Seismic Hazards 5 - Mitigation

Seismic Hazard Reduction Strategies and Anti-Seismic Building Design

Learning Outcomes

  • Assess strategies for reducing vulnerability to seismic hazards.
  • Describe basic principles of anti-seismic building design (new builds or retrofits).
  • Gain perspective on seismic and geological hazards from a political perspective.

Lecture Structure

  • Land use planning and building regulations.
  • Principles of anti-seismic design.
  • Strengthening structures and materials.
  • Insurance (briefly).
  • Next Section: Early warning systems (functionality, limitations, strengths) and emergency response.
  • Tsunami risk mitigation.

Building on Previous Lectures

  • Previous sessions covered probabilistic models (based on earthquake catalogs).
  • Objective: Prediction doesn't work, so we use probabilistic models.
  • Hazard maps: color-coded probabilities of ground acceleration within specific time windows (e.g., 30 years, 475 years, 1 century).
  • Risk assessment: relative levels of seismic risk in an area.
  • Consideration of liquefaction through local surveys (e.g., Dhaka, Bangladesh).

Hazard Microzonation

  • Hazard microzonation work should be micro-scale.
  • Maps include the probability of exceeding a seismic intensity or peak ground acceleration threshold within a time window.
  • Used for resource investment decisions.
  • Identify areas prone to:
    • Liquefaction.
    • Landslides.
    • Tsunamis.
    • Fire potential (e.g., timber buildings).
    • Infrastructure vulnerability (power lines, water).
    • Nuclear power stations.
  • Demographics: population characteristics (age, affluence).
  • All factors combined ideally determine policy.
  • Real-world policies are often not ideal, leading to suffering.

Example: Italy and Microzonation

  • Focus: Central Italy (Apennine Mountains, faulting).
  • Close to the L'Aquila earthquake zone (2009).

Seismic Hazard Map

  • Purple zone: 10% chance of exceeding ground acceleration within 475-year return period.
  • Corresponds to intensity 7-8.
  • Village: Built on sediments at the foot of mountains thrust up by faulting.

Geology

  • Variable geology: strong crystalline rocks (mountains) and soft sediments (village).
  • Similar to volcanic contexts where sediments surround the volcano.
  • Soft sediments amplify seismic waves.

Recent Study

  • Lithological map: crystalline rocks vs. soft sediments.
  • Boreholes: sunk to 30-35 meters.
  • Measured in-situ S-wave velocity (V_s).
  • Extremely slow V_s: Around 200 m/s in some zones.
  • Crystalline rocks: V_s in kilometers per second.
  • Contrast between fast and slow S-waves amplifies seismic signal.

2D Slice Model

  • Mountains: higher S-wave velocities (around 1000 m/s).
  • Sediments: far lower S-wave velocities.

Amplification Model

  • Amplification ratio (AR) up to 3.5.
  • Significant amplification in specific areas.
  • Due to seismic wave wavelength and frequency.
  • Strongest amplification: roughly 1 second period.
  • Frequency-dependent and localized amplification.

Detailed Peak Ground Acceleration Model

  • Red zones: 0.4g (very strong shaking).
  • Caused by shallow sediments.

Regional Context

  • Located in the highest seismic risk zone in Italy.
  • 10% chance of exceeding 0.25g within 50 years.
  • National maps used for decisions on enforcing seismic codes.

European Coordination

  • Similar images constructed for many European countries coordinated by the European Commission.
  • Each country has its own way of representing seismic hazard.
  • Combined at a continental scale with integrated language.
  • Grunenthal et al.: Probabilistic model showing peak ground acceleration with a certain return period.
  • Red areas around Romania, Turkey, Italy, Greece.

Regulatory Building Codes

  • Codes stipulate construction types based on anticipated seismic shaking strength.
  • By 2002, many countries had established codes.
  • European codes cover new buildings, bridges, retrofitting, tanks, pipelines.
  • Mandatory level depends on seismic hazard.
  • Codes are based on acceptable risk (pragmatic approach).

PBSD (Performance Based Seismic Design)

  • Balances cost with safety.

  • Minor earthquakes: no damage.

  • Strongest earthquakes: some degree of resilience.

  • Matrix: Seismic shaking intensity (very rare to frequent) vs. Performance level (fully operational to near collapse).

  • Acceptability thresholds:

    • Frequent events: no damage.
    • Rarest events: near collapse.
    • Next level: life safety in strongest events.
    • Safety-critical: full operation even in very rare events.

Consultancy Reports

  • Ninhus report for the UK government: global and regional overview.
  • Focuses on low-income countries and their vulnerability due to lack of resources.
  • Reviews regulation and effectiveness of seismic building codes.
  • Addresses code enforcement.

Key Idea

  • Earthquakes don't kill people, buildings do.
  • No natural disasters, but governmental failures.

Example: New Zealand

  • Seismically active and affluent country with a great system.
  • National system categorizes areas into high, medium, and low seismic risk.
  • Sets timeframes for seismic work on prone buildings.

Anti-Seismic Design Principles

  • Understand ground shaking and building response.
  • Consider amplitudes and frequencies.
  • Link structure and building response.
  • Buildings have their fundamental frequency (related to dimensions).
  • Resonance = Damage.
  • Strengthening processes or isolation/damping systems can reduce damage.

Fundamental Frequency

  • Matterhorn example: edifice shakes and moves slightly in response to earthquakes.

Building Resonance

  • Taller buildings have lower fundamental frequencies.
  • Rooftop pools slosh due to building resonance.
  • Fundamental frequency ≈ 10 Hz / number of floors.
  • High-rise buildings have periods of multiple seconds.

Demonstration

  • Different building types have different fundamental frequencies of oscillation.
  • Local amplification of different wavelengths and frequencies of seismic energy.

Strengthening Structures

Ideal Structure

  • Symmetrical with balanced forces.
  • Continuous structure with given height-width ratio (less than 4).
  • Avoid pencil-thin buildings.
  • Avoid extensions with different mechanical behavior.
  • Well-connected structural elements (e.g., steel frame binding walls).
  • Continuous panels.
  • Cross bracing (triangular style linking adjacent columns).

Soft Story

  • Particularly dangerous; lacking cross bracing.

Building Materials

  • Timber: high strength, high ductility, lightweight (good for earthquakes).
  • Steel: expensive but good (modern buildings in wealthy countries).
  • Reinforced concrete: concrete pillars with steel struts (okay-ish).
  • Unreinforced concrete/brick/clay/adobe: heavy and weak (prone to failure).

Examples of Damaged Buildings

  • Adobe structure in Peru.
  • Brick school building in Utah.

Upgrading Old Buildings

  • Retrofitting is cheaper than rebuilding.
  • Considers cultural importance of buildings.
  • Outdated construction is responsible for multiple deaths.

Low-Cost Structures for Developing Countries

  • Smart Shelter Foundation: hollow concrete blocks with horizontal bands.
  • Used for school buildings in Nepal that survived the 2015 Gorkha earthquake.

High-Budget Solutions

  • Skyscrapers balanced on rubber pads (decoupling).
  • Kinks in pipelines to accommodate seismic slip.
  • Damping systems (syringe with sticky fluid).
  • Dynamic control systems (e.g., 730-ton pendulum in Taipei).

Earthquake Insurance

  • Sold separately with a big deductible (typically 15%).
  • Less than 20% of households in Japan and California have earthquake insurance.
  • Insurance industry employs hazard assessors.

Culture of Preparedness

  • Public awareness through schoolwork, public TV, social media, etc.
  • Drills (e.g., under tables in schools).
  • Posters with instructions (e.g., Dominica).
  • Gap between seismic knowledge and action.

Next steps

  • Lecture by Guninch on political economy and community impact.