7.2 Seismic waves: P-waves, S-waves, and surface waves
4 min read•august 16, 2024
Earthquakes release energy in the form of seismic waves. These waves come in different types, each with unique properties that help scientists understand Earth's structure. , , and travel at different speeds and interact with Earth's layers in distinct ways.
Seismic waves provide crucial information about our planet's interior. By studying how these waves move through Earth, scientists can map out its layers, from the crust to the inner core. This knowledge is essential for understanding plate tectonics and earthquake behavior.
Seismic Wave Types
Body Waves and Surface Waves
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Seismic waves divide into body waves and surface waves
Body waves include P-waves and S-waves
Surface waves include and
P-waves propagate through solid and liquid media as compressional waves
Particles oscillate parallel to wave propagation direction
Move fastest among all seismic waves
S-waves propagate only through solid media as shear waves
Particles oscillate perpendicular to wave propagation direction
Travel slower than P-waves but faster than surface waves
Surface waves travel along Earth's surface
Love waves cause horizontal shearing of ground surface
P-waves travel through all Earth layers including the core
S-waves cannot propagate through liquid outer core
Creates "" on opposite side of Earth
Surface waves confined to outermost Earth layers
Body waves refract and reflect at layer boundaries
Due to changes in density and elastic properties
Body decreases with distance from source
Results from and
Surface waves attenuate more slowly with distance
Makes them more destructive in earthquakes
Seismic Wave Characteristics
Wave Velocities and Material Properties
Seismic velocities relate directly to elastic properties and density of propagation medium
P-wave velocity (Vp) determined by:
Bulk modulus
Shear modulus
Density of material
S-wave velocity (Vs) determined by:
Shear modulus
Density of material
provides information about:
Composition of Earth materials
Physical state of Earth materials
Velocity discontinuities indicate composition or property changes
Examples include and
Seismic velocities generally increase with depth
Due to increasing pressure and density
Can decrease in areas of partial melting or liquid outer core
Wave Behavior and Attenuation
Body waves refract and reflect at layer boundaries
Caused by changes in density and elastic properties
P-wave and S-wave amplitude decreases with distance from source
Due to geometric spreading as waves propagate outward
Attenuation from energy loss in the medium
Surface waves attenuate more slowly compared to body waves
Contributes to their destructive potential in earthquakes
Love waves cause horizontal shearing motion of ground surface
Rayleigh waves produce elliptical particle motion at surface
Wave Velocities and Earth's Layers
Velocity Structure of Earth's Interior
Seismic velocities generally increase with depth
Result of increasing pressure and density
Velocity discontinuities mark significant layer boundaries
Mohorovičić discontinuity between crust and mantle
Core-mantle boundary between mantle and outer core
Velocity decreases can occur in specific regions
Partial melting zones in upper mantle
Liquid outer core
P-wave velocities higher than S-wave velocities in all solid layers
S-waves cannot propagate through liquid outer core
Creates S-wave shadow zone on opposite side of Earth
Surface wave velocities vary with frequency (dispersion)
Provides information about shallow Earth structure
Relationship to Earth Properties
Vp/Vs ratio indicates material properties
Higher ratios suggest more mafic composition
Lower ratios indicate more felsic composition
Velocity changes reflect variations in:
Temperature
Pressure
Composition
Physical state (solid vs liquid)
Low velocity zones may indicate:
Partial melting
Change in mineral structure
High velocity zones often represent:
Colder, denser material
Subducted slabs in mantle
Seismic Waves for Earth's Structure
Seismic Imaging Techniques
creates 3D images of Earth's interior
Uses travel times and amplitudes of seismic waves
Reveals velocity variations in crust, mantle, and core
P-wave and S-wave arrival time analysis determines:
Earthquake locations
Velocity models of Earth's interior
Seismic reflection and refraction methods study:
Detailed structure of crust
Upper mantle structure
Surface wave dispersion analysis constrains:
velocity structure of crust
Upper mantle velocity structure
uses converted seismic waves
Provides information on sharp velocity contrasts
Helps image crust-mantle boundary (Moho)
Insights into Earth Structure
S-wave shadow zones provide evidence for liquid outer core
Body wave travel times reveal:
Overall radial structure of Earth
Major discontinuities between layers
Surface wave dispersion shows:
Variations in crustal thickness
Upper mantle structure
indicates:
Mantle flow patterns
Preferred mineral orientations
(S waves reflected off core) constrain:
Core-mantle boundary structure
D" layer properties
(P waves traveling through core) reveal:
Inner core structure
Inner core anisotropy
Key Terms to Review (31)
Attenuation: Attenuation refers to the reduction in strength or intensity of seismic waves as they travel through different materials in the Earth's interior. This phenomenon occurs due to scattering, absorption, and geometrical spreading, impacting how seismic waves are detected and analyzed. Understanding attenuation is crucial for interpreting seismic data, as it affects the amplitude and frequency of waves reaching the surface after an earthquake.
Compressional wave: A compressional wave, also known as a P-wave or primary wave, is a type of seismic wave that travels through a medium by compressing and expanding the material in the direction of the wave's propagation. These waves are the fastest seismic waves and can move through solids, liquids, and gases, making them crucial for understanding the internal structure of the Earth.
Core-mantle boundary: The core-mantle boundary is the interface that separates the Earth's outer core, which is composed mainly of liquid iron and nickel, from the overlying mantle, made primarily of silicate rocks. This boundary plays a crucial role in understanding seismic wave behavior, as it marks a significant change in material properties that affects how seismic waves propagate through the Earth.
Earthquake intensity: Earthquake intensity is a measure of the effects of an earthquake at specific locations, describing how strongly the ground shakes and the level of damage caused. This concept helps differentiate between the physical characteristics of seismic waves and their impact on people and structures, providing insight into how earthquakes affect various environments.
Elastic Rebound Theory: Elastic rebound theory explains how energy accumulates in rocks as they are deformed by tectonic forces until they reach a breaking point, leading to an earthquake. This theory illustrates the process where rocks bend elastically under stress, and when they eventually fracture, the stored energy is released as seismic waves, causing ground shaking. Understanding this process connects various aspects of seismic activity and plate tectonics, showing how stresses build up along faults and lead to earthquakes.
Epicenter: The epicenter is the point on the Earth's surface directly above the location where an earthquake originates, known as the focus or hypocenter. This point is crucial in understanding earthquakes as it helps in identifying the area that experiences the strongest shaking and potential damage during seismic events. Knowing the epicenter allows scientists to analyze how seismic waves travel and can inform emergency response efforts in affected regions.
Fault Line: A fault line is a crack or fracture in the Earth's crust where tectonic plates meet, leading to the potential for earthquakes as stress builds up and is released. These lines are critical in understanding seismic activity, as they represent zones of weakness where movement occurs, linking directly to the causes of earthquakes, the behavior of seismic waves, and the characteristics of transform faults.
Focus: In the context of seismic waves, the focus refers to the exact point within the Earth where an earthquake originates. This point is critical because it determines how seismic energy radiates outward, influencing the intensity and impact of the earthquake felt at the surface. Understanding the focus helps scientists identify not just where an earthquake begins, but also provides insights into the geology and tectonic activity of the region.
Geometric spreading: Geometric spreading refers to the way seismic waves lose energy as they propagate through the Earth, primarily due to the increasing area over which the energy is distributed. As these waves travel outward from their source, they spread out over a larger volume, causing a decrease in amplitude and intensity. This concept is crucial for understanding how different types of seismic waves behave as they move through various geological materials.
Ground shaking: Ground shaking is the vibration of the Earth's surface caused by seismic waves generated during an earthquake. This phenomenon is crucial as it directly impacts the level of destruction experienced in an area, influenced by factors such as the type of seismic waves, local geology, and building structures. Understanding ground shaking helps in assessing earthquake hazards and designing buildings that can withstand these forces.
Love Waves: Love waves are a type of surface seismic wave that travels along the Earth's surface, causing horizontal shaking. They are named after the British mathematician A.E.H. Love, who developed the mathematical model for these waves. Love waves are particularly destructive during earthquakes, as their horizontal motion can cause significant damage to structures.
Mohorovičić discontinuity: The mohorovičić discontinuity, often referred to as the Moho, is the boundary between the Earth's crust and the underlying mantle, characterized by a sudden change in seismic wave velocities. This discontinuity is significant because it marks the transition from the less dense, silicate-rich crust to the denser mantle, and plays a crucial role in understanding seismic wave behavior and Earth's internal structure.
P-waves: P-waves, or primary waves, are the fastest type of seismic wave generated by earthquakes, traveling through the Earth’s interior. These waves are compressional, meaning they move by compressing and expanding the material they pass through, allowing them to travel through both solid and liquid layers of the Earth.
Pkp phases: The pkp phases are seismic waves that are part of the P-wave family, specifically the primary waves that travel through the Earth's interior after an earthquake. These waves can travel through both solid and liquid materials, making them crucial for understanding the Earth's internal structure. The pkp designation indicates that these waves have passed through the outer core, allowing scientists to gather information about the composition and behavior of the Earth's layers.
Rayleigh waves: Rayleigh waves are a type of surface seismic wave that travels along the Earth's surface, causing both vertical and horizontal ground movement. These waves result in a rolling motion, similar to ocean waves, and are crucial for understanding how seismic energy is released during earthquakes. Rayleigh waves are slower than P-waves and S-waves, but they can cause significant damage due to their larger amplitudes as they propagate through the Earth's crust.
Receiver function analysis: Receiver function analysis is a seismic technique used to investigate the Earth's subsurface structure by analyzing the conversion of seismic waves as they travel through different layers of material. This method primarily focuses on the P-waves and S-waves generated by earthquakes, allowing scientists to infer information about the crust and upper mantle based on how these waves interact with various geological features. By using data from seismometers, receiver function analysis helps to enhance our understanding of tectonic processes and subsurface composition.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are shear waves that only travel through solid materials, making them slower than P-waves and responsible for much of the damage associated with earthquakes due to their side-to-side motion.
Scs phases: SCS phases refer to the different stages of seismic wave propagation, specifically within the context of how seismic waves travel through the Earth during an earthquake. Understanding these phases helps in analyzing the behavior and impact of seismic waves, which include primary waves (P-waves), secondary waves (S-waves), and surface waves, each with unique characteristics and effects on the Earth's materials.
Seismic anisotropy: Seismic anisotropy refers to the variation in seismic wave speeds in different directions within a material, influenced by the structure and composition of the Earth's subsurface. This phenomenon is crucial for understanding the Earth's internal properties because it affects how P-waves, S-waves, and surface waves propagate through various geological formations, revealing insights about tectonic processes and material characteristics.
Seismic tomography: Seismic tomography is an imaging technique that uses seismic waves to create detailed pictures of the Earth's interior. By analyzing how different types of seismic waves—like P-waves, S-waves, and surface waves—travel through the Earth, scientists can infer the composition, structure, and dynamics of geological formations. This technique plays a crucial role in understanding the Earth's internal structure and can also help reconstruct past plate positions by revealing information about historical tectonic activity.
Seismogram: A seismogram is a record produced by a seismograph that captures the motion of the ground during an earthquake or seismic event. This record shows the amplitude and duration of seismic waves, providing crucial information about the intensity and characteristics of the seismic activity, including P-waves, S-waves, and surface waves.
Seismograph: A seismograph is an instrument that detects and records the motion of the ground caused by seismic waves generated during an earthquake. It works by measuring the vibrations of the Earth's surface, allowing scientists to analyze the characteristics of earthquakes and their origins, which are often linked to the movement of tectonic plates. The data obtained from seismographs is essential for understanding earthquake mechanics and assessing potential hazards in different regions.
Shadow zone: A shadow zone is an area on the Earth's surface where seismic waves are not detected after an earthquake, specifically due to the properties of P-waves and S-waves as they travel through the Earth. This phenomenon occurs because P-waves can travel through both solid and liquid materials, while S-waves can only travel through solids, creating regions where certain seismic waves cannot be detected. The existence of shadow zones helps scientists understand the internal structure of the Earth.
Shear wave: A shear wave, also known as an S-wave, is a type of seismic wave that moves through the Earth by shearing or shaking particles perpendicular to the direction of wave travel. These waves are crucial in understanding the Earth's internal structure because they can only travel through solid materials, revealing important information about the composition and state of the Earth's layers.
Surface wave properties: Surface wave properties refer to the characteristics of seismic waves that travel along the Earth's surface, primarily generated by the energy released during an earthquake. These waves are crucial for understanding how seismic energy propagates and how it affects structures and the Earth’s surface. Surface waves typically have slower velocities compared to P-waves and S-waves but can cause more damage due to their larger amplitudes and longer durations.
Surface waves: Surface waves are seismic waves that travel along the Earth's surface, causing most of the damage during an earthquake. Unlike P-waves and S-waves, which travel through the Earth's interior, surface waves move more slowly but can produce significant shaking and rolling motions, making them particularly destructive in populated areas. Their characteristics are crucial for understanding how energy is released during earthquakes and how it impacts buildings and infrastructure.
Tectonic boundary: A tectonic boundary is a region where two tectonic plates meet, leading to various geological phenomena such as earthquakes, volcanic activity, and mountain formation. These boundaries are classified into three main types: convergent, divergent, and transform, each having distinct characteristics that influence seismic waves generated during tectonic events.
Vp/vs ratio: The vp/vs ratio is the ratio of the velocities of primary waves (P-waves) to secondary waves (S-waves) as they travel through geological materials. This ratio is significant because it helps geologists understand the properties of the materials within the Earth, such as density and elastic modulus, and can provide insights into the types of rocks or fluids present in subsurface formations.
Wave amplitude: Wave amplitude refers to the maximum displacement of points on a wave from their rest position, representing the height of the wave. In the context of seismic waves, it indicates the energy released during an earthquake and affects how strongly the waves can shake the ground. Larger amplitudes correspond to more intense seismic events and can lead to greater destruction.
Wave speed: Wave speed refers to the velocity at which seismic waves travel through different materials within the Earth. This speed is influenced by factors such as the type of wave, the medium it travels through, and its physical properties like density and elasticity. Understanding wave speed is crucial for interpreting seismic data and assessing the Earth's internal structure.
Wave theory: Wave theory describes the behavior of seismic waves as they propagate through the Earth. It explains how these waves, including P-waves, S-waves, and surface waves, carry energy from an earthquake's source to various points on the Earth's surface. This concept is essential for understanding how seismic waves travel, their characteristics, and their effects on structures during an earthquake.