🌋Seismology Unit 8 – Earthquake Sources and Focal Mechanisms

What's This Unit All About?

• Explores the origins and causes of earthquakes, focusing on the geological processes and mechanisms that generate seismic events
• Examines the different types of earthquake sources, including tectonic earthquakes, volcanic earthquakes, and induced seismicity
• Introduces the concept of focal mechanisms, which describe the orientation and movement of the fault plane during an earthquake
• Discusses the various types of seismic waves generated by earthquakes and their behavior as they propagate through the Earth's interior
• Covers the tools and techniques used by seismologists to analyze and interpret seismic data, such as seismograms and focal mechanism solutions
• Highlights the real-world applications of understanding earthquake sources and focal mechanisms, including seismic hazard assessment and risk mitigation strategies
• Addresses common misconceptions and frequently asked questions related to earthquake sources and focal mechanisms

Key Concepts and Definitions

• Earthquake source: The location and mechanism responsible for generating seismic waves during an earthquake
• Focal mechanism: A description of the orientation and movement of the fault plane during an earthquake, represented by a beach ball diagram
• Seismic waves: Elastic waves generated by earthquakes that propagate through the Earth's interior, including P-waves, S-waves, and surface waves
• Fault plane: The surface along which the rock ruptures during an earthquake, characterized by its orientation (strike and dip) and the direction of slip (rake)
• Moment tensor: A mathematical representation of the forces acting at the earthquake source, used to determine the focal mechanism
• Double-couple: A simplified model of the earthquake source, representing the forces acting on the fault plane as a pair of equal and opposite forces
• Seismic moment ($M_0$): A measure of the size of an earthquake, calculated from the product of the rock rigidity, fault area, and average slip

Types of Earthquake Sources

• Tectonic earthquakes: The most common type of earthquake, caused by the sudden release of stored elastic strain energy along faults due to plate tectonic movements
  • Occur along plate boundaries (interplate) and within plates (intraplate)
  • Examples include the San Andreas Fault in California and the Himalayan Frontal Thrust
• Volcanic earthquakes: Seismic events associated with volcanic activity, often caused by the movement of magma or the collapse of volcanic structures
  • Can be categorized into long-period (LP) events, volcano-tectonic (VT) events, and hybrid events
  • Examples include earthquakes at Mount St. Helens and Kilauea volcanoes
• Induced seismicity: Earthquakes triggered by human activities, such as fluid injection, reservoir impoundment, and mining operations
  • Can be caused by changes in stress or pore pressure in the subsurface
  • Examples include earthquakes related to wastewater injection in Oklahoma and the Three Gorges Dam in China
• Slow slip events: Gradual, aseismic slip along faults that can last from days to years, often accompanied by low-frequency tremor
  • Occur in subduction zones and along the deep extensions of faults
  • Examples include the Cascadia subduction zone and the San Andreas Fault

Understanding Focal Mechanisms

• Beach ball diagrams: A graphical representation of the focal mechanism, showing the orientation of the fault plane and the direction of slip
  • Compressional quadrants (usually shaded) represent the areas of compression, while dilatational quadrants (usually white) represent areas of tension
  • The nodal planes (lines separating the quadrants) represent the fault plane and the auxiliary plane
• Fault plane solutions: The process of determining the focal mechanism from seismic data, using the polarities and amplitudes of P-wave first motions
  • Involves plotting the P-wave first motions on a stereographic projection and finding the nodal planes that best fit the data
  • Can be determined using manual methods or automated algorithms
• Moment tensor inversion: A technique for determining the full moment tensor from seismic waveform data, providing a more complete description of the earthquake source
  • Involves fitting synthetic seismograms to observed data and minimizing the misfit
  • Allows for the determination of non-double-couple components, such as isotropic and compensated linear vector dipole (CLVD) components
• Stress inversion: The process of determining the orientation and relative magnitudes of the principal stress axes from a set of focal mechanisms
  • Assumes that earthquakes occur on pre-existing faults oriented favorably with respect to the regional stress field
  • Provides insights into the tectonic stress regime and the likelihood of future earthquakes

Seismic Waves and Their Behavior

• P-waves (Primary waves): Compressional waves that travel fastest through the Earth's interior, with particle motion parallel to the direction of wave propagation
  • Can travel through both solid and liquid materials
  • Velocity depends on the elastic properties and density of the medium
• S-waves (Secondary waves): Shear waves that travel slower than P-waves, with particle motion perpendicular to the direction of wave propagation
  • Can only travel through solid materials
  • Velocity depends on the shear modulus and density of the medium
• Surface waves: Seismic waves that propagate along the Earth's surface, with amplitudes that decrease with depth
  • Rayleigh waves: Surface waves with elliptical particle motion in the vertical plane, similar to ocean waves
  • Love waves: Surface waves with horizontal particle motion perpendicular to the direction of wave propagation
• Seismic wave propagation: The behavior of seismic waves as they travel through the Earth's interior, influenced by factors such as velocity structure, attenuation, and scattering
  • Reflection and refraction occur at boundaries between materials with different elastic properties
  • Attenuation causes the amplitude of seismic waves to decrease with distance due to energy dissipation
  • Scattering occurs when seismic waves encounter heterogeneities in the Earth's interior, causing energy to be redistributed in different directions

Tools and Techniques for Analysis

• Seismograms: Recordings of ground motion as a function of time, obtained from seismometers or accelerometers
  • Provide information about the arrival times, amplitudes, and polarities of seismic waves
  • Can be analyzed to determine the location, magnitude, and focal mechanism of earthquakes
• Seismic networks: Arrays of seismometers deployed to monitor seismic activity in a region
  • Allow for the detection and location of earthquakes using the arrival times of seismic waves at multiple stations
  • Examples include the Global Seismographic Network (GSN) and regional networks like the Southern California Seismic Network (SCSN)
• Waveform modeling: The process of generating synthetic seismograms based on a given Earth model and earthquake source parameters
  • Used to test hypotheses about the Earth's structure and earthquake source properties
  • Involves solving the elastic wave equation using numerical methods, such as finite differences or spectral elements
• Focal mechanism catalogs: Databases of focal mechanism solutions for earthquakes in a given region or time period
  • Provide information about the dominant faulting styles and stress orientations in a region
  • Examples include the Global Centroid Moment Tensor (GCMT) catalog and regional catalogs like the Southern California Earthquake Center (SCEC) catalog

Real-World Applications

• Seismic hazard assessment: The process of evaluating the likelihood and potential consequences of future earthquakes in a region
  • Involves characterizing the seismic sources, ground motion prediction, and site effects
  • Used to develop building codes, land-use planning, and emergency response plans
• Earthquake early warning systems: Networks of seismometers and algorithms designed to rapidly detect and characterize earthquakes, providing warning of imminent ground shaking
  • Rely on the faster speed of electronic signals compared to seismic waves
  • Examples include the ShakeAlert system in the United States and the Earthquake Early Warning system in Japan
• Induced seismicity monitoring: The practice of monitoring and managing seismic activity related to human activities, such as fluid injection and mining
  • Involves real-time seismic monitoring, traffic light protocols, and risk mitigation strategies
  • Examples include the monitoring of geothermal energy production in Iceland and the regulation of wastewater injection in Oklahoma
• Tectonic studies: The use of earthquake source information to investigate the structure and dynamics of the Earth's crust and upper mantle
  • Focal mechanisms provide insights into the stress orientation and faulting styles in a region
  • Seismic tomography uses the travel times and amplitudes of seismic waves to image the Earth's interior structure

Common Misconceptions and FAQs

• Can earthquakes be predicted? While short-term earthquake prediction remains a challenge, long-term forecasting based on probabilistic seismic hazard assessment is possible
• Are all earthquakes caused by plate tectonics? While most earthquakes are related to plate tectonic processes, some can be caused by other factors such as volcanic activity or human-induced stress changes
• Do small earthquakes relieve stress and prevent larger ones? Small earthquakes do release stress, but they do not necessarily prevent larger earthquakes from occurring
• Can animals predict earthquakes? There is no scientific evidence that animals can reliably predict earthquakes, although some unusual animal behavior has been reported before seismic events
• Are deep earthquakes more dangerous than shallow ones? Deep earthquakes can be felt over larger areas due to less attenuation, but shallow earthquakes often cause more damage due to stronger ground motion near the surface
• What is the difference between magnitude and intensity? Magnitude is a measure of the energy released by an earthquake at its source, while intensity describes the effects of the earthquake on people, structures, and the environment at a given location
• How do seismologists locate earthquakes? Seismologists use the arrival times of seismic waves at multiple seismic stations to triangulate the location of the earthquake source, a process known as hypocenter determination