2.4 Earthquake seismology and seismic hazard assessment
5 min read•Last Updated on August 14, 2024
Earthquakes shake up our world, literally. They're caused by sudden energy releases in Earth's crust, mainly along fault lines. Understanding how they work is key to predicting and preparing for these powerful events.
Seismic hazards pose real risks to communities worldwide. By studying tectonic settings, soil conditions, and building vulnerabilities, we can assess and mitigate these risks. This knowledge helps create safer, more resilient cities in earthquake-prone areas.
Earthquake generation mechanisms
Causes and processes of earthquake occurrence
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Earthquakes are caused by the sudden release of stored elastic strain energy in the Earth's crust and upper mantle, primarily along pre-existing fault zones
The elastic rebound theory explains that earthquakes occur when the accumulated stress on a fault exceeds the frictional resistance, causing the fault to rupture and the two sides to move relative to each other
Factors controlling earthquake occurrence include the local and regional stress field, the frictional properties and geometry of the fault, the rate of stress accumulation, and the presence of fluids in the crust
The frequency and magnitude of earthquakes in a given region are influenced by the tectonic setting, with the largest and most frequent events occurring along plate boundaries, particularly in subduction zones (e.g., Japan, Chile) and continental collision zones (e.g., Himalayan region)
Earthquake magnitude and energy release
Earthquake magnitude is a measure of the energy released during an earthquake, and it is determined by the size of the rupture area, the amount of slip on the fault, and the rigidity of the rock
The moment magnitude scale (Mw) is the most widely used measure of earthquake size, as it provides a more accurate estimate of the energy released compared to other scales, such as the Richter scale
Moment magnitude is calculated using the formula: Mw=32log10(M0)−10.7, where M0 is the seismic moment in dyne-cm, which is a function of the fault area, average slip, and rock rigidity
The energy released by an earthquake increases exponentially with magnitude; for example, a magnitude 7 earthquake releases about 32 times more energy than a magnitude 6 earthquake
Locating earthquake epicenters
Seismic waves and their propagation
Seismic waves generated by earthquakes propagate through the Earth's interior and can be recorded by seismometers at various locations on the surface
P-waves (primary or compressional waves) and S-waves (secondary or shear waves) are the main types of body waves used in earthquake location and characterization
P-waves travel faster than S-waves and can pass through both solid and liquid materials, while S-waves can only propagate through solid materials
The difference in arrival times of P- and S-waves at multiple seismic stations can be used to determine the location of the earthquake epicenter through a process called triangulation
Focal mechanisms and beach ball diagrams
The focal mechanism of an earthquake describes the orientation and sense of motion of the fault plane during the rupture, and it can be determined by analyzing the polarities and amplitudes of seismic waves recorded at different stations
Beach ball diagrams, also known as focal mechanism solutions, provide a graphical representation of the focal mechanism, indicating the orientation of the fault plane and the direction of slip
Focal mechanisms can be classified into three main types: normal (extensional), reverse (compressional), and strike-slip (lateral), each associated with specific tectonic settings and stress regimes
For example, normal faulting is common in rift zones (e.g., East African Rift), reverse faulting occurs in subduction zones and collision zones (e.g., Andes, Himalayas), and strike-slip faulting is found along transform plate boundaries (e.g., San Andreas Fault)
Seismic hazards and risk
Tectonic settings and their seismic hazard characteristics
Seismic hazard refers to the probability of a given level of ground shaking or other earthquake-related effects (e.g., liquefaction, landslides) occurring at a specific location within a certain time period
Tectonic settings, such as subduction zones, continental collision zones, and intraplate regions, have distinct seismic hazard characteristics and associated risks
Subduction zones, where oceanic plates descend beneath continental or other oceanic plates, are capable of generating the world's largest earthquakes (Mw > 9) and often pose significant tsunami hazards to coastal communities
Continental collision zones, such as the Himalayan region, are characterized by frequent, moderate to large earthquakes and high seismic risk due to the dense population and vulnerable infrastructure in these areas
Intraplate regions, located within tectonic plates far from plate boundaries, experience less frequent but potentially damaging earthquakes, often with longer recurrence intervals and higher uncertainty in hazard assessment
Factors influencing seismic risk
Seismic risk is the potential for social and economic losses due to earthquakes, and it depends on the exposure and vulnerability of the built environment and population to seismic hazards
Geographic factors, such as the proximity to active faults, site conditions (e.g., soil type, bedrock depth), and topography, can significantly influence the local seismic hazard and risk
Soft soil sites (e.g., alluvial basins) can amplify seismic waves and increase the intensity of ground shaking, while hard rock sites generally experience lower amplification
Topographic effects, such as ridges and steep slopes, can also amplify ground motions and increase the risk of earthquake-triggered landslides
The vulnerability of buildings and infrastructure depends on factors such as construction type, age, design, and maintenance, with older and poorly constructed structures being more susceptible to damage during earthquakes
Seismic hazard assessment and mitigation
Probabilistic and deterministic hazard analysis methods
Probabilistic Seismic Hazard Analysis (PSHA) is a widely used approach that quantifies the probability of exceeding a given level of ground motion at a specific site over a certain time period, considering the combined effect of all potential earthquake sources in the region
PSHA involves characterizing seismic sources (faults and seismogenic zones), defining their earthquake recurrence rates and maximum magnitudes, selecting appropriate ground motion prediction equations (GMPEs), and integrating these components to obtain hazard curves and maps
Deterministic Seismic Hazard Analysis (DSHA) focuses on the worst-case scenario by considering the largest credible earthquake that could occur on a specific fault or within a seismic source zone, and estimating the resulting ground motions at a site of interest
DSHA is often used for critical facilities, such as nuclear power plants or large dams, where the consequences of failure are severe, and a more conservative approach to hazard assessment is required
Risk reduction and mitigation strategies
Seismic hazard maps, developed using PSHA or DSHA, provide the basis for risk assessment, land-use planning, and the development of building codes and design standards for earthquake-resistant structures
Mitigation strategies aim to reduce seismic risk by improving the resilience of the built environment and enhancing community preparedness
Implementing and enforcing seismic building codes and retrofitting existing structures to improve their earthquake resistance are crucial steps in reducing the vulnerability of buildings and infrastructure
Conducting site-specific hazard assessments and microzonation studies can guide land-use planning and development decisions, helping to avoid construction in high-risk areas or ensuring appropriate design measures are taken
Developing and maintaining early warning systems and emergency response plans can minimize the impact of earthquakes on communities by providing critical time for evacuation and preparedness
Promoting public awareness and education about seismic hazards and risk reduction measures is essential for fostering a culture of earthquake preparedness and resilience