🤙🏼Earthquake Engineering Unit 12 – Seismic Risk Assessment and Mitigation

Key Concepts and Terminology

• Seismic risk assessment evaluates potential consequences of earthquakes on buildings, infrastructure, and populations
• Seismic hazard analysis estimates likelihood and intensity of ground shaking at a specific site based on seismicity, geology, and attenuation relationships
• Vulnerability assessment determines susceptibility of structures and systems to damage from earthquake ground motion
• Risk calculation methods combine seismic hazard and vulnerability data to quantify potential losses in terms of casualties, economic impacts, and downtime
• Mitigation strategies aim to reduce seismic risk through structural retrofitting, building codes, land-use planning, and emergency preparedness
  • Structural retrofitting strengthens existing buildings to withstand earthquake forces (base isolation, shear walls)
  • Building codes establish minimum design and construction standards for new structures in seismic zones
• Resilience refers to ability of a community to withstand, adapt to, and recover from earthquake impacts
• Exposure represents value of assets (buildings, infrastructure, population) in an area that could be affected by an earthquake

Seismic Hazard Analysis

• Probabilistic Seismic Hazard Analysis (PSHA) estimates probability of exceeding various ground motion levels at a site over a specified time period
  • PSHA considers multiple earthquake sources, magnitudes, and ground motion prediction equations
  • Results are expressed as hazard curves showing annual probability of exceedance vs. ground motion intensity
• Deterministic Seismic Hazard Analysis (DSHA) evaluates worst-case scenario based on maximum credible earthquake from nearest fault
• Ground motion prediction equations (GMPEs) estimate peak ground acceleration (PGA), peak ground velocity (PGV), and spectral accelerations as a function of magnitude, distance, and site conditions
• Site effects influence ground motion characteristics based on local soil conditions and topography
  • Soft soils can amplify ground motions and increase duration of shaking
• Seismic source characterization involves identifying active faults, determining their geometry, slip rates, and maximum magnitudes
• Deaggregation identifies relative contributions of different earthquake scenarios to overall hazard at a site
• Time-dependent hazard analysis accounts for changes in earthquake probability over time due to stress buildup and release on faults

Vulnerability Assessment

• Fragility curves describe probability of a structure or component reaching or exceeding a damage state as a function of ground motion intensity
• Capacity curves represent lateral load-deformation relationship of a structure, indicating yield and ultimate strength
• Nonlinear static pushover analysis determines capacity curve by applying increasing lateral loads until failure
  • Pushover results help identify weak links and potential failure mechanisms in a structure
• Rapid visual screening assesses seismic vulnerability of large building inventories based on observable characteristics (age, height, irregularities)
• Detailed structural analysis using finite element models can simulate response of complex structures to earthquake loading
• Damage indices quantify extent of structural damage on a scale from 0 (no damage) to 1 (collapse)
  • Examples include Park-Ang index and FEMA P-58 damage states
• Vulnerability modifiers account for factors that increase or decrease seismic risk, such as construction quality, maintenance, and retrofitting

Risk Calculation Methods

• Scenario-based risk assessment estimates losses for a single hypothetical earthquake event
  • Useful for emergency planning and response exercises
• Probabilistic risk assessment (PRA) quantifies expected losses over a specified time period considering all possible earthquake scenarios
  • PRA results can be expressed as average annual loss (AAL) or probable maximum loss (PML)
• Event tree analysis models progression of an earthquake through a series of branching events and consequences
• Consequence functions relate damage states to losses such as repair costs, casualties, and downtime
• Indirect losses from business interruption and supply chain disruption can exceed direct physical damage costs
• Uncertainty analysis quantifies variability and confidence intervals in risk estimates due to incomplete data and modeling assumptions
• Benefit-cost analysis compares expected risk reduction benefits of mitigation measures to their implementation costs

Mitigation Strategies

• Performance-based design sets specific performance objectives for a structure under different earthquake hazard levels
  • Objectives can range from collapse prevention to immediate occupancy post-earthquake
• Seismic isolation decouples a structure from ground motion using flexible bearings or dampers
  • Reduces seismic forces and interstory drifts in the superstructure
• Energy dissipation devices (dampers) absorb earthquake energy and reduce structural response
  • Examples include viscous fluid dampers, friction dampers, and yielding metal dampers
• Structural health monitoring uses sensors to continuously assess condition and detect damage in structures
• Seismic early warning systems detect P-waves from an earthquake and provide alerts before damaging S-waves arrive
  • Can trigger automated safety measures such as shutting off gas lines and slowing trains
• Non-structural mitigation secures building contents and equipment to prevent injuries and property damage
  • Includes anchoring bookshelves, restraining hazardous materials, and using safety glass
• Community resilience planning engages stakeholders to identify and prioritize actions for reducing seismic risk and improving recovery capabilities

Case Studies and Real-World Applications

• 1994 Northridge earthquake in Los Angeles resulted in $20 billion in losses and exposed vulnerabilities in steel moment frame buildings
  • Led to changes in building codes and retrofit programs for existing structures
• 2011 Christchurch earthquake in New Zealand caused widespread liquefaction and damage to unreinforced masonry buildings
  • Highlighted importance of soil-structure interaction and performance of retrofitted structures
• Seismic risk assessment of nuclear power plants ensures safety and prevents release of radioactive materials during earthquakes
• Lifeline infrastructure systems (water, power, transportation) require special consideration due to their network characteristics and cascading failure potential
• Seismic design of bridges and overpasses prevents collapse and ensures post-earthquake functionality for emergency response and recovery
• Retrofit of historic buildings balances preservation of cultural heritage with life safety and damage reduction
• Risk assessment of port facilities and container cranes is critical for maintaining global supply chains after earthquakes

Tools and Technologies

• Geographic Information Systems (GIS) integrate and visualize seismic hazard, vulnerability, and risk data spatially
  • Enable mapping of earthquake scenarios and loss estimates for decision support
• Remote sensing techniques such as LiDAR and satellite imagery can rapidly assess damage and ground deformation after earthquakes
• Shake maps display observed and predicted ground motions in real-time for situational awareness and emergency response
• Building Information Modeling (BIM) creates digital representations of structures for seismic analysis, retrofit design, and asset management
• Artificial intelligence and machine learning techniques can automate damage assessment from post-earthquake images and data
• Cloud computing and high-performance computing enable large-scale seismic risk simulations and uncertainty quantification
• Wireless sensor networks monitor structural health and detect damage for rapid post-earthquake assessment and decision-making

Future Trends and Challenges

• Integration of seismic risk assessment with multi-hazard resilience planning for comprehensive risk management
  • Considers interactions and cascading effects between earthquakes, tsunamis, landslides, and other hazards
• Incorporation of socioeconomic vulnerability and social justice considerations in seismic risk assessment and mitigation prioritization
• Development of performance-based earthquake engineering (PBEE) frameworks that explicitly link seismic hazard, structural response, damage, and losses
• Advancement of physics-based ground motion simulations that capture complex source, path, and site effects
• Utilization of big data analytics and crowdsourcing for rapid damage assessment and situational awareness after earthquakes
• Adaptation of seismic risk assessment methods for developing countries with limited data and resources
• Consideration of aging infrastructure and deterioration in seismic vulnerability assessment and retrofit prioritization
• Integration of seismic risk assessment into sustainable design and lifecycle cost analysis of structures and infrastructure systems