3.2 Characteristics and propagation of S-waves
3 min read•august 9, 2024
are the second type of body waves to arrive during earthquakes. They move slower than and cause shearing deformation in the medium they pass through, with perpendicular to wave propagation.
These waves can't travel through liquids, making them crucial for identifying Earth's structure. S-waves carry more energy than P-waves, causing larger ground motions and potentially more damage to structures during seismic events.
Types and Characteristics
Secondary Waves and Shear Motion
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- S-waves, also known as secondary waves, arrive after P-waves in seismic recordings
- Classified as shear waves due to their motion causing shearing deformation in the medium they pass through
- Exhibit characteristics with particle motion perpendicular to the direction of wave propagation
- Particle motion in S-waves creates oscillations similar to a vibrating string
- S-waves can be polarized into two components: SH (horizontally polarized) and SV (vertically polarized)
- SH waves move particles in a horizontal plane perpendicular to the direction of wave propagation
- SV waves move particles in a vertical plane perpendicular to the direction of wave propagation
- Polarization of S-waves provides valuable information about the source mechanism and path effects in seismology
Particle Motion and Wave Behavior
- S-waves cause particles to move back and forth perpendicular to the wave's travel direction
- Particle displacement in S-waves forms a sinusoidal pattern when viewed from the side
- Amplitude of S-waves represents the maximum displacement of particles from their equilibrium position
- Wavelength of S-waves measures the distance between two consecutive peaks or troughs in the wave
- Frequency of S-waves determines the number of oscillations per unit time (typically measured in Hertz)
- S-waves carry more energy than P-waves, resulting in larger ground motions and potential for structural damage
- Energy of S-waves decreases with distance from the source due to geometric spreading and attenuation
Propagation
Wave Velocity and Medium Properties
- S-wave velocity (Vs) depends on the elastic properties and density of the medium they travel through
- Vs can be calculated using the formula:
- μ represents the shear modulus of the medium
- ρ represents the density of the medium
- S-waves travel slower than P-waves, with typical velocities ranging from 3-4 km/s in the Earth's crust
- Velocity of S-waves increases with depth in the Earth due to increasing pressure and changes in rock properties
- Vs/Vp ratio (typically around 0.5-0.6) provides information about the composition and physical state of rocks
- S-wave velocities vary in different geological materials (granite: ~3.0-3.3 km/s, sedimentary rocks: ~1.5-2.5 km/s)
Propagation Characteristics and Limitations
- S-waves propagate exclusively through solid materials, unable to travel through liquids or gases
- Inability to propagate through fluids results in the absence of S-waves in the Earth's outer core
- S-wave shadow zone exists on the opposite side of the Earth from an due to the liquid outer core
- Propagation of S-waves affected by changes in material properties, leading to and at boundaries
- S-waves experience mode conversion at interfaces, potentially transforming into P-waves or surface waves
- Anisotropy in Earth materials can cause S-wave splitting, where a single S-wave splits into two waves with different velocities
- S-waves play a crucial role in determining the internal structure of the Earth, particularly in identifying the solid inner core
Key Terms to Review (18)
Asthenosphere: The asthenosphere is a semi-fluid layer of the Earth's mantle, located beneath the lithosphere, which plays a crucial role in tectonic plate movement. It is characterized by its ability to flow and deform under pressure, enabling the lithospheric plates to glide over it. This property connects it to the generation of seismic waves, the reflection and refraction of body waves, and the overall dynamics of the Earth's internal structure.
Charles Francis Richter: Charles Francis Richter was an American seismologist best known for developing the Richter scale, a logarithmic scale used to measure the magnitude of earthquakes. His work fundamentally changed how we quantify seismic events, providing a standardized way to compare their size and impact, influencing the understanding of earthquake characteristics, seismic instrumentation, stress and strain in earthquake regions, and the structure of the Earth's mantle and core.
Earthquake source: An earthquake source refers to the specific location and mechanism where an earthquake originates, typically involving the sudden release of energy in the Earth's crust. This energy release occurs due to accumulated stress along geological faults, resulting in seismic waves that propagate through the Earth. Understanding the earthquake source is crucial for analyzing the characteristics of seismic waves, particularly S-waves, as they provide insight into the behavior and impact of earthquakes.
Elastic wave theory: Elastic wave theory describes how waves propagate through elastic materials, like rocks and soil, by creating stress and strain in response to external forces. This theory is essential for understanding how seismic waves, including Love waves and S-waves, travel through the Earth during an earthquake, helping scientists analyze and interpret seismic data effectively.
Fault line: A fault line is a fracture or zone of fractures between two blocks of rock, which can lead to seismic activity such as earthquakes. These lines are critical as they mark the boundaries where tectonic plates interact, causing stress accumulation and release, which is manifested in the form of seismic waves. Understanding fault lines is essential for interpreting wave characteristics, revealing Earth’s internal structure, and assessing potential hazards associated with seismic events.
Mantle: The mantle is a thick layer of rock located between the Earth's crust and the outer core, making up about 84% of Earth's total volume. It plays a critical role in seismic wave propagation and the dynamics of plate tectonics, influencing everything from travel time calculations to the generation of seismic waves.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves that travel through the Earth, moving in a compressional manner. They can propagate through both solid and liquid materials, making them essential for understanding the Earth's internal structure and behavior during seismic events.
Particle motion: Particle motion refers to the movement of particles within a medium, particularly in the context of seismic waves. In seismology, understanding how particles move helps in analyzing the propagation and characteristics of different types of seismic waves, including S-waves, where the motion is perpendicular to the direction of wave travel.
Reflection: In seismology, reflection refers to the bouncing back of seismic waves when they encounter a boundary between different types of geological materials. This process is crucial for understanding the internal structure of the Earth, as it helps identify different layers and their properties by analyzing how seismic waves behave at these boundaries.
Refraction: Refraction is the bending of seismic waves as they pass through different layers of the Earth's interior, caused by variations in wave speed due to changes in material properties. This phenomenon is crucial for understanding how seismic waves travel and interact with different geological structures, which aids in identifying seismic phases, analyzing travel time curves, and interpreting seismograms.
Richard Dixon Oldham: Richard Dixon Oldham was a British geologist and seismologist known for his pioneering work in seismology, particularly for discovering S-waves and their significance in understanding the Earth's interior. His research laid the groundwork for modern seismic studies, connecting the behavior of S-waves to the composition and structure of the Earth’s layers.
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 transverse motion, which means they move the ground perpendicular to the direction of wave propagation, and are only able to travel through solid materials, making them crucial for understanding Earth's internal structure.
Seismograph: A seismograph is an instrument that measures and records the vibrations of the ground caused by seismic waves, such as those generated by earthquakes. It captures the intensity, duration, and frequency of these vibrations, which are crucial for understanding seismic events and the Earth's internal structure.
Seismometer: A seismometer is an instrument that detects and records the motion of the ground caused by seismic waves from earthquakes or other vibrations. It plays a crucial role in understanding seismic activity by capturing the details of seismic waves, enabling scientists to analyze their characteristics and origins.
Shear wave: A shear wave, also known as an S-wave, is a type of elastic wave that moves through a medium by displacing particles perpendicular to the direction of wave propagation. This characteristic allows S-waves to transmit energy through solid materials, making them critical in the study of seismic activity and earthquake dynamics.
Transverse wave: A transverse wave is a type of wave where the particle displacement is perpendicular to the direction of wave propagation. This characteristic means that as the wave travels, particles move up and down or side to side while the wave itself moves forward. Transverse waves are essential in understanding how seismic waves travel through the Earth, particularly in how they are recorded in seismograms and their unique properties during propagation.
Wave propagation theory: Wave propagation theory is the study of how waves travel through different media, explaining the behavior and characteristics of waves as they move. This theory is crucial for understanding how seismic waves, including surface waves and S-waves, interact with geological structures, influencing both their speed and amplitude. The insights gained from this theory help to analyze phenomena such as dispersion characteristics in surface waves and the distinct propagation patterns of S-waves in various environments.
Wave speed: Wave speed refers to the speed at which a wave travels through a medium, which is influenced by the medium's properties and the type of wave. Understanding wave speed is crucial for analyzing how seismic waves, such as surface waves and S-waves, propagate through the Earth and for interpreting the data used in studies of Earth's structure.