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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=23log10(M0)10.7Mw = \frac{2}{3} \log_{10} (M_0) - 10.7, where M0M_0 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


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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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