12.3 3D velocity structure of the Earth

Seismic tomography unveils Earth's 3D velocity structure, revealing hidden features within our planet. By mapping velocity anomalies, we can see subduction zones, mantle plumes, and the asthenosphere, giving us a window into Earth's dynamic interior.

This technique helps us understand how Earth's layers interact and move. From the rigid lithosphere to the partially molten asthenosphere, seismic tomography shows us the complex dance of plate tectonics and mantle convection shaping our world.

Mantle Structures

Velocity Anomalies and Subduction Zones

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• Velocity anomalies represent deviations from average seismic wave speeds in Earth's mantle
• Positive velocity anomalies indicate faster-than-average seismic wave propagation (colder, denser regions)
• Negative velocity anomalies signify slower-than-average seismic wave propagation (hotter, less dense regions)
• Subduction zones exhibit distinctive velocity patterns in seismic tomography images
  • Cold, dense subducting slabs appear as positive velocity anomalies
  • Surrounding mantle wedge often shows negative velocity anomalies due to partial melting and hydration
• Velocity contrasts in subduction zones help geologists map plate boundaries and study mantle dynamics
• Subduction zones play a crucial role in plate tectonics and mantle convection processes

Mantle Plumes and Asthenosphere

• Mantle plumes manifest as columnar upwellings of hot, buoyant material from deep within the Earth
• Seismic tomography reveals mantle plumes as negative velocity anomalies extending from the core-mantle boundary
• Plumes contribute to intraplate volcanism and the formation of hotspot chains (Hawaii)
• The asthenosphere represents a mechanically weak layer beneath the lithosphere
• Characterized by lower seismic velocities compared to the overlying lithosphere and underlying mantle
• Asthenosphere facilitates plate tectonic movements and isostatic adjustments
• Typically located between depths of 100-400 km, varying with tectonic setting
• Plays a crucial role in mantle convection and heat transfer within the Earth

Earth's Layers

Lithosphere Characteristics and Composition

• Lithosphere comprises the Earth's rigid outer layer, including the crust and uppermost mantle
• Thickness varies from ~5-70 km for oceanic lithosphere to ~100-250 km for continental lithosphere
• Exhibits higher seismic velocities compared to the underlying asthenosphere
• Composed of various rock types:
  • Oceanic lithosphere: basaltic crust overlying ultramafic mantle rocks
  • Continental lithosphere: diverse crustal rocks (granites, metamorphic rocks) overlying mantle rocks
• Lithosphere breaks into tectonic plates, driving global geological processes
• Undergoes deformation through brittle failure and ductile flow, depending on depth and temperature

Asthenosphere Properties and Significance

• Asthenosphere represents a partially molten, mechanically weak layer beneath the lithosphere
• Characterized by lower seismic velocities and higher attenuation compared to surrounding mantle
• Typically located between depths of 100-400 km, with variations depending on tectonic setting
• Plays a crucial role in mantle convection and plate tectonic processes
• Facilitates isostatic adjustments and provides a "lubricating" layer for plate movements
• Composition includes upper mantle rocks with small amounts of partial melt
• Temperature and pressure conditions in the asthenosphere promote ductile deformation
• Acts as a source region for mid-ocean ridge basalts and some intraplate volcanism

Seismic Tomography

Global Tomography Models and Applications

• Global tomography models provide 3D images of Earth's interior velocity structure
• Utilize data from worldwide seismic networks to create comprehensive velocity maps
• Resolution varies with depth and geographical location, depending on data coverage
• Global models reveal large-scale mantle structures:
  • Subduction zones (Pacific Ring of Fire)
  • Mantle plumes (Hawaii, Iceland)
  • Large low shear velocity provinces (LLSVPs) at the core-mantle boundary
• Applications of global tomography include:
  • Studying mantle convection patterns and heat transfer mechanisms
  • Investigating the thermal and compositional state of the deep Earth
  • Constraining geodynamic models of plate tectonics and mantle evolution

Regional Tomography and Velocity Anomalies

• Regional tomography focuses on smaller-scale structures with higher resolution
• Employs dense local seismic networks to image specific tectonic features or regions
• Provides detailed information on crustal and upper mantle structure
• Regional studies reveal:
  • Subduction zone geometries and slab morphologies
  • Magma chamber locations beneath volcanoes
  • Crustal thickness variations and sedimentary basin structures
• Velocity anomalies in regional tomography help identify:
  • Zones of partial melting or fluid accumulation (negative anomalies)
  • Ancient cratonic roots or subducted slabs (positive anomalies)
• Regional tomography contributes to:
  • Earthquake hazard assessment and monitoring
  • Mineral and hydrocarbon resource exploration
  • Understanding local tectonic processes and crustal evolution