1 / 29

Earthquakes

Earthquakes. Seismology – the study of Earthquakes Sudden release of pent-up energy in the Earth’s crust Strain energy – crustal rocks store stress until they fail or rupture Focus – precise spot BELOW the Earth’s surface where rupture occurs

saber
Download Presentation

Earthquakes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Earthquakes • Seismology – the study of Earthquakes • Sudden release of pent-up energy in the Earth’s crust • Strain energy – crustal rocks store stress until they fail or rupture • Focus – precise spot BELOW the Earth’s surface where rupture occurs • Epicenter – point ON the Earth’s surface directly above the focus • Foreshocks – possible tremors foreshadowing a coming major event (often associated w/magmatic movement) • Aftershocks – tremors associated as rocks adjust to new positions

  2. Focus and Epicenter

  3. Seismic Waves • Earthquake energy transmitted through the Earth • Waves travel and are deflected/reflected along internal boundaries of the Earth • Body Waves • P waves – Primary waves, fastest (4mi/sec), arrive first at seismic stations, compression waves, parallel to direction of wave propagation, change in shape and volume of rock • S waves – Secondary waves, slower (2mi/sec), arrive next at seismic stations, shearing waves, perpendicular to direction of wave propagation (up and down), change in shape but not volume • Cannot travel through liquid medium • Indirect evidence for liquid outer core • Surface Waves • Rolling • Side-to-side • Travel in upper few km of crust, slowest (1.5mi/sec), cause most damage to rigid structures

  4. Primary and secondary seismic wave paths • Location of Earth layer boundaries mapped by behavior of seismic waves • Note secondary waves do not travel through the outer core evidence for the liquid nature of this layer • Much that is known about the physical properties of the Earth’s interior have been derived through study of seismic wave behavior as they travel through the Earth

  5. Earthquake Frequency vs Magnitude • Inverse correlation between magnitude of EQ’s and frequency of occurrence • Most EQ’s are never felt even by those who live nearby • Valle, Az. area is most active seismic zone • Intersection of Bright Angel and Cataract Creek fault systems

  6. Measuring Earthquakes • Richter Scale – defined magnitude of largest peak traced on a seismograph • Logarithmic (i.e. each successive unit is 10 times greater than previous one • Actually a 33 fold increase • Not accurate for events over 7.0 magnitude • Moment –Magnitude Scale • Length of fault rupture X depth X slip X strength of rock • More accurate because it can be calculated directly

  7. Cylindrical Seismograph

  8. Seismograms

  9. Relative time lag between seismic waves • Single station recording

  10. Seismic Waves • Delay of shock arrival at different stations • Note attenuation of surface waves

  11. Depth and Magnitude • Shallow • < 70 km • 90% of all quakes occur at depths of < 100 km • Can accumulate large amounts of strain energy • Brittle failure • Intermediate • 70 – 300 km • Deep • > 300 km • Heat weakens rocks ability to store strain energy • Less brittle failure

  12. Historical record of largest EQ’s in U.S.

  13. Potential Seismic Shake Map • Color contour map of potential EQ hazard in the continental U.S. • Red indicates zones where hazard potential if greatest • Note red traces plate boundary and associated volcanic area related to plate subduction • The New Madrid seismic zone is somewhat of an anomaly • Old plate boundary where lithosphere is thinned • Highest potential for event occurrence east of Miss. River • Perhaps largest EQ ever in U. S.

  14. Foci of New Madrid EQ’s • Location of EQ’s recorded since 1974 • Note location relative to breakup of Pangaea ~200mya • Mississippi River Valley probably the inactive 3rd arm (aulocogen) rift zone

  15. Earthquake Consequences • Consequences depend on combination of factors • Magnitude • Depth • Distance from pop centers • Geology of area • Structural soundness

  16. Effect of substrate composition on seismic wave amplitude

  17. Earthquake Effects • Ground Displacement – along fault plane • Strike-slip • Dip-slip • Can be meters at a time • Can account for 1000’s meters over time • Landslides • Rock fragments detach from bedrock • Sedimentary rocks slip along bedding planes • Metamorphic rocks shift along foliation planes • Loose sediment moves down-slope • Liquefaction • Converts saturated ground with some cohesiveness into sediment that can flow like water (mud slurry – water pressure forces grains out of contact) • Seiches • The back and forth movement of water in enclosed areas

  18. Effects cont’d • Changes in elevation • Uplift • Subsidence • Tsunamis • Caused by submarine landslides and faulting • Can travel at speeds > 500mph • About 1m in height • Can travel across large distances • Speed, shallow water and departure of water near coast responsible for most damage • Fires • Gas mains, electrical power lines, oil/gas storage • Disease • Broken sewer and water lines • Disruption of emergency or medical services • Disruption of natural environment

  19. Tsunamis • Caused by submarine landslides and faulting • Can travel at speeds > 500mph • About 1m in height • Can travel across large distances • Speed, shallow water and departure of water near coast responsible for most damage • Fires • Gas mains, electrical power lines, oil/gas storage

  20. Tsunami

  21. Banda Ache shoreline before Dec. 26, 2004

  22. Banda Ache shoreline after Dec. 26, 2004

  23. Earthquake Zones • Majority occur along plate boundaries (fig. 11-13) • Shallow at mid-ocean ridges (thinner crust) • Deeper at subduction zones (thicker crust) • Benioff-Wadati Zone – progression of quake depth along the descending plate • Japan, Mexico and Central America, Western N.A. and S.A.

  24. > M 6 earthquakes in the Indian Ocean Region

  25. Sumatra-Andaman Earthquake2005/12/26 • 9.15 Magnitude • Indian/Burma Plate boundary • 1200km of faultline slipped 15m • Two phases • 400km (250mi) x 100km (60mi) rupture (largest ever known) • Traveled NW @ 2.8km/s (6,300mph) for 100 secs • 100 sec pause and then second pulse • 21 km/s (4,700mph) to plate boundary • Sea-floor rose several meters • Displacing 30km3 of ocean water

  26. Sumatra-Andaman Earthquake2005/12/26 • Increased possibility of Toba Eruption (some activity recorded since in area) • Activated two other volcanoes in the Aceh province • 0.8 gigatons of TNT • As much total energy as the U.S. uses in 11 days • Earth’s surface oscillated 20-30cm • Vertical movement of 3mm as far away as Oklahoma • Entire Earth’s surface estimated to have risen 1cm • Shortened our day by ~2.68 microseconds • Caused 5-6cm wobble in rotation due to decrease in Earth’s oblateness

  27. Prediction • Seismic gaps – accumulation of pent-up strain as opposed to tectonic creep • Micro-quake swarms – micro cracks cause rocks to dilate • Tilt or Bulges – can be measured by tiltmeters or lasers • Change is seismic velocity – due to air pockets in micro-cracks • Variations in electrical conductivity – air lowers, water increases • Changes in ground water – level and chemistry • Animal behavior

More Related