3.2 Types of seismic waves and their properties
5 min read•august 14, 2024
Seismic waves are the earth's messengers, revealing its hidden secrets. Body waves travel through the planet's interior, while surface waves ripple along its skin. Each type has unique properties, helping scientists understand earthquakes and Earth's structure.
, , , and all play crucial roles in seismology. By studying their behavior, arrival times, and amplitudes, geophysicists can locate earthquakes, map the Earth's layers, and assess potential damage from seismic events.
Body Waves vs Surface Waves
Distinguishing Characteristics
Top images from around the web for Distinguishing Characteristics
9.1 Understanding Earth through Seismology | Physical Geology View original
Is this image relevant?
geophysics - How to distinguish P, S, Love, and Rayleigh waves in a seismogram? - Earth Science ... View original
Is this image relevant?
9.1 Understanding Earth Through Seismology – Physical Geology – 2nd Edition View original
Is this image relevant?
9.1 Understanding Earth through Seismology | Physical Geology View original
Is this image relevant?
geophysics - How to distinguish P, S, Love, and Rayleigh waves in a seismogram? - Earth Science ... View original
Is this image relevant?
1 of 3
Top images from around the web for Distinguishing Characteristics
9.1 Understanding Earth through Seismology | Physical Geology View original
Is this image relevant?
geophysics - How to distinguish P, S, Love, and Rayleigh waves in a seismogram? - Earth Science ... View original
Is this image relevant?
9.1 Understanding Earth Through Seismology – Physical Geology – 2nd Edition View original
Is this image relevant?
9.1 Understanding Earth through Seismology | Physical Geology View original
Is this image relevant?
geophysics - How to distinguish P, S, Love, and Rayleigh waves in a seismogram? - Earth Science ... View original
Is this image relevant?
1 of 3
- Body waves are seismic waves that travel through the interior of the Earth, while surface waves propagate along the Earth's surface
- P-waves (primary waves) and S-waves (secondary waves) are the two types of body waves
- Rayleigh waves and Love waves are the two types of surface waves
- Body waves generally travel faster than surface waves and arrive at seismic stations earlier
- Surface waves are typically more destructive than body waves due to their larger amplitudes and longer durations
Propagation and Destructive Potential
- Body waves propagate through the Earth's interior, providing information about the Earth's internal structure
- P-waves can travel through both solid and liquid materials, while S-waves can only travel through solids
- Body waves are less destructive than surface waves due to their smaller amplitudes and shorter durations
- Surface waves propagate along the Earth's surface and are more destructive than body waves
- Surface waves have larger amplitudes and longer durations compared to body waves
- Rayleigh and Love waves can cause significant damage to structures and infrastructure near the Earth's surface
Seismic Wave Characteristics
P-Waves (Compressional Waves)
- P-waves are the fastest seismic waves and can travel through both solid and liquid materials
- P-waves cause particles to oscillate parallel to the direction of wave propagation, resulting in compression and rarefaction of the material
- P-wave depends on the elastic properties and of the material through which they propagate
- Example: P-waves are the first waves to arrive at seismic stations due to their high velocity
S-Waves (Shear Waves)
- S-waves are slower than P-waves and can only travel through solid materials
- S-waves cause particles to oscillate perpendicular to the direction of wave propagation
- S-wave velocity is related to the shear modulus and density of the material
- Example: S-waves cannot travel through the Earth's outer core, indicating that it is liquid
Rayleigh Waves
- Rayleigh waves are surface waves that cause particles to move in an elliptical motion, with both vertical and horizontal components
- Rayleigh waves have a retrograde motion at the surface, which transitions to prograde motion at depth
- Rayleigh wave velocity is approximately 0.92 times the S-wave velocity in the material
- Example: Rayleigh waves can cause the ground to roll and undulate, similar to ocean waves
Love Waves
- Love waves are surface waves that cause particles to oscillate horizontally, perpendicular to the direction of wave propagation
- Love waves are observed when there is a low-velocity layer overlying a high-velocity layer
- Love wave velocity depends on the thickness and elastic properties of the layers through which they propagate
- Example: Love waves can cause significant lateral movement of the ground, leading to the collapse of structures
Seismic Waves and Earth's Structure
Seismic Wave Behavior at Boundaries
- Seismic waves change velocity and direction when they encounter boundaries between materials with different elastic properties, a phenomenon known as
- Refraction occurs when seismic waves pass through boundaries such as the Moho discontinuity (crust-mantle boundary) or the mantle-core boundary
- Refraction of seismic waves can cause shadow zones, where certain types of waves are not observed
- Example: The absence of S-waves in the Earth's outer core indicates that it is liquid, as S-waves cannot propagate through fluids
Seismic Anisotropy and Earth's Deformation
- Seismic anisotropy is the variation of seismic wave velocity with direction, providing insights into the Earth's internal structure and deformation processes
- Seismic anisotropy can be caused by the alignment of minerals, the presence of fractures or faults, or the flow of materials in the Earth's interior
- Studying seismic anisotropy helps understand the deformation and flow patterns in the Earth's mantle and crust
- Example: Seismic anisotropy in the Earth's upper mantle can be caused by the alignment of olivine crystals due to mantle flow
Interpreting Seismograms
Seismogram Components
- Seismograms are graphical representations of ground motion recorded by seismometers, displaying and arrival times of seismic waves
- The vertical axis of a seismogram represents the amplitude of ground motion, while the horizontal axis represents time
- Seismograms can have multiple components (vertical, north-south, and east-west) to capture the full motion of the ground
- Example: A seismogram from a station close to an epicenter will show larger amplitudes compared to a station farther away
Identifying Seismic Phases
- P-waves appear as the first arrivals on a seismogram due to their higher velocity compared to other seismic waves
- S-waves arrive after P-waves and are often characterized by larger amplitudes than P-waves
- Surface waves (Rayleigh and Love waves) have the largest amplitudes and longest durations on seismograms, arriving after body waves
- Rayleigh waves appear on the vertical and radial components of a seismogram, while Love waves appear on the transverse component
- Example: The time difference between the arrivals of P-waves and S-waves (S-P time) can be used to estimate the distance between the seismic station and the earthquake epicenter
Earthquake Location and Magnitude
- Seismogram interpretation involves identifying the characteristic waveforms and arrival times of different seismic phases to determine the earthquake location and magnitude
- The arrival times of P-waves and S-waves at multiple seismic stations can be used to triangulate the location of an earthquake epicenter
- The amplitude of seismic waves can be used to estimate the magnitude of an earthquake, which is a measure of the energy released during the event
- Example: The Richter scale is a logarithmic scale used to quantify the magnitude of an earthquake based on the amplitude of seismic waves recorded by seismometers
Key Terms to Review (23)
Amplitude: Amplitude refers to the maximum displacement of a wave from its rest position, indicating the strength or intensity of the wave. In geophysics, amplitude is crucial for interpreting seismic data, as it relates directly to the energy released during an earthquake and the characteristics of seismic waves. Understanding amplitude helps in assessing the potential impact of seismic events and integrating this information with geological and geochemical data.
Density: Density is a physical property defined as mass per unit volume, often expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). It plays a crucial role in understanding the composition and behavior of Earth's layers, influences gravitational fields, and is essential in the exploration of natural resources and the analysis of seismic waves.
Diffraction: Diffraction is the bending of waves around obstacles and the spreading of waves as they pass through openings. This phenomenon is particularly significant in geophysics as it affects the propagation of seismic waves, impacting how they travel through different materials and interact with geological structures. Understanding diffraction helps in interpreting seismic data and analyzing subsurface features, enhancing our ability to study the Earth's interior.
Earthquake: An earthquake is a sudden and intense shaking of the ground caused by the movement of tectonic plates beneath the Earth's surface. This movement can release energy that has accumulated over time, creating seismic waves that travel through the Earth. The relationship between earthquakes and tectonic activity highlights how stress builds along fault lines, leading to various magnitudes and impacts depending on the location and geological conditions.
Elasticity: Elasticity refers to the ability of a material to deform when subjected to stress and return to its original shape upon the removal of that stress. In the context of geophysics, elasticity is crucial as it governs how seismic waves travel through the Earth and is fundamental to understanding the behavior of geological materials under various conditions.
Frequency: Frequency refers to the number of occurrences of a repeating event per unit time, typically measured in Hertz (Hz), which indicates cycles per second. In the context of various geophysical methods, frequency plays a critical role in determining the resolution and penetration depth of signals used to analyze subsurface structures and properties.
Ground shaking: Ground shaking refers to the seismic vibrations of the Earth's surface caused by the release of energy during an earthquake. This phenomenon is a critical factor in determining the level of damage that can occur to structures and infrastructure, as it influences how forces are transmitted through the ground and into buildings. Understanding ground shaking helps in evaluating seismic hazards and implementing risk mitigation strategies to protect lives and property.
Longitudinal waves: Longitudinal waves are a type of mechanical wave in which the particle displacement is parallel to the direction of wave propagation. In the context of seismic waves, these waves travel through materials by compressing and expanding the medium, which allows them to transmit energy effectively. Understanding longitudinal waves is essential as they form the basis for primary seismic waves (P-waves), which are the fastest and first to be detected during an earthquake.
Love Waves: Love waves are a type of surface seismic wave that travel along the Earth's surface, characterized by a horizontal shear motion that causes the ground to move side to side. They are named after A.E.H. Love, who developed a mathematical model for these waves. Love waves are one of the most destructive types of seismic waves, often responsible for significant damage during earthquakes due to their horizontal motion, which can cause buildings and structures to sway and ultimately collapse.
P-waves: P-waves, or primary waves, are a type of seismic wave that compresses and expands material as they travel through the Earth. They are the fastest seismic waves, which allows them to be the first to arrive at seismic recording stations during an earthquake, making them crucial for understanding Earth's internal structure and the dynamics of seismic events.
Rayleigh waves: Rayleigh waves are a type of surface seismic wave that travel along the Earth's surface, causing both vertical and horizontal ground motion. They are named after Lord Rayleigh, who mathematically described their properties. These waves are significant in understanding seismic wave propagation, as they typically have longer wavelengths and can produce more damage during earthquakes compared to other types of seismic waves.
Reflection: Reflection refers to the process where seismic waves bounce back after encountering a boundary between different geological materials. This phenomenon is crucial for understanding how elastic waves propagate through the Earth and helps in interpreting seismic data, as reflected waves can provide insights into the Earth's structure and composition.
Refraction: Refraction is the bending of waves as they pass from one medium to another with different densities. This phenomenon is crucial in understanding how seismic waves travel through the Earth, as their speed changes when they encounter materials like rock or sediment, impacting the pathways of energy release during seismic events.
Refraction Seismology: Refraction seismology is a geophysical method that uses seismic waves to determine the structure of the subsurface by analyzing how these waves bend or refract as they encounter different geological layers. This technique is particularly useful in understanding the depth and type of materials beneath the surface, as variations in wave speed provide crucial information about the Earth's internal structure.
Rigidity: Rigidity is a property of materials that describes their ability to resist deformation when subjected to stress. This characteristic is crucial in understanding how different seismic waves travel through various geological materials, influencing their speed and behavior. The rigidity of a material affects the propagation of seismic waves, determining whether they can move through solid structures or are refracted or absorbed.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth and are characterized by their transverse motion, meaning they oscillate perpendicular to the direction of wave propagation. These waves play a crucial role in understanding the Earth's internal structure, as they cannot travel through liquids, providing insights into the composition of the Earth's layers and how seismic waves behave when encountering different materials.
Seismic Tomography: Seismic tomography is a technique used to visualize the Earth's internal structure by analyzing seismic wave data from earthquakes and artificial sources. This method allows scientists to create detailed images of the Earth's layers, revealing features such as subducting plates, magma chambers, and other geological structures, which are crucial for understanding tectonic processes, the distribution of heat, and the overall dynamics of our planet.
Seismograph: A seismograph is an instrument that detects and records the vibrations caused by seismic waves as they travel through the Earth. These instruments are essential for understanding earthquakes and geophysical processes, providing data that helps determine the location, magnitude, and characteristics of seismic events.
Seismometer: A seismometer is a sensitive instrument used to detect and record seismic waves generated by earthquakes, explosions, and other ground movements. These devices play a crucial role in understanding seismic activity by measuring the vibrations of the ground, which can then be analyzed to locate earthquakes and determine their magnitude, as well as to study the properties of seismic waves.
Surface rupture: Surface rupture refers to the visible displacement of the Earth's surface that occurs during an earthquake, typically along a fault line. This phenomenon is a direct result of the seismic waves generated by the sudden release of stress accumulated along geological faults, which can result in ground shaking and breaking of the surface, creating fractures and offsets.
Transverse waves: Transverse waves are a type of wave where the particle motion is perpendicular to the direction of wave propagation. In geophysics, these waves play a crucial role in understanding seismic activity, as they are one of the main types of seismic waves generated by earthquakes. Transverse waves, also known as shear waves, are essential in exploring the Earth's interior and provide important information about its structure and composition.
Tsunami: A tsunami is a series of ocean waves generated by large disturbances, typically associated with underwater earthquakes, volcanic eruptions, or landslides. These waves can travel across entire ocean basins and cause devastating impacts when they reach coastal areas, making understanding the processes behind their formation crucial in the context of seismic wave types and properties.
Velocity: Velocity is a measure of the speed and direction of an object in motion, often expressed as distance traveled per unit of time. In the context of seismic waves, velocity is crucial because it determines how quickly seismic energy travels through different geological materials, affecting the timing and intensity of ground shaking during an earthquake.