Earthquake Notes

How Earthquakes Occur

• Elastic Rebound
  • Rocks store energy elastically
  • When stored stress exceeds the strength of the friction on the fault OR the strength of the rock… snap.
  • The longer between quakes, the more energy is stored. The deformation can be measured as a way to predict the size of the next quake.
• Earthquakes and plates
  • Diverging: small, shallow
  • Subduction zone: large, shallow to deep
  • Suture zone: large, shallow to medium depth
  • Transform: small to large (large are rare), shallow

Basic Earthquake Terms

• Focus: where the fault ruptures, where the earthquake actually begins.
  • Usually underground
  • Rupture can propagate along the surface – size of quake depends on the amount of rupture
• Epicenter: point on Earth’s surface above the focus, where surface waves begin

Earthquake Waves

• Body waves: run through interior of Earth
  • P (primary) wave – compressional, fast
  • S (secondary) wave – transverse, slower
• Surface waves: ripples at surface of Earth, much slower

Finding the Epicenter

• Use difference in travel time between P and S wave to find distance to epicenter
• Use several stations to triangulate the one place that is the right distance from each station

Measuring Earthquakes

• Magnitude v. intensity
  • Magnitude: amount of energy released in the earthquake
    • Moment magnitude - depends on the area of the surface that ruptures and the average displacement
    • Estimated by amplitude of seismogram
  • What happened to Richter?
    • Richter magnitude estimated energy from amplitude of the seismogram
    • Could not distinguish between very large magnitudes
    • Required use of a specific seismometer
  • Intensity depends on
    • distance from epicenter
    • geological substrate

Earthquake Hazards

• Ground motion - Pakistan 2004
  • depends on type of rock (SF, 1906)
• Liquefaction
  • Water-soaked sediment; during earthquake, sediment sinks
  • SF 1906, Loma Prieta highway collapse, Niigata, Japan, apartments sink
• Landslides - Anchorage, 1964
• Fire - SF 1906, Loma Prieta
• Tsunamis
  • displacement on ocean floor
  • can cause water to retreat before wave hits
  • long wavelength, wave height jow in open water
  • Indonesia Dec. 26, 2004: M9.1 earthquake, 1300 km rupture
  • arrival time of tsunami immediate in Indonesia, 4 hours in Sri Lanka, 9 hours in Africa
  • wave height up to 25 meters

Regions of Earthquake Hazard

• California
• Alaska
• New Madrid, Missouri
• Hawaii

Earthquake Safety before, during and after earthquakes

Earthquake Safety and Buildings

• Unsafe structures:
  • Unreinforced masonry (brick)
  • Pre-1954 houses not bolted to foundation
  • Insufficient bracing of cripple wall
  • Porch failures
  • Concrete frame structures
  • “Soft-story” structures - missing a wall on the first floor
• Safe structures
  • Wood frame
  • Steel frame
  • Shear walls
  • Cross-braced
  • Base isolators

Earthquake prediction

• Precursors
  • Ground tilt
  • Seismic-wave velocities
  • Electrical resistivity
  • Well water levels
  • Radon
• Seismic Gaps
• Areas along fault with lower-than-expected frequency of earthquakes. May indicate the fault is locked up and preparing to fail.
• Parkfield Experiment
  • Parkfield (near Cholame Valley) had earthquakes about every 20 years for decades, all about M6.
  • Earthquake expected 1988
  • An earthquake finally happened 9/28/2004
  • Using data from quake to examine precursors

Public Policy and Earthquakes

• Zoning for hazard
  • Alquist-Priolo Act, 1972
  • Set out “earthquake fault zones: 100 cities, 36 counties
  • Prohibited building on active rupture
  • Required identification and retrofit of at-risk buildings