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# GEOG 100--Lecture 13--Earthquakes

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### GEOG 100--Lecture 13--Earthquakes

1. 1. 11Folding, Faulting,Folding, Faulting,and Earthquakesand EarthquakesChapter 9Chapter 9
2. 2. 22DiastrophismDiastrophism• Folding and faulting causing deformationof Earth’s crust on a large scale is calleddiastrophism
3. 3. 33Compression, Tension, andCompression, Tension, andShearing stressesShearing stresses• Compression is force exerted inward  • Tension is a pulling apart  • Shearing occurs when force is exerted inopposite directions, but parallel to oneanother 
4. 4. 44Crustal Fold StructuresCrustal Fold Structures• monocline—a one-sided slope• syncline—a downfold that creates a U-shape• anticline—an upfold that creates an n-shape• overturned fold—similar to an anticline, but tipped to oneside• overthrust fold—an overturned fold pushed completelyover on its side, so that the entire fold lays on top of thesection in front of it
5. 5. 55FaultsFaultsEventually, even the plastic crust will break…
7. 7. 77Strike-slip/Transform/Transcurrent FaultStrike-slip/Transform/Transcurrent Fault(shear stress)(shear stress)
8. 8. 88The San Andreas runs rightThe San Andreas runs rightthrough the town of Hollister, CA…through the town of Hollister, CA…
9. 9. 99Common Structures AssociatedCommon Structures Associatedwith Transform Faultswith Transform Faults
10. 10. 1010EarthquakesEarthquakesor Hypocenter
11. 11. Earthquake Measurement Seismic waves—Energy waves propagated during anearthquake Seismograph—Recording device for measuring the amount ofshaking which occurs during an earthquake Seismogram—The printed record made by the seismograph
12. 12. 1212Quantitative vs. QualitativeQuantitative vs. QualitativeSeismic MeasurementsSeismic Measurements• Quantitative—Objective, fact-basedmeasurement; mathematical• Qualitative—Subjective; each person’sinterpretation of the same event may bedifferent
13. 13. 1313Measuring Seismic WavesMeasuring Seismic Waves• Richter Scale—A numerical expression of theamount of energy released during anearthquake event (Quantitative)– Based on the physical force exerted by the surfacemovement of earthquake waves– Logarithmic scale…the difference between one orderof magnitude and the next represents 10 times theamount of force– Only useful for expressing surface motion• Moment Magnitude Scale—More accurate athigher magnitudes (Quantitative)– Equations used to compare larger quakes (>4.0)--best for 7.0+
14. 14. 1414Measuring Seismic WavesMeasuring Seismic Waves• Mercalli Scale– Measures an earthquake’s intensity(Qualitative)– Based on what people feel– Uses questionnaires and personal accounts• Good for:– Earthquakes of the past for which no other records exist– Areas where existing development makes geologicstudies more difficult– Can help urban agencies plan for future earthquakes inareas needing upgrades or retrofitting
15. 15. 1515Loma Prieta Quake, 1989Loma Prieta Quake, 1989
16. 16. 1616Loma Prieta QuakeLoma Prieta Quake• On October 17, 1989, at 5:04:15 p.m. (PDT)• Magnitude 6.9 (moment magnitude)• Severely shook the San Francisco and Monterey Bayregions• Epicenter located near Loma Prieta peak in the SantaCruz Mountains, approximately 14 km (9 mi) NE ofSanta Cruz and 96 km (60 mi) S-SE of San Francisco• The Pacific and North American Plates abruptly slippedas much as 2 meters (7 ft) along the San Andreas fault• The rupture began at a depth of 18 km (11 mi) andextended 35 km (22 mi) along the fault, but it did notbreak the surface of the Earth
17. 17. 1717Loma Prieta Quake, 1989Loma Prieta Quake, 1989
18. 18. 1818Seismic WavesSeismic Waves• Body waves– Travel deep beneath the surface• P-waves• S-waves• Surface waves– Travel at or near the surface• L-waves• R-waves
19. 19. Seismic Waves P-waves: Pressure orPrimary waves Travel fastest First to arrive Travel through all mediums(solid, liquid, gas) If big enough, they can be felton the other side of the planet
20. 20. Seismic Waves S-waves: Secondary orShear waves Slower than P-waves Second to arrive Travel only through solids
21. 21. 2121Seismic WavesSeismic Waves• Love waves– Push rocks from side to side as the motion of thewave follows a horizontal ellipse which travelsforward
22. 22. 2222Seismic WavesSeismic Waves• Rayleigh waves produce an up and downmotion created by a forward roll, much like thatof an oscillating water wave on the open ocean– Slower than Love waves• 10 times the speed of sound
23. 23. 2323Both L-waves and R-waves:Both L-waves and R-waves:• Can travel only through solids• May not be felt at every earthquake event• Motion decreases exponentially the further thefocus is from the surface• Motions do not dissipate quickly• May continue for an extended period at the tailend of a quake• Can travel much longer distances than bodywaves• Tend cause the most damage to structures andto landforms prone to mass movement
24. 24. 2424Earthquakes and theEarthquakes and theRelationship to Plate TectonicsRelationship to Plate Tectonics
25. 25. 2525Pinpointing an EarthquakePinpointing an Earthquake
26. 26. 2626Pinpointing an EarthquakePinpointing an Earthquake
27. 27. 2727Earthquake Hazard MapEarthquake Hazard Map
28. 28. 2828Earthquake HazardsEarthquake Hazards• Falling debris and rock material• Crumbling buildings• Ground cracks• Broken bridges• Landslides• Liquefaction• Tsunamis
29. 29. 2929LiquefactionLiquefaction• Liquefaction– (from Latin liquefacere meaning “to liquefy”)– Settling of solid material and rising of water normallystored between the pore spaces, turning surfacematerial into quicksand– Danger where the water table is near the surface andsurface material consists of loose, unconsolidated,water-saturated sediments• Land may subside• Structures (and people!) sink– Only occurs during shaking• After shaking, settled material becomes solid again and anywater on the surface either percolates back down or runs offinto streams.
30. 30. 3030LiquefactionLiquefaction
31. 31. 3131Liquefaction under Kawagishi-choLiquefaction under Kawagishi-choapartment buildings, Niigata quake, 1964apartment buildings, Niigata quake, 1964
32. 32. 3232San Francisco: Original ShorelineSan Francisco: Original ShorelineLiquefaction Potential?Liquefaction Potential?
33. 33. 3333The Pacific Ring of FireThe Pacific Ring of Fire
34. 34. 3434The Pacific Ring of FireThe Pacific Ring of Fire
35. 35. 3535
36. 36. 3636What Does a Tidal WaveWhat Does a Tidal WaveLook Like?Look Like?Truro, Nova Scotia, Canada
37. 37. 3737You meant aYou meant a TsunamiTsunami,, right?right?
38. 38. 3838Produces:– Big earthquakes and volcanic islands, called“island arcs”– A deep ocean trench–High potential for tsunamisOceanic-Oceanic SubductionOceanic-Oceanic Subduction
39. 39. 3939TsunamisTsunamis• Waves caused by undersea volcanic or tectonicevents (earthquakes)• Unnoticed by observers on the open ocean• Can reach up to 100 feet in height when theyenter a coastal zone• Can be anticipated wherever deep-focusundersea earthquakes occur• Can be detected with special sensors, allowingtime for evacuation
40. 40. 4040Sumatra Quake, Dec. 26, 2004Sumatra Quake, Dec. 26, 2004http://www.nytimes.com/packages/khtml/2004/12/26/international/20041227_QUAKE_FEATURE.html