia, 1968, M6.8

36. Armenia, 1988, M6.9

37. Borah Peak, ID: 1983 M7.3

38. California, 1979, M6.9

39. Landslides caused by 2002 Denali Fault earthquake

40. 1965 Seattle quake M6.5

41. Liquifaction Water in sediment causes solid rock to behave like a liquid.

42. This residential and commercial building sank more than three feet into the partially liquefied soil.

43. Liquifaction: Niigata, Japan, 1964 :

44. San Francisco 1906 M8.3 Secondary effects: Fire

45. Summary • Earthquake wave types • Locating earthquakes • Intensity and scales • Primary Effects: Shaking • Secondary Effects: Landslides, scarps, liquefaction