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Environmental Geology

Environmental Geology. Earthquakes. Directivity. The amplitude of seismic waves is greater in the direction of fault rupture. Building Damage and Ground-Structure Interactions. Damage depends on the ground motion and duration of shaking Ground motion is related to:

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Environmental Geology

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  1. Environmental Geology Earthquakes

  2. Directivity • The amplitude of seismic waves is greater in the direction of fault rupture

  3. Building Damage and Ground-Structure Interactions • Damage depends on the ground motion and duration of shaking • Ground motion is related to: • -the magnitude of the eq and characteristics of the seismic waves • -Proximity of the epicenter to the site • Soil conditions at the site

  4. Resonance buildings suffer the greatest damage from ground motion at a frequency close or equal to their own natural frequency. • Amplifying effect produced when the natural vibration frequency of groundor structure ismatched by the frequency of seismic waves. • Building Height / Typical Natural Period 2 story .2 seconds 5 story .5 seconds 10 story 1.0 seconds 20 story 2.0 seconds 30 story 3.0 seconds

  5. Resonance • Building vibration periods roughly = the number of stories • If building and soil have same frequency of vibration, resonation occurs (amplification) • Typically, low-rise buildings (<5 Stories) located on shallow soils (<50 feet) and high-rise buildings (>14 Stories) on deep soils (>150 feet) sustain the most structural damage

  6. Earthquake Damage A) Direct shaking & ground rupture B) Secondary effects Liquefaction Landslides Fires Tsunamis Flooding due to changes in land elevation

  7. What can we do? • Structural protection • Land use planning • Earthquake warning system • Effective emergency response plan • Increased insurance/recovery measures

  8. B) Short-term prediction: Precursory phenomena: Ground deformation Seismic gaps Patterns and frequency of small earthquakes foreshocks Anomalous animal behavior

  9. Frequency & Period of Earthquake waves (clarification) • Frequency (Hz) = 1/Period (sec) • Higher frequency/lower period = more rapid attenuation (examples?) • Body waves (usually .5-20 Hz = .05-2 sec): • higher frequencies (lower periods) than surface waves • Surface waves (usually less than 1 Hz, greater than 1 sec) • Lower frequencies (higher periods)

  10. Effect of waves • Buildings have a natural vibrational frequency • Low buildings have higher frequencies (lower periods) than high buildings • Low buildings shaken by body waves (high freq.) • High buildings shaken by surface waves (low freq.) • High frequency waves attenuate more quickly • High buildings shaken at longer distance from the epicenter • REALITY: MANY FACTORS GOVERN EARTHQUAKE DAMAGE!!!

  11. Earthquake Damage A) Direct shaking & ground rupture B) Secondary effects Liquefaction Landslides Fires Tsunamis Flooding due to changes in land elevation

  12. Human activity that affects e.q.: 1) Underground nuclear explosions 2) Loading/unloading of the earth’s crust Dam or reservoir 3) Deep waste disposal Ex: Rocky Mnt. Arsenal (1962-1965) Liquid waste pumped 3.6 km

  13. What can we do? • Structural protection • Land use planning • Earthquake warning system • Effective emergency response plan • Increased insurance/recovery measures

  14. Earthquake prediction A) Long-term prediction: Estimate relative seismic hazard Estimate conditional probabilities B) Short-term prediction Precursory phenomena

  15. A) Long-term prediction: Estimate relative seismic hazard Active faults? Active: Holocene potentially active: Quaternary Inactive: Pre Quaternary History of fault activity: Paleoseismology (Pallet Creek study & Coyote Creek study; page) Estimate average recurrence interval

  16. B) Short-term prediction: Precursory phenomena: Ground deformation Seismic gaps Patterns and frequency of small earthquakes foreshocks Anomalous animal behavior

  17. The San Andreas Fault Zone in Southern California Faults in San Diego

  18. California Regulations pertaining to earthquakes • Alquist-Priolo Fault Zoning Act (1972) (California law) • direct result of the 1971 San Fernando Earthquake • requires that zones along active faults with well-defined surface features be established • (http://www.consrv.ca.gov/CGS/rghm/ap/) • Seismic Hazards Mapping Act (1990) • addresses non-surface fault rupture earthquake hazards, including liquefaction and seismically induced landslides • The Natural Hazards Disclosure Act (1998) • sellers of real property must provide prospective buyers with a "Natural Hazard Disclosure Statement" when the property being sold lies within one or more state-mapped hazard areas.

  19. Alquist-Priolo Fault Zoning Act • 1972 – Alquist-Priolo Special Studies Zones Act - Passed in 1972 as a direct result of the 1971 San Fernando Earthquake. • Many cities have their own amendments • Purpose • Provides policies and criteria to assist cities, counties, and state agencies in the exercise of their responsibility to prohibit the locations of developments and structures for human occupancy across the trace of active faults • Fault must have “well-defined” Holocene surface rupture (Blind-thrusts are exempt)

  20. Alquist-Priolo Fault Zoning Act • Summary of Specific Criteria • No structure for human occupancy shall be placed within 50 feet of an active fault • Area within 50 feet of fault trace presumed to be underlain by active branches of the fault unless proven otherwise • Lead agencies (State Geologist, State Mining and Geology Board) shall provide public disclosure of delineated fault zones to the public • Development permit applications for any project within a delineated zone must be accompanied by a geologic report • The seller’s agent or seller of real property located within a delineated zone shall disclose to the buyer that fact

  21. Alquist – Priolo Fault Zoning Act • Exemptions • Any structure built before May 4, 1975 • Single-Family wood-framed dwellings (2 stories or under) • Conversions or alterations of existing structures (under 50% the value of the structure).

  22. Seismic Hazards Mapping Act • Passed in 1990, addresses non-surface fault rupture earthquake hazards, including liquefaction and seismically induced landslides.

  23. Seismic Zones • In Seismic Zone 4, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.4g (4/10 the acceleration of gravity) will occur within the next fifty years. • In Zone 1, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.1g (1/10 the acceleration of gravity) will occur within the next fifty years.

  24. Seismic Zones • The Uniform Building Code places San Diego in Seismic Zone 4 • Buildings are required to withstand 1/3 more of the lateral force from earthquakes that Seismic Zone 3 mandates

  25. San Diego Seismic Standards • San Diego has been required to enforce the State Earthquake Protection Law (Riley Act) since its enactment in 1933. However, the seismic resistance requirements of the law were minimal for many years and San Diego did not embrace more restrictive seismic design standards until its first adoption of the Uniform Building Code in 1951. • It is estimated that about 1,000 (mainly nonresidential) masonry buildings within the City may constitute structural hazards.

  26. Structural Design and Seismic Performance • Ductile steel and ductile reinforced concrete frame buildings (as defined in Uniform Building Code) - highly resistant to structural damage; may suffer nonstructural damage. • Vertical load-bearing steel and reinforced concrete frame buildings braced against lateral forces - perform well but may suffer some structural as well as nonstructural damage. • Unreinforced masonry buildings of all types - highly vulnerable to damage.

  27. Structural Design and Seismic Performance • Reinforced brick and concrete block masonry buildings - perform well but may suffer some structural as well as nonstructural damage. • Pre-engineered and other light steel and sheet metal buildings - usually perform extremely well. • Residential buildings - Traditional wood frames with wood or stucco siding usually behave well but may suffer damage. Modern design open-type houses with large glass openings, split-level houses, and two-story houses or apartments with large garage openings in the first story are vulnerable to earthquake damage.

  28. probability of being exceeded in 50 years

  29. Tsunami • Seismic sea wave (not a tidal wave) • Disturbing forces: vertical movement of seafloor • Offshore faults; subduction zone earthquakes • Submarine landslides • Volcanic eruptions • Meteorite impact • Size: • Wavelength: 125 miles (200 km) • Wave height: • Coast: 10’s of meters (largest are ~30-40 m or 100+ ft.) • Open ocean: 1 meter (3 ft) • Velocity: • Open ocean: 450 mph • Slow as they approach coastlines • Distance of travel: across oceans (1000’s of km)

  30. Waves • Deep Water Waves • Those waves traveling with a water depth greater than ½ the wavelength • The speed of deep-water waves depends on the wavelength of the waves. Waves with a longer wavelength travels at higher speed.

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