Earthquake Hazards and Mitigation

Earthquake Hazards

Earthquake Hazards: Ground Rupture

  • Fault Scarp:
    • Rupture along the fault dip (dip-slip fault).
    • Cliff created by vertical movement along fault.
  • Subsidence:
    • Ground sinks.
    • Associated with normal faults due to tension (divergent plate boundaries) or transtension (transform plate boundaries).
    • Examples: Subsidence trough, graben.
    • NOT the same as a sinkhole.
      Normal Fault: Stress = Tension
      Subsidence occurs due to rock blocks dropping down at normal faults from extension at divergent plate boundaries.
      Subsidence also occurs due to rock blocks dropping down at normal faults from extension at transform plate boundaries, where stress = transtension.
  • Uplift:
    • Ground is pushed up.
    • Associated with reverse and thrust faults due to compression (convergent plate boundaries) or transpression (transform plate boundaries).
      Reverse Fault: Stress = Compression
      Uplift occurs when rock blocks are pushed up at reverse faults from compression at convergent plate boundaries.
      Uplift also occurs due to stress = transpression at transform plate boundaries like restraining bends.
      Example: 2016 Kaikōura, NZ Earthquake (M7.8) - 80 km of coast lifted above sea level! Fault scarp
      *Uplifted dock on Hinchinbrook Island rose by 8ft!
      *Uplifted sea floor at Cape Cleare, Montague Island by 33 ft!
      *1964 Prince William Sound, Alaska Earthquake (M9.2)
  • Horizontal Rupture:
    • Rupture along the fault strike (strike-slip fault).
    • Example: 1964 Denali Fault Earthquake (M7.9) Rupture = 4.3m (~20ft).
    • Crosses Trans-Alaska Oil Pipeline

Sinkholes

  • Caused by the collapse of an underground cave.
  • Cave cavity formed as minerals are dissolved by water.
  • NOT due to passing seismic waves or earthquakes.
  • Also known as cenotes.

Earthquake Hazards: Ground Liquefaction

  • Pre-Earthquake:
    • Water-saturated sediment.
    • Loosely-packed but firm due to downward pressure from overlying material.
  • During Earthquake:
    • Shaking loosens packing between sand grains.
    • Ground becomes soft and liquefied.
    • Seismic waves increase water pressure between ground particles increasing the space between them.
    • Eventually a slurry forms that can no longer support any weight.
  • Post-Earthquake:
    • Sediment re-solidifies.
    • Ground becomes firm again.
  • Layers of solid, firm sediment saturated by water liquefies into soupy mixture as seismic waves pass through during an earthquake

Earthquake Hazards: Ground Motion

  • Damage depends on:
    • Magnitude and intensity of shaking
    • Distance from epicenter, focus depth, local geology, and direction of rupture (i.e., along fault strike).
    • Duration of shaking and resonance.
    • Building structure and local infrastructure.
  • Buildings experience shear stress from earthquake from passing S-, Love, and Rayleigh waves.
  • Buildings weak to shear are more likely to collapse.

Shaking Amplification via Resonance Frequency

  • Every object vibrates at a specific frequency known as its resonance frequency.
  • If the object is pushed in sync with the vibration, then the vibration gets stronger.
  • If seismic waves resonate with the natural resonance frequency of a building, then it may collapse from too much shaking.
  • Small buildings vibrate at a higher frequency, while tall buildings vibrate at a lower frequency.
  • Seismic waves cause higher frequency vibrations through solid rock (high consolidation) and lower frequency vibrations through loose ground (low consolidation).
    YouTube link to archival footage of Tacoma Bridge collapse circa 1940: https://www.youtube.com/watch?v= XggxeuFDaDU&t=27s
    Earthquake + Building Resonance Video on earthquakes and resonance. Available on CANVAS/ https://www.iris.edu/hq/inclass/animation/buildingresonancetheresonantfre quencyofdifferentseismicwaves

Infrastructure Damage Exacerbates Earthquake Casualties

  • Fire caused most of the damage from the 1906 San Francisco earthquake.
  • Damaged buildings/bridges hinder rescue efforts.
  • Damaged water pipes/dams hinder fire-fighting capability.
  • Damaged sewers/dams lead to disease from poor sanitation.

Building Better Buildings

Goal

  • Reduce risk from building collapse.
  • Reduce shaking/swaying of buildings by reducing shear on building.

Approach

  • Build stronger.
  • Build smarter (apply science).
  • Build wiser (enact policy).

Build Stronger with Reinforced Concrete

  • Concrete with embedded metal rods (or other materials).
  • Stronger than concrete
  • Cracks appear long before failure of concrete so you know in advance.

Build Smarter by Applying Science

  • Problem: Buildings collapse from shear caused by too much shaking.
  • Problem: Buildings sink from ground liquefaction
  • Problem: Ground amplification increases ground shaking
  • Solution: Strengthen buildings against shear à shear walls and braced frames.
    *Solution: Reduce shear and shaking à seismically isolated foundation and mass dampener
    *Solution: Reduce ground shaking around building à seismic cloaking and planting trees
    *Solution: Reduce ground amplification and risk of ground liquefaction à attaching building’s foundation to bedrock

Build Smarter with Shear Walls

  • Made with concrete or steel plate.
  • Wide, flat, solid walls that run down the length of the building.
  • Strengthens building against shear.

Build Smarter with Braced Frames

  • Made with steel.
  • Frame around the outside of the building.
  • Strengthens building against shear (from earthquakes and strong winds).

Build Smarter with Seismically Isolated Foundation

  • The building’s foundation is separated from the ground by a movable platform.
  • Platform moves via ball bearings on tracks and springs.
  • Reduces building shaking from earthquake.

Build Smarter with Mass Dampeners

  • Heavy pendulum inside building.
  • Pendulum opposes building motion.
  • Reduces building shaking from earthquake.
    Gif of pendulum reducing building motion from Practical Engineering Link to original video: https://www.youtube.com/watch?app=desktop&v=f1U4SAgy60c

Build Smarter with Seismic Cloaking

  • Bore holes and/or embedded pillars alter the density of the ground surrounding building.
  • The path of passing seismic waves is altered by a change in density.
  • Reduces building shaking by deflecting seismic waves.
  • Still under development.

Build Smarter by Planting Trees

  • Surround buildings with tall trees.
  • Ground shaking is reduced because some seismic wave energy is lost to tree shaking.

Build Smarter by Attaching Foundation to Bedrock

  • Bedrock is a single, solid rock beneath layers of dirt and sediment.
  • Bypasses layers of loosely packed dirt and sediment.
  • Bypasses water-saturated layers of dirt and sediment.

Build Wiser by Enacting Policy

  • Focus on mitigating probable effects of an earthquake:
    • Require use of stronger building materials.
    • Require use of technologies that reduce collapse of buildings from ground shaking.
    • Retrofit older structures with new tech.
    • Study the local geology for faults and loosely-packed ground that amplify shaking.
    • Bolster emergency response.

Building Construction and Policy Mitigates Damage

  • Examples:
    • M6.9 Armenia (1988) earthquake - 25,000 casualties.
    • M6.7 Northridge (1994) earthquake - 60 casualties.
    • M8.8 Chile (2010) earthquake - 0.1% of Chileans who experienced severe shaking perished.
    • M7.0 Haiti (2010) earthquake - 11% of Haitians who experienced severe shaking perished.