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Earthquakes

Earthquakes

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Earthquakes

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  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. Seismic Waves • Relative time lag between types of seismic waves • Single station recording provides a measure of distance from the focus to the station by timing lag

  10. Triangulation Location of EQ Epicenters • Three stations allow for precise location of EQ epicenters through triangulation • Timing of arrival of P-waves at different stations can be mapped

  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 • Magnitude can vary with depth • Brittle rocks fail suddenly • Hot, plastic rocks can absorb more strain and may reduce the energy released through less sudden failure • Impact varies with depth and magnitude • Large & shallow = significant impact • Large & deep = less destructive impact

  12. Most seismically active zone in Arizona • Valle, Az. • Intersection of the Cataract Creek – Bright Angel Fault System • Most EQ’s are too small to be felt • ~50% chance a 6.0 Mag or larger EQ occurring in the Flag area in the next 30yrs • Largest recent EQ’s were 4.9 & 5.3 M occurring in 1993 in the Cataract Creek area outside of Flagstaff

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

  14. 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.

  15. 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

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

  17. Effect of substrate composition on seismic wave amplitude • Seismic waves can be amplified by differences in substrate composition • The less rigid, less coherent, the less consolidated the material the higher the amplification

  18. Soil Type & Amplification

  19. 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

  20. Effects cont’d • Changes in ground elevation • Uplift • Subsidence - flooding • 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

  21. Minimizing Earthquake Hazards • National EQ Hazard Reduction Program • USGS in coop with state and universities • Develop knowledge base for physical aspects and behavior of EQ’s • Determine EQ potential • Predict consequences • Application of “lessons learned” • Estimation of seismic risk • Compilation of hazard maps • Seismic gaps • Deformation maps • Micro-quake monitoring • Classification of faults • Estimation of maximum moment magnitude • Slip rate

  22. Minimizing Earthquake Hazards • Short-term prediction • Precursor based • Specifies a shorter time period of occurrence, est. magnitude, localized area of event and probability • Developing area of research with only limited success • EQ warning systems • Of such short advanced notice not a feasible tool at this time • Japan does use a system of limited success

  23. 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 • Lunar/solar alignments • Animal behavior

  24. Tsunami

  25. Banda Ache shoreline before Dec. 26, 2004

  26. Banda Ache shoreline after Dec. 26, 2004

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

  28. > M 6 earthquakes in the Indian Ocean Region

  29. Sumatra-Andaman Earthquake2004/12/26 • 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

  30. Sumatra-Andaman Earthquake2004/12/26 • 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