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Earthquakes&seismology butler

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Earthquakes&seismology butler

  1. 1. Part 2: Earthquakes & Seismology (See Part 1: The Earth & Plate Tectonics for background) Video lectures of this presentation can be viewed at: www.iris.edu/hq/programs/education_and_outreach/videos • “Normal” view has a Notes panel below with lecture notes and links to video lectures, activities, or web sites. •“Slide Show” view has elements that pop up on clicking •Video & animations were removed to increase speed. download information is in the Notes panel in Normal view www.iris.edu/hq/programs/education_and_outreach/animations • Modify slide show liberally for your own needs! Slide show prepared by Dr. Robert Butler, University of Portland, and Jenda Johnson.
  2. 2. Focus: or hypocenter; point within Earth where the EQ occurred. Epicenter—Location directly above EQ on Earth’s surface. Epicenter & Focus of Earthquakes
  3. 3. Why are there earthquakes? Brittle vs. ductile Example: Subduction-zone earthquakes occur in discrete areas on and between plates. Why? Watch the video lectures!
  4. 4. Body waves (P and S) travel inside Earth. Surface waves travel along Earth’s surface. Body Waves and Surface Waves While P- and S- waves radiate outward in all directions, surface waves travel along the surface of the earth and decrease in amplitude with depth.
  5. 5. P waves Fast S waves Intermediate Surface Slow waves Types of seismic waves
  6. 6. Body waves P waves are compressional Activity: “Seismic Slinky—Generating P & S waves”; see notes. ACTIVITY: Screen Grab Animation link in notes
  7. 7. S waves are shear waves Body wavesACTIVITY: Screen Grab Animation link in notes
  8. 8. Rayleigh & Love Waves Surface waves
  9. 9. A seismograph detects and records EQs. A seismogram is the EQ record. How do scientists detect earthquakes? When an earthquake occurs the seismic waves travel through the Earth to the seismic station where the information is transmitted to distant computers.
  10. 10. Seismometers Animations of horizontal and vertical seismographs and a movie of an actual seismogram being recorded during an earthquake are on the website: http://www.iris.edu/hq/programs/education_and_outreach/animations Screen Grab Animation link in notes Download one-pager, “No. 7. How Does a Seismometer Work?” http://www.iris.edu/hq/publications/brochures_and_ onepagers/edu
  11. 11. Narrated animations of basic effect of P, S, and Surface waves on a seismic station are on: http://www.iris.edu/hq/programs/education_and_outreach/animatio ns SeismicWaveBehavior— Effect on Building: 3-Component Seismograph: Screen Grabs Animation link in notes
  12. 12. Download the impressive interactive P.C. program with other earthquakes from Alan L. Jones’ website: http://www.geol.binghamton.edu/faculty/jones/ If you don’t have a PC, you can watch the progression (and how complicated it can be) in an animated replication of a part of the computer program: http://www.iris.edu/hq/programs/education_and_outreach/animations ACTIVITY: Seismic waves from the 2004 Sumatran earthquake were recorded at stations all over the globe. Screen Grab Animation link in notes
  13. 13. Travel-time curves Seismic waves from large earthquakes arrive at seismic stations around the world at predictable times. The graph on the right (turned 90° and includes only major direct waves) shows the arrival times as continuous lines for each of the waves reflecting the pattern in the seismograms above. (see animation “Travel-time curves”) Animation link in notes
  14. 14. A seismogram is the record of an earthquake. P waves travel about 1.7 times faster than S waves. Surface waves take their time rolling along at shallow levels. The difference between the arrival times of the seismic waves indicates of how far away the earthquake was. QUESTION: If a P wave arrived at a station 5 minutes and 45 seconds after the earthquake occurred; and the S wave arrived at the station 10 min and 30 seconds minutes after the
  15. 15. How far away was the earthquake? Calculate S-arrival time minus P-arrival time using this graph of travel-time curves. ACTIVITY:
  16. 16. How can you use this information to locate an earthquake? ACTIVITY: Graph of travel-time curves
  17. 17. Locating an Earthquake 1) Determine distance of EQ from three seismic stations by calculating the S minus P arrival times. 2) Plot them on the travel- time graph. 3) Intersection of the circles gives the location. 1 23 ACTIVITY: See: http://www.iris.edu/hq/programs/education_and_outreach/animations#N
  18. 18. How big was it? The Richter Scale What is the Richter magnitude of this EQ?
  19. 19. S — P = 26 sec Amplitude = 23mm Magnitude = 5 How big was it? The Richter Scale What is the Richter magnitude of this EQ?
  20. 20. Magnitudes and Energy of Earthquakes Download: How Often do Earthquakes Occur? Annual Numbers of EQs What’s the message? MOST of the energy is released by around 20 magnitude-7 and larger EQs every year.
  21. 21. Magnitudes and Energy of Earthquakes Download: How Often do Earthquakes Occur? Annual Numbers of EQs What’s the message? MOST of the energy is released by around 20 magnitude-7 and larger EQs every year.
  22. 22. The Earthquake Machine Think forces, faults, and friction. How could pulling a block of wood with a string of rubber bands have anything to do with earthquakes? There are no rubber bands in the Earth, but all solids are elastic, including the Earth’s crust. ACTIVITY: ACTIVITY & ANIMATIONS are on http://www.iris.edu/hq/programs/education_and_outreach/animations/1
  23. 23. Earthquake Intensity is what you feel. What Controls the Level of Shaking? • Magnitude—More energy released • Distance—Shaking decays with distance • Geology—Local soils amplify the shaking • Building style—Construction, not height • Duration of shaking Example: 1994 Northridge EQ, M=6.7
  24. 24. Seismic intensity is affected by rock type. Amplitude of oscillation How would you expect the houses to react during an EQ?
  25. 25. Seismic intensity is affected by rock type. Amplitude of oscillation increasing
  26. 26. Liquefaction experiment What happens to a structure built on a weak foundation when an earthquake strikes? Photos: Lily Rodriguez From website below For the description of this liquefaction activity: http://www.exploratorium.edu/faultline/activezone/liquefaction.html Niigata, Japan 1964 Source: National Geophysical Data Center
  27. 27. Building design: If the resonant frequency of a building is equal to the frequency of ground oscillation, then damage or collapse is likely. What is resonant frequency? Discussion: Why is building damage selective?
  28. 28. Resonance Oscillating buildings demonstration—3 methods. BOSS model Video demonstrations by John Lahr are online: http://www.iris.edu/hq/programs/education_and_outreach/videos Two similar demonstrations use simpler methods: Spaghetti & Manilla file raisins……. “Boss lite”… Screen Grabs
  29. 29. Building design is critical to earthquake safety Resistance to shear is critical. See the video lecture on Building Strength on: http://www.iris.edu/hq/programs/education_and_outreach/videos

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