14.2 Probabilistic seismic hazard analysis
4 min read•august 9, 2024
(PSHA) is a crucial tool for assessing earthquake risks. It combines data on seismic sources, ground motion predictions, and site conditions to estimate the likelihood of different levels of at a specific location.
PSHA plays a vital role in earthquake prediction and hazard assessment. By considering various earthquake scenarios and their probabilities, it provides a comprehensive view of potential seismic hazards, informing building codes, infrastructure design, and emergency planning efforts.
Seismic Source and Ground Motion
Characterizing Seismic Sources and Magnitude
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- Seismic source characterization involves identifying potential earthquake sources and their properties
- Includes fault geometry, slip rate, and
- Requires geological, seismological, and geodetic data analysis
- describe the occurrence rates of earthquakes of different sizes
- expresses the logarithm of earthquake occurrence rate versus magnitude
- Formula: , where N is the number of events, M is magnitude, a and b are constants
- b-value in the Gutenberg-Richter relationship typically ranges from 0.8 to 1.2
- Higher b-values indicate a larger proportion of small earthquakes relative to large ones
- Maximum magnitude (Mmax) estimation crucial for hazard assessment
- Based on fault dimensions, historical seismicity, and tectonic setting
- Affects the upper limit of potential ground motions
Ground Motion Prediction and Attenuation
- (GMPEs) estimate expected ground shaking at a site
- Account for source characteristics, path effects, and site conditions
- Typically express as a function of magnitude, distance, and site class
- Common ground motion parameters include (PGA) and (SA)
- GMPEs often incorporate through standard deviation terms
- Accounts for natural randomness in ground motions
- Regional variations in GMPEs exist due to differences in tectonic settings and crustal properties
- Western North America GMPEs differ from those for stable continental regions (Eastern North America)
- factors modify GMPEs to account for local soil conditions
- Soft soils generally amplify ground motions compared to bedrock sites
Hazard Analysis Techniques
Probabilistic Seismic Hazard Curves
- Hazard curves represent the annual probability of exceeding specific ground motion levels
- X-axis typically shows ground motion parameter (PGA or SA)
- Y-axis shows annual or
- Developed by integrating over all possible earthquake scenarios
- Considers magnitude-frequency distributions and ground motion predictions
- Hazard curves form the basis for designing structures to withstand specific levels of ground shaking
- Building codes often specify design ground motions based on hazard curves
- Different hazard levels correspond to various return periods
- 2% in 50 years (approximately 2475-year return period) often used for critical structures
- 10% in 50 years (475-year return period) common for ordinary buildings
Uniform Hazard Spectra and Deaggregation
- (UHS) show spectral accelerations with equal probability of exceedance across all periods
- Useful for designing structures with different natural periods
- Constructed by combining hazard curves for multiple spectral periods
- UHS provide a consistent level of hazard across the frequency range of interest
- Often used in building codes and structural design
- breaks down the hazard contributions from different sources
- Identifies dominant earthquake scenarios (magnitude-distance pairs) contributing to hazard
- Helps in selecting appropriate ground motion records for dynamic analysis
- Deaggregation results often presented as 3D plots or bar charts
- Shows relative contributions from different magnitude-distance combinations
- Useful for understanding which earthquake scenarios drive the hazard at a site
Monte Carlo Simulations in Hazard Analysis
- Monte Carlo simulations generate synthetic catalogs of earthquakes based on source models
- Allows for explicit modeling of spatial and temporal dependencies
- Can incorporate complex rupture scenarios and time-dependent probabilities
- Advantages of Monte Carlo approach include:
- Ability to model complex source geometries and rupture behaviors
- Direct incorporation of uncertainties in input parameters
- Generation of ground motion fields for multiple sites simultaneously
- Monte Carlo results can be used to develop hazard curves and maps
- Requires many simulations to achieve stable results
- Computationally intensive but becoming more feasible with increasing computing power
- Useful for assessing hazards from multiple, interacting faults or subduction zones
- Can model complex rupture scenarios not easily captured by traditional PSHA methods
Uncertainty and Modeling
Logic Trees and Epistemic Uncertainty
- Logic trees provide a framework for incorporating epistemic uncertainties in PSHA
- Represent alternative models or parameter values as branches
- Assign weights to branches based on expert judgment or data support
- Common components modeled in logic trees include:
- Seismic source characterization (fault geometry, slip rates)
- Ground motion prediction equations
- Maximum magnitude estimates
- Logic tree results yield a distribution of hazard curves rather than a single curve
- Allows for quantification of epistemic uncertainty in hazard estimates
- Mean hazard curve often used for design purposes
- Represents the average across all logic tree branches
- Fractile hazard curves (16th, 50th, 84th percentiles) provide insight into uncertainty range
Aleatory Variability and Uncertainty Analysis
- Aleatory variability represents the inherent randomness in earthquake processes
- Included in ground motion prediction equations as sigma terms
- Affects the shape and values of hazard curves
- Sensitivity analyses assess the impact of different uncertainties on hazard results
- Tornado diagrams visualize the relative importance of different parameters
- Help prioritize research efforts to reduce uncertainties with the largest impact
- Total uncertainty in PSHA combines both epistemic and aleatory components
- Epistemic uncertainty potentially reducible with more knowledge or data
- Aleatory variability considered irreducible due to the complex nature of earthquakes
- Uncertainty analysis crucial for risk-informed decision making
- Provides a range of possible outcomes for consideration in policy and design
- Helps communicate the limitations and confidence in hazard estimates to stakeholders
Key Terms to Review (26)
Aleatory Variability: Aleatory variability refers to the inherent randomness and unpredictability associated with natural phenomena, such as earthquakes. It captures the natural fluctuations in seismic events, including variations in ground motion intensity, duration, and frequency, making it a critical concept in assessing seismic risk. Understanding aleatory variability is essential for probabilistic seismic hazard analysis, as it helps quantify the uncertainty in seismic events and their impacts on structures and communities.
Attenuation Relationship: An attenuation relationship is a mathematical model that describes how seismic waves decrease in amplitude as they travel through the Earth. This relationship is crucial for estimating ground shaking intensity at specific locations, which helps in assessing seismic hazards and risks associated with earthquakes.
Building code: A building code is a set of regulations that specify the minimum standards for constructed objects such as buildings and non-building structures. These codes are designed to ensure safety, health, and environmental protection for occupants and the general public, particularly in areas prone to natural hazards like earthquakes. The relevance of building codes becomes even more crucial in probabilistic seismic hazard analysis, where understanding the risks associated with seismic events informs the development of these regulations.
Deaggregation: Deaggregation refers to the process of breaking down seismic hazard estimates into more detailed components, focusing on specific seismic sources and ground shaking levels. This concept is essential in understanding the probabilities of different ground motion levels occurring due to various seismic events, allowing for more accurate risk assessments in probabilistic seismic hazard analysis.
Exceedance probability: Exceedance probability is the likelihood that a certain seismic event, such as an earthquake of a specified magnitude or intensity, will occur at least once within a given time frame at a specific location. This concept is essential for understanding seismic risk and helps in planning for potential earthquake impacts by quantifying how often certain levels of shaking are expected to be exceeded, based on historical data and seismic models.
Fault line: A fault line is a fracture or zone of fractures between two blocks of rock, which can lead to seismic activity such as earthquakes. These lines are critical as they mark the boundaries where tectonic plates interact, causing stress accumulation and release, which is manifested in the form of seismic waves. Understanding fault lines is essential for interpreting wave characteristics, revealing Earth’s internal structure, and assessing potential hazards associated with seismic events.
Ground motion parameters: Ground motion parameters are quantitative measures that describe the characteristics of ground shaking during an earthquake, including aspects like amplitude, frequency, and duration. These parameters are crucial for understanding how seismic waves propagate and impact structures and populations, forming the backbone of probabilistic seismic hazard analysis.
Ground Motion Prediction Equations: Ground motion prediction equations (GMPEs) are mathematical models used to estimate the expected ground shaking at a site during an earthquake based on various parameters like earthquake magnitude, distance from the source, and local geological conditions. These equations play a crucial role in assessing seismic hazard by providing a way to quantify how much shaking might be experienced at different locations during seismic events, aiding in engineering design and risk management.
Ground shaking: Ground shaking refers to the vibration of the Earth's surface caused by seismic waves generated during an earthquake. This phenomenon is crucial for understanding the impact of earthquakes on structures and populations, as it can lead to significant damage depending on the intensity, duration, and frequency of the shaking. Analyzing ground shaking helps in identifying different seismic phases and measuring various magnitudes, which are essential for assessing hazards and risks associated with earthquakes.
Gutenberg-Richter Relationship: The Gutenberg-Richter Relationship is a mathematical formula that describes the frequency-magnitude distribution of earthquakes, indicating that the number of earthquakes decreases exponentially with increasing magnitude. This relationship is crucial for understanding the energy release during seismic events and helps in assessing the likelihood of various magnitudes occurring in a specific region, thereby aiding in risk assessment and preparedness efforts.
Logic Tree Analysis: Logic tree analysis is a systematic method used to assess the various factors and uncertainties involved in seismic hazard assessments by breaking down complex problems into simpler, more manageable components. This approach helps to visualize the relationships between different variables, such as earthquake sources, ground motion prediction equations, and site effects, providing a structured framework for probabilistic seismic hazard analysis.
Loma Prieta Earthquake: The Loma Prieta Earthquake was a significant seismic event that struck Northern California on October 17, 1989, measuring 6.9 on the moment magnitude scale. It caused extensive damage in the San Francisco Bay Area and is known for its impact on urban infrastructure, leading to improved seismic hazard assessments and better preparedness strategies in the region.
Magnitude-frequency relationships: Magnitude-frequency relationships describe the connection between the size (magnitude) of earthquakes and how often they occur (frequency). This concept is crucial for understanding seismic hazard and risk, as it helps estimate the likelihood of various magnitudes of earthquakes over a given time period, influencing building codes and preparedness measures.
Maximum magnitude: Maximum magnitude refers to the largest earthquake that can occur in a specific area, based on geological and historical data. It helps in understanding the potential risk of seismic events by establishing a benchmark for the worst-case scenario that might be expected in a given region.
Monte Carlo Simulation: Monte Carlo simulation is a statistical technique that uses random sampling and repeated computation to model and understand complex systems or processes. By generating a large number of simulations based on random inputs, it helps quantify uncertainties and provides probabilistic outcomes for various scenarios, making it a valuable tool in fields like error analysis and seismic hazard assessment.
Northridge Earthquake: The Northridge Earthquake was a significant seismic event that struck the San Fernando Valley region of Los Angeles, California, on January 17, 1994. With a magnitude of 6.7, it caused extensive damage and resulted in significant loss of life, highlighting vulnerabilities in infrastructure and leading to changes in building codes and earthquake preparedness.
Peak Ground Acceleration: Peak ground acceleration (PGA) is a measure of the maximum ground acceleration experienced during an earthquake, typically expressed in units of g (gravitational acceleration). It is a crucial parameter used in understanding the intensity of ground shaking and assessing the potential impact on structures and human safety during seismic events.
Probabilistic seismic hazard analysis: Probabilistic seismic hazard analysis (PSHA) is a method used to estimate the likelihood of different levels of ground shaking at a site over a specified time period, considering the uncertainties associated with earthquake occurrence and ground motion. This approach incorporates various factors, such as seismic sources, geological conditions, and local site effects, to provide a statistical framework for understanding potential seismic risks. By using this analysis, engineers and planners can make informed decisions about building design and safety measures in earthquake-prone areas.
Return Period: The return period is the average time interval between events of a certain size or intensity, often used in risk assessment and statistical analyses to estimate the frequency of seismic events. It connects the likelihood of an earthquake occurring within a given timeframe to the statistical data on past seismic events, helping to inform building codes and hazard mitigation strategies.
Risk mitigation: Risk mitigation refers to the strategies and measures taken to reduce the potential impact of hazardous events, particularly in the context of natural disasters such as earthquakes. It encompasses various approaches to minimizing risks through preparedness, response, recovery, and mitigation techniques that can help protect communities and infrastructure. Effective risk mitigation involves understanding the vulnerabilities and probabilities associated with seismic activity, leading to informed decision-making and resource allocation.
Seismic source model: A seismic source model is a representation of the locations, sizes, and characteristics of potential seismic sources that can generate earthquakes in a specific region. It forms a crucial part of seismic hazard assessments by helping to estimate the likelihood of different magnitudes of earthquakes occurring over a given time period. These models integrate geological and geophysical data to characterize faults and other seismic sources, thus enabling better risk management and preparedness strategies.
Seismic zone: A seismic zone is a geographic area characterized by a certain level of seismic activity, which is defined based on the frequency and intensity of earthquakes that can occur in that region. Understanding seismic zones is crucial for assessing the earthquake risk to structures and populations, as they help engineers and planners design buildings and infrastructure that can withstand potential seismic events.
Site Amplification: Site amplification refers to the increase in ground motion intensity at a specific location due to local geological and environmental conditions during an earthquake. This phenomenon can significantly enhance the shaking experienced at the surface compared to the shaking observed at a reference site, typically located on bedrock. Factors such as soil type, thickness, and underlying geological structures can greatly influence site amplification, making it crucial for assessing seismic hazards and designing earthquake-resistant structures.
Spectral Acceleration: Spectral acceleration is a measure of the maximum response of a structure to ground motion, specifically reflecting how much a structure accelerates during an earthquake at different frequencies. It is essential for understanding the potential impact of seismic activity on buildings and infrastructure, as it helps quantify the forces exerted on structures during seismic events. By analyzing spectral acceleration, engineers can design buildings that better withstand earthquakes, ensuring safety and minimizing damage.
Uniform Hazard Spectra: Uniform hazard spectra (UHS) represent the relationship between ground motion intensity and the probability of exceeding that intensity during a specific time period. This concept is crucial in probabilistic seismic hazard analysis, where it helps quantify how likely certain levels of ground shaking are, taking into account various seismic sources and their associated uncertainties. UHS is instrumental in designing structures that can withstand potential earthquake forces based on defined hazard levels.
Vulnerability assessment: A vulnerability assessment is a systematic process used to identify, evaluate, and prioritize the weaknesses or potential hazards in a system or area that could be adversely affected by seismic events. This assessment helps in understanding the susceptibility of structures, infrastructure, and communities to earthquakes, guiding the development of strategies for mitigation and preparedness.