Seismic waves are Earth's messengers, revealing its hidden layers and structures. They come in two main types: body waves that travel through Earth's interior, and that ripple along its surface.
These waves behave differently based on the materials they encounter. By studying their speeds, paths, and changes, scientists can map out Earth's inner workings, from its crust to its core.
Seismic Wave Types and Characteristics
Body Waves
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(primary or compressional waves) are the fastest seismic waves and can travel through both solid and liquid materials
Cause particles to oscillate parallel to the direction of wave propagation, resulting in compression and rarefaction
typically ranges from 1.5 km/s in unconsolidated sediments to over 13 km/s in the Earth's inner core
(secondary or shear waves) are slower than P-waves and can only travel through solid materials
Cause particles to oscillate perpendicular to the direction of wave propagation, resulting in shearing motion
Velocity is typically about 60% of the corresponding P-wave velocity in solid materials
Cannot propagate through liquids, such as the Earth's outer core
Surface Waves
Rayleigh waves cause particles to move in an elliptical motion in the vertical plane, with both vertical and horizontal components
Slowest seismic waves and cause the most damage during earthquakes (ground rolling motion)
Velocity is slightly slower than S-waves in the same material
Love waves cause particles to move side-to-side in a horizontal plane, perpendicular to the direction of wave propagation
Faster than Rayleigh waves but slower than body waves
Confined to the Earth's surface and shallow depths
Require a low-velocity layer overlying a high-velocity layer to propagate (waveguide effect)
Factors Influencing Seismic Wave Propagation
Elastic Properties and Density
Seismic wave velocity is primarily controlled by the elastic moduli (bulk modulus and shear modulus) and density of the material
Higher elastic moduli and lower density result in faster wave velocities
P-wave velocity: Vp=ρK+4/3μ, where K is bulk modulus, μ is shear modulus, and ρ is density
S-wave velocity: Vs=ρμ
Temperature and pressure increase with depth in the Earth, affecting the elastic properties and density of materials
Increasing temperature generally reduces the elastic moduli (thermal expansion and weakening of atomic bonds)
Increasing pressure increases the elastic moduli and density (compression and closure of pores and cracks)
Anisotropy and Heterogeneity
Anisotropy is the variation of elastic properties with direction, causing seismic waves to travel at different velocities depending on their direction of propagation
Common in layered or foliated rocks (sedimentary bedding, metamorphic foliation)
Can lead to shear wave splitting (birefringence) and azimuthal variations in velocity
Heterogeneities, such as inclusions, fractures, and compositional variations, can cause scattering and of seismic waves
Scattering redistributes energy and can generate coda waves (late-arriving, scattered energy)
Attenuation is the loss of energy as waves propagate through a material, resulting in decreased and higher frequencies being attenuated more rapidly than lower frequencies
Seismic Wave Velocity and Attenuation
Velocity Variations in Earth Materials
Seismic wave velocities vary depending on the type of material and its properties
Velocity generally increases with depth due to increasing pressure and changes in material properties
Crustal velocity structure: gradual increase with depth, with discontinuities at major compositional boundaries (e.g., Moho)
Mantle velocity structure: gradual increase with depth, with discontinuities at phase transitions (e.g., 410 km and 660 km discontinuities)
Attenuation Mechanisms and Quality Factor
Attenuation is quantified by the quality factor (Q), which is inversely proportional to the energy loss per cycle
Higher Q values indicate lower attenuation and more efficient wave propagation
Q is -dependent: Q(ω)=2α(ω)Vω, where ω is angular frequency, α is attenuation coefficient, and V is wave velocity
Intrinsic absorption is the conversion of elastic energy to heat due to anelastic processes (e.g., grain boundary sliding, dislocation motion)
Increases with temperature and decreases with pressure
More significant in partially molten or fluid-rich regions (asthenosphere, magma chambers)
Scattering is the redistribution of energy due to heterogeneities in the medium
Depends on the size, shape, and contrast of the heterogeneities relative to the seismic
More pronounced in highly fractured or heterogeneous regions (fault zones, volcanic areas)
Geometrical spreading is the decrease in energy density with distance from the source due to the expansion of the wavefront
Causes amplitude to decrease with distance even in the absence of other attenuation mechanisms
Amplitude decays as 1/r for body waves and 1/r for surface waves, where r is distance from the source
Earth's Internal Structure from Seismic Waves
Seismic Wave Arrival Times and Amplitudes
The difference in arrival times between P-waves and S-waves (S-P time) increases with distance from the source
Allows determination of the distance to the earthquake epicenter using travel-time curves or tables
S-P time increases sharply at distances corresponding to major discontinuities (e.g., core-mantle boundary)
Seismic wave amplitudes provide information about the attenuation properties of the materials they pass through
Lower amplitudes indicate higher attenuation and can be used to identify regions of partial melting or fluid content
Amplitude variations with distance can also reveal the presence of discontinuities and velocity gradients
Seismic Discontinuities and Velocity Structure
Seismic discontinuities are characterized by abrupt changes in seismic wave velocities and cause reflections and conversions between wave types
Mohorovičić discontinuity (Moho): boundary between the crust and mantle, marked by a sharp increase in P-wave velocity (6.0-7.5 km/s to 7.5-8.5 km/s)
Core-mantle boundary (CMB): boundary between the mantle and outer core, marked by a sharp decrease in P-wave velocity (13.0 km/s to 8.0 km/s) and disappearance of S-waves
Inner core boundary (ICB): boundary between the outer and inner core, marked by a sharp increase in P-wave velocity (10.5 km/s to 11.0 km/s) and reappearance of S-waves
Seismic wave velocities increase with depth due to increasing pressure and changes in material properties
Causes seismic waves to refract (bend) according to Snell's law: V1sinθ1=V2sinθ2, where θ is the angle of incidence/ and V is the wave velocity
Results in curved ray paths and the formation of shadow zones (regions where direct seismic waves are not observed)
The absence of direct S-waves beyond about 100° from the epicenter suggests the presence of a liquid outer core
Seismic Tomography and 3D Earth Structure
uses the arrival times and amplitudes of seismic waves from multiple sources and receivers to create 3D images of the Earth's interior
Travel-time tomography: uses the difference between observed and predicted travel times to invert for velocity variations
Attenuation tomography: uses the decay of seismic wave amplitudes to invert for attenuation variations
Waveform tomography: uses the complete waveform (shape and amplitude) to invert for velocity and attenuation variations
Tomographic models reveal lateral variations in seismic wave velocity and attenuation, providing insights into the Earth's 3D structure
Mantle plumes: low-velocity, high-attenuation regions extending from the core-mantle boundary to the surface (e.g., Hawaii, Iceland)
Subducting slabs: high-velocity, low-attenuation regions extending from the surface to the lower mantle (e.g., Pacific Ring of Fire)
Large low-shear-velocity provinces (LLSVPs): broad, low-velocity regions in the lowermost mantle, possibly related to thermal or compositional variations
Key Terms to Review (21)
Accelerometer: An accelerometer is a device that measures the acceleration forces acting on an object, including gravity, to determine its motion and orientation. In the context of seismic studies, accelerometers play a vital role in detecting and recording ground motions caused by seismic waves, providing critical data for analyzing seismic events and their effects on structures and the Earth.
Amplitude: Amplitude refers to the maximum extent of a vibration or oscillation, measured from the position of equilibrium. In the context of seismic waves, amplitude is directly related to the energy released during an earthquake, with larger amplitudes indicating more intense seismic activity. Similarly, in acoustic and seismic logging, amplitude helps in assessing the properties of subsurface materials and can provide insights into geological formations.
Attenuation: Attenuation refers to the reduction in the amplitude and intensity of seismic waves as they propagate through different geological materials. This decrease can be caused by various factors such as scattering, absorption, and geometric spreading, impacting how effectively seismic waves transmit energy. Understanding attenuation is crucial for interpreting seismic data, determining subsurface structures, and assessing material properties in geophysical studies.
Diffraction: Diffraction is the bending of waves around obstacles and the spreading out of waves when they pass through small openings. This phenomenon is particularly important in understanding how seismic waves travel through the Earth, as their behavior changes when encountering different materials, interfaces, and geological structures. It helps in interpreting seismic data by providing insights into subsurface features and wave propagation characteristics.
Earthquake source: An earthquake source refers to the specific location and mechanism where an earthquake originates, typically associated with the sudden release of energy along a fault line in the Earth's crust. Understanding the earthquake source is crucial for interpreting seismic waves and their properties, as it determines how the energy propagates through the Earth, influencing factors like wave type, intensity, and duration.
Elasticity: Elasticity refers to the ability of a material to deform under stress and return to its original shape once the stress is removed. This property is crucial in understanding how Earth's materials respond to forces, influencing their behavior during geological processes, including those associated with seismic waves and Earth's structure. Elasticity helps in predicting how different materials within the Earth will react when subjected to pressure, which is essential for various applications in geophysics.
Fault Line: A fault line is a fracture or zone of fractures between two blocks of rock that allows for relative movement, typically occurring in the Earth's crust. These lines are critical in understanding tectonic activity, as they indicate where stress is building up and can lead to earthquakes when released. Fault lines are shaped by the dynamics of plate tectonics, which drive the movement of the Earth's lithospheric plates and contribute to geodynamic processes.
Frequency: Frequency refers to the number of cycles of a wave that occur in a given unit of time, typically measured in hertz (Hz). In the context of seismic waves, frequency is crucial because it helps to determine the energy and behavior of the waves as they propagate through different materials in the Earth's crust. Higher frequencies are associated with shorter wavelengths and can provide more detailed information about subsurface structures, while lower frequencies can penetrate deeper into the Earth, revealing broader geological features.
Intensity: In the context of seismic waves, intensity refers to the measure of the energy released by an earthquake as it propagates through the Earth and is felt on the surface. This energy is usually measured in terms of the damage caused to structures and the effects experienced by people, making intensity a crucial factor in understanding how earthquakes impact communities and environments.
Linear elasticity theory: Linear elasticity theory is a mathematical framework that describes how materials deform and return to their original shape when subjected to external forces, assuming the relationship between stress and strain is linear. This theory simplifies the analysis of material behavior by assuming small deformations and uniform material properties, making it essential in understanding the propagation of seismic waves through solid earth materials.
P-waves: P-waves, or primary waves, are a type of seismic wave that travels through the Earth during an earthquake. They are compressional waves, meaning they cause particles in the material they move through to vibrate in the same direction as the wave itself, which allows them to travel through solids, liquids, and gases. Understanding p-waves is crucial for studying the Earth's internal structure, seismic wave properties, and assessing earthquake hazards.
Reflection: Reflection is the bouncing back of waves, such as seismic waves, when they encounter a boundary between different materials. This phenomenon is crucial in understanding Earth's structure, as it provides insights into the composition and properties of various geological layers. The way seismic waves reflect off these boundaries helps geophysicists interpret subsurface features, revealing vital information about the Earth's internal makeup.
Refraction: Refraction is the bending of waves when they enter a medium where their speed is different, resulting in a change in direction. This phenomenon occurs with seismic waves as they travel through different layers of the Earth, such as from the crust to the mantle, which have varying densities and elastic properties. Refraction helps geophysicists understand the internal structure of the Earth and the behavior of seismic waves as they interact with geological formations.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are characterized by their ability to move the ground up and down or side to side and are slower than primary waves, making them crucial for understanding Earth's internal structure and the dynamics of seismic activity.
Seismic tomography: Seismic tomography is a geophysical imaging technique that uses seismic waves to create detailed three-dimensional images of the Earth's internal structure, including its composition and properties. This technique is essential for understanding the distribution of different materials within the Earth, such as the crust, mantle, and core, and plays a significant role in studying tectonic processes and earthquake behavior.
Seismometer: A seismometer is an instrument that detects and records the motion of the ground caused by seismic waves during events like earthquakes. It plays a crucial role in understanding the nature of seismic waves and helps gather essential data for assessing hazards, predicting earthquakes, and conducting various geophysical surveys.
Surface Waves: Surface waves are seismic waves that travel along the Earth's surface, typically resulting from the energy released during an earthquake. They are known for causing the most damage during seismic events due to their high amplitude and prolonged duration. These waves can be classified mainly into two types: Love waves and Rayleigh waves, which each have distinct motion characteristics and effects on structures.
Velocity: Velocity is a vector quantity that refers to the rate at which an object changes its position in a specific direction. In geophysics, understanding velocity is crucial as it influences how seismic waves propagate through different materials, which in turn affects data interpretation during geological surveys and planning. The measurement of velocity helps determine subsurface structures and can be essential for making informed decisions during exploration and resource management.
Wave Theory: Wave theory explains the behavior and characteristics of waves as they propagate through different mediums, which is fundamental for understanding seismic waves. In this context, it describes how energy is transmitted through the Earth's materials, leading to the generation of seismic waves during events like earthquakes. The theory also encompasses the distinctions between various types of seismic waves and their interactions with geological structures.
Waveform analysis: Waveform analysis is the process of examining seismic waveforms to extract meaningful information about the properties and behavior of seismic waves as they travel through different geological materials. This technique helps in understanding how waves interact with structures and can be crucial in assessing subsurface conditions, making it vital for applications like oil and gas exploration, earthquake studies, and geotechnical engineering.
Wavelength: Wavelength is the distance between consecutive peaks (or troughs) of a wave, typically measured in meters. It is a critical property of waves that helps to determine their behavior, including how they propagate through different media and how they interact with matter. In the context of seismic waves, wavelength influences the ability to resolve geological features and affects the energy carried by the waves.