10.3 Seismic hazards and risk assessment

Earthquakes pose significant threats to human life and infrastructure. From ground shaking to tsunamis, these seismic hazards can cause widespread destruction. Understanding these risks is crucial for developing effective mitigation strategies.

Seismic risk assessment considers factors like population density, building codes, and local geology. By creating hazard maps and implementing risk reduction strategies, communities can better prepare for and respond to earthquake events, ultimately saving lives and minimizing damage.

Seismic Hazards

Primary seismic hazards

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• Ground shaking propagates through Earth's crust via seismic waves measured by Modified Mercalli Intensity Scale influenced by epicenter distance and local geology

• Liquefaction transforms water-saturated unconsolidated sediments into liquid-like state causing buildings to sink or tilt especially in coastal areas and river valleys (San Francisco Bay Area)

• Landslides triggered by ground shaking or liquefaction more likely on steep slopes or unstable geology include rock falls, debris flows, and slumps (1970 Peru earthquake)

• Surface rupture along fault line creates scarps and ground displacements damaging structures built across the fault (San Andreas Fault)

• Tsunamis generated by underwater earthquakes or landslides flood coastal areas and travel long distances across oceans (2004 Indian Ocean tsunami)

Risk Assessment and Mitigation

Factors influencing seismic risk

• Population density increases potential casualties and economic losses in urban areas with complex infrastructure systems (Tokyo)

• Building codes vary by region and change over time older buildings may not meet current seismic standards (California Building Standards Code)

• Infrastructure damage to transportation water and power networks leads to cascading effects and prolonged recovery periods (2011 Christchurch earthquake)

• Local geology affects ground motion amplification sedimentary basins increase shaking intensity and duration (Mexico City)

• Building design and construction quality determine structural resilience soft-story buildings and unreinforced masonry particularly vulnerable (1989 Loma Prieta earthquake)

• Socioeconomic factors like poverty levels and access to resources affect community resilience and recovery ability (Haiti vs Chile earthquakes)

Creation of seismic hazard maps

• Seismic hazard maps show likelihood of ground shaking levels based on historical earthquake data and geological information

• Creation process involves:

  1. Compiling earthquake catalogs and fault maps
  2. Developing seismotectonic models of the region
  3. Using probabilistic seismic hazard analysis (PSHA) to calculate hazard levels

• PSHA components include earthquake source characterization ground motion prediction equations and site response analysis

• Map outputs often show peak ground acceleration (PGA) or spectral acceleration for different probability levels (2% in 50 years)

• Applications inform building codes guide infrastructure planning assist emergency response planning and help insurance companies assess risk

Strategies for seismic risk reduction

• Earthquake-resistant construction uses base isolation systems shear walls and moment-resisting frames for lateral stability and ductile design (Taipei 101)

• Retrofitting existing structures strengthens foundations adds bracing to walls and secures non-structural elements (Golden Gate Bridge seismic retrofit)

• Land use planning avoids construction in high-risk areas and implements buffer zones around known faults (Alquist-Priolo Earthquake Fault Zoning Act)

• Early warning systems use seismometer networks to provide seconds to minutes of warning before strong shaking arrives (ShakeAlert)

• Emergency preparedness involves developing evacuation plans stockpiling supplies and training first responders (Japan's nationwide earthquake drills)

• Public education informs communities about local seismic risks teaches proper response actions and encourages household preparedness (Drop Cover and Hold On)

• Infrastructure resilience designs redundant systems implements automatic shut-off valves and strengthens lifeline systems (San Francisco's Auxiliary Water Supply System)