3.4 Body wave interactions with Earth's internal structure
4 min read•august 9, 2024
Body waves reveal Earth's hidden layers as they travel through its interior. These seismic waves interact with different structures, providing crucial information about our planet's and dynamics.
Understanding how and behave inside Earth is key to unraveling its mysteries. From core-mantle boundaries to low- zones, these interactions paint a picture of our planet's inner workings.
Seismic Discontinuities
Core-Mantle Boundary and Mohorovičić Discontinuity
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- Core-mantle boundary (CMB) marks the transition between Earth's liquid outer core and solid mantle
- Located approximately 2900 km below Earth's surface
- Characterized by a significant change in seismic wave velocities
- Plays crucial role in Earth's magnetic field generation and heat transfer
- Mohorovičić discontinuity (Moho) separates Earth's crust from the underlying mantle
- Depth varies from 5-10 km beneath oceans to 30-50 km beneath continents
- Identified by abrupt increase in seismic wave velocities
- Represents change in rock composition and physical properties
- Both discontinuities cause and of seismic waves
- Reflected waves provide information about discontinuity depths and properties
- Refracted waves follow complex paths through Earth's interior
Seismic Discontinuities and Low Velocity Zones
- Seismic discontinuities represent abrupt changes in seismic wave velocities within Earth
- Can be caused by changes in composition, phase, or physical properties of materials
- Major discontinuities include 410 km and 660 km boundaries in the mantle transition zone
- Low velocity zones (LVZs) exhibit decreased seismic wave speeds compared to surrounding areas
- Asthenosphere (upper mantle) contains prominent LVZ
- Caused by partial melting, changes in mineral structure, or presence of volatiles
- Affect seismic wave propagation and can lead to wave attenuation
- Discontinuities and LVZs provide insights into Earth's internal structure and dynamics
- Help constrain models of Earth's composition and thermal state
- Used to study mantle convection and plate tectonic processes
Seismic Wave Behavior
Shadow Zones and Wave Paths
- represent areas where specific seismic waves cannot be detected
- P-wave shadow zone extends from 103° to 142° angular distance from earthquake
- S-wave shadow zone covers entire area beyond 103° from epicenter
- Caused by refraction and reflection of waves at the core-mantle boundary
- Wave paths through Earth's interior depend on velocity structure and discontinuities
- Direct waves travel along great circle paths near Earth's surface
- Refracted waves bend at velocity discontinuities ()
- Reflected waves bounce off major discontinuities (CMB, Moho)
- Complex wave paths result in multiple arrivals at seismic stations
- Primary waves (P, S) arrive first, followed by reflected and refracted phases
- Travel-time curves used to analyze wave arrivals and determine Earth structure
Triplication and Wave Interactions
- Triplication occurs when seismic waves encounter sharp velocity increases with depth
- Results in three different wave paths arriving at same location
- Produces complex seismic records with multiple arrivals for same wave type
- Commonly observed for waves interacting with mantle transition zone
- Wave interactions at discontinuities generate converted phases
- P-to-S and S-to-P conversions occur at major boundaries (CMB, Moho)
- Converted phases provide additional information about discontinuity properties
- Scattered waves result from interactions with small-scale heterogeneities
- Contribute to seismic coda (tail of seismogram)
- Used to study fine-scale structure of Earth's interior
Imaging Earth's Interior
Seismic Tomography Techniques and Applications
- uses seismic wave travel times to create 3D images of Earth's interior
- Analogous to medical CT scans, but using seismic waves instead of X-rays
- Requires large datasets of seismic wave arrival times from many earthquakes and stations
- Involves complex mathematical inversion techniques to reconstruct velocity structure
- Different types of seismic tomography target various aspects of Earth structure
- Body wave tomography uses P and S wave travel times
- Surface wave tomography utilizes dispersion characteristics of surface waves
- Full waveform inversion incorporates entire seismic waveforms for higher resolution
- Tomographic images reveal Earth's internal structure at various scales
- Global tomography maps large-scale mantle convection patterns and subducting slabs
- Regional tomography provides detailed images of crust and upper mantle structure
- Local tomography used for high-resolution imaging of specific geological targets
Interpreting Tomographic Results
- Tomographic images typically display velocity anomalies relative to reference Earth model
- Red colors often indicate slower velocities (potentially hotter or partially molten regions)
- Blue colors suggest faster velocities (potentially colder or denser material)
- Interpretation of tomographic results requires consideration of multiple factors
- Temperature effects on seismic velocities
- Compositional variations and their impact on wave speeds
- Presence of fluids or partial melt
- Anisotropy and preferred mineral orientations
- Tomography results integrated with other geophysical and geological data
- Provides comprehensive understanding of Earth's dynamics and evolution
- Used to study mantle plumes, subduction zones, and continental structure
- Contributes to models of plate tectonics and mantle convection
Key Terms to Review (19)
Amplitude: Amplitude refers to the maximum displacement of a wave from its rest position, essentially measuring how strong or intense the wave is. In seismology, it’s crucial because it helps indicate the energy released during an earthquake and can influence the interpretation of seismic data. Amplitude is not only important for understanding the strength of seismic waves but also plays a role in distinguishing between different types of waves and their behavior as they propagate through various geological structures.
Composition: Composition refers to the different materials and elements that make up the Earth’s interior layers, such as the crust, mantle, and core. Understanding the composition is essential for deciphering how body waves interact with these layers, as different materials can affect wave speed and behavior. By examining the composition of the Earth’s internal structure, we gain insights into its physical properties, dynamics, and the geological processes that shape our planet.
Density: Density is defined as the mass per unit volume of a material, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). This property is crucial for understanding how seismic waves travel through different materials within the Earth, as density affects wave speed and behavior.
Earthquake focal point: The earthquake focal point, or focus, is the precise location within the Earth where an earthquake originates. It represents the initial point of failure on a fault line, where stress has accumulated and is released in the form of seismic energy, causing waves that propagate through the Earth's interior and along its surface. Understanding the focal point is crucial for studying how seismic waves interact with the Earth's internal structure, influencing both the propagation characteristics of body waves and their detection by seismic instruments.
Elasticity: Elasticity refers to the ability of materials, including rocks and minerals in the Earth, to deform and return to their original shape when stress is applied. This property is crucial for understanding how seismic waves travel through different layers of the Earth and interact with its internal structure, as well as how waves reflect and refract at boundaries between different materials.
Epicenter: The epicenter is the point on the Earth's surface directly above the focus of an earthquake, where seismic waves first reach the surface. Understanding the epicenter is crucial for identifying seismic phases, analyzing seismograms, and studying how body waves interact with Earth’s internal structure.
Frequency: Frequency refers to the number of oscillations or cycles that occur in a given time period, typically measured in Hertz (Hz). In seismology, frequency is critical for understanding the characteristics of seismic waves and how they interact with different geological structures, influencing everything from wave behavior to the interpretation of seismic data.
Mode conversion: Mode conversion refers to the process by which seismic waves change from one type to another as they encounter different materials within the Earth's interior. This phenomenon is crucial in understanding how seismic waves propagate through the Earth, as it can significantly alter wave speed and behavior, which is important for interpreting seismic data and analyzing Earth's structure.
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.
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.
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.
Seismic Tomography: Seismic tomography is an imaging technique used to visualize the Earth's internal structure by analyzing seismic waves generated by earthquakes or artificial sources. This method allows scientists to create detailed three-dimensional models of the Earth's subsurface, revealing variations in material properties, such as density and seismic wave speed, which are essential for understanding geological processes and tectonic activities.
Seismograms: Seismograms are graphical records produced by seismographs that display the vibrations of the Earth caused by seismic waves. These recordings are crucial for understanding how body waves travel through different layers of the Earth’s internal structure, revealing details about their properties and behavior during seismic events.
Shadow zones: Shadow zones are regions on the Earth's surface where seismic waves from an earthquake are not detected. This phenomenon occurs due to the way body waves, such as P-waves and S-waves, interact with the Earth's internal structure, particularly when they encounter different materials and layers within the Earth. The existence of shadow zones provides crucial information about the composition and behavior of the Earth's interior, as these areas reveal insights into the properties of materials that seismic waves cannot traverse or penetrate.
Snell's Law: Snell's Law describes how seismic waves change direction when they pass through different layers of material with varying properties, specifically concerning their velocities. This fundamental principle is crucial for understanding how waves refract and reflect as they encounter boundaries within the Earth's subsurface, influencing methods of data interpretation in seismology.
Velocity: In seismology, velocity refers to the speed at which seismic waves travel through different materials in the Earth. This concept is crucial for understanding how waves propagate and interact with geological structures, influencing methods used to interpret subsurface conditions and locate seismic events.
Wave equation: The wave equation is a fundamental mathematical representation that describes how waves propagate through different media. It captures the relationship between wave speed, wavelength, frequency, and the characteristics of the medium, making it crucial for understanding various types of seismic waves as they travel through the Earth's layers.
Wavefronts: Wavefronts are surfaces that represent the points of a wave that are in the same phase of vibration, essentially showing the leading edge of a wave as it propagates through a medium. In seismology, understanding wavefronts is crucial for visualizing how seismic waves travel through different materials within the Earth, and how they interact with various layers of the planet's internal structure.