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Rupture, Seismic Waves, and Shaking

Rupture, Seismic Waves, and Shaking. Earthquake Origins and Seismic Waves. Focus point where earthquake rupture occurs Shallow focus - 70 km or less (80% or more) Deep focus - down to 700 km; often along subduction zones

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Rupture, Seismic Waves, and Shaking

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  1. Rupture, Seismic Waves, and Shaking

  2. Earthquake Origins and Seismic Waves • Focuspoint where earthquake rupture occurs • Shallow focus -70 km or less (80% or more) • Deep focus -down to 700 km; often along subduction zones • Epicenterpoint projected onto the surface from the center of the Earth through the focus

  3. Types of Seismic Waves • Body waves -travel within the Earth • Primary or P-waves • Compression • Faster of the two types (5.5-1.5 km/sec) • Passes through solids, liquids, or gases (sound at 15 Hz) • Secondary or S-waves • Shear waves • Travels only in solids • Velocity ~60% of P-wave velocity

  4. Shaking Amplification Shaking is affected by: Seismic wave frequency Earthquake magnitude Directivity – direction of fault rupture Rock material through which seismic wave propagates

  5. Comparing Earthquakes • Earthquake Magnitude (1935) • Richter Magnitude (MW) • Measures of energy released (30 X increase/ magnitude) • Maximum amplitude 100 km from the epicenter • Logarithmic scale; a 10-fold increase per cm of amplitude • mb = log10(A/T) + Q(D,h) • MS = log10 (A/T) + 1.66 log10 (D) + 3.30 • Seismograph • S-and P-waves • Magnitude (wave amplitude) • Distance (P-S) Charles Richter

  6. Richter-TNT Seismic Magnitudes • 1.0 30 pounds Large Blast at a Construction Site • 2.0 1 ton Large Quarry or Mine Blast • 4.0 1,000 tons Small Nuclear Weapon • 4.5 5,100 tons Average Tornado (total energy) • 6.0 1 million tons Double Spring Flat, NV Quake, 1994 • 6.5 5 million tons Northridge, CA Quake, 1994 • 7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon • 7.5 160 million tons Landers, CA Quake, 1992 • 8.0 1 billion tons San Francisco, CA Quake, 1906 • 8.5 5 billion tons Anchorage, AK Quake, 1964 • 9.0 32 billion tons Chilean Quake, 1960 • 10.0 1 trillion tons San-Andreas type fault circling Earth • 12.0 160 trillion tons Fault Earth in half through center, OR Earth's daily receipt of solar energy

  7. Calculating Earthquake Magnitude • Moment method • Essentially the same as Richter at higher magnitudes • May be applicable over a wider range of ground motions than Richter Mo = µ A D (seismic moment) µ = shear modulus = 32 GPa in crust, 75 GPa in mantle A = LW = area D = average displacement during rupture MW = 2/3 log10(M0) - 10.7

  8. Determining Earthquake LocationPakistan Event (2005)

  9. Determining Earthquake Location -P-wave Travel Map From Pakistan

  10. Determining Earthquake Location

  11. Comparing Earthquakes • Earthquake Intensity • Modified Mercalli Scale • Qualitative way of comparing earthquakes • Based on perception by people • Damage to buildings • Varies with distance from the epicenter (unlike Richter or Moment scales) • Radio wave model

  12. Comparison of Earthquake Scales Richter Modified MagnitudeMercalli Maximum Intensity (at epicenter) 2 I - II Usually detected only by instruments 3 III Felt indoors 4 IV - V Felt by most people; slight damage 5 VI - VII Felt by all; many frightened and run outdoors; damage minor to moderate 6 VII - VIII Everybody runs outdoors; damage moderate to major 7 IX - X Major damage 8+ X - XII Total and major damage After Charles F. Richter, 1958, Elementary Seismology.

  13. Ground Acceleration Rate of change in horizontal and vertical velocity Measure with respect to gravity (1 g = 9.8 m/s2) Building resistance varies with construction (adobe vs.concrete)

  14. Earthquake Risk and Prediction • Regional Changes in Land Elevation • 1964 Alaskan Quake • 1992 Mendocino, CA • Long and short term prediction • Estimation of Seismic Risk (maps) • Conditional Probabilities of Future Earthquakes • Probability maps • The Loma Prieta quake as probable before 1989 • Failed at San Bernardino in 1992

  15. Earthquake Hazard Map

  16. Florida’s Earthquake History • Basically aseismic(i.e., little activity) since Paleozoic • Only about 6 "real" earthquakes (until 2006!) • 1879-location unknown; felt in Fla. and Ga. • 1893-Jacksonville • 1933-Palatka • 1945-offshore Miami • 1973-Merritt Island • 1975-Daytona Beach (most recent) • 1998 Panhandle (Jay fault; epicenter near Brewton,Alabama; MR = 4.5)

  17. 2006 Gulf of Mexico Event More information available at: http://www.clas.ufl.edu/users/rrusso/florida_eq.html

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