L9 - Imaging the Earth's Interior

Seismology and Earth's Interior

We've discussed polarizing waves and seismic sources. Today, we'll explore how we use these waves to investigate the Earth's interior and the features we image.

Importance of Studying Earth's Interior

Earth exhibits unique surface features compared to other planetary bodies like Mars and the moon due to dynamic processes such as volcanism, plate tectonics, earthquakes, and its magnetic field. These processes are crucial for life on Earth; a habitable planet requires a magnetic field and likely plate tectonics. Seismology is the most important method for studying these processes and the Earth's interior.

Other methods to study Earth's interior:

  • Topography.

  • Isostasy.

  • Magnetism.

  • Gravity.

Seismic Imaging of Earth

Our goal is to understand the Earth's internal structure including its layers.

Number of Layers in Earth

At a basic level, the Earth has four layers: crust, mantle (upper and lower), outer core, and inner core. However, with detailed analysis, we can identify second-order layers as well. Key discontinuities include: 410 km, 520 km, and 660 km depth discontinuities, along with upper and lower crustal divisions.

Determining Earth's Layers

Seismology is the primary tool for determining the structure and composition of these layers.

The analogy provided is that by using globes with different material (glass, metal, wood) inside, one can infer the internal composition without opening merely by assessing weight and the sound they make. Weight relates to gravity and density, while the sounds are analogous to seismic waves.

Two Stages of Seismic Imaging

  1. One-Dimensional (1D) Structure: Focuses on how the Earth's properties vary with depth (radius).

    • The relationship between data and structure is nonlinear, making it more complex.

    • Involves combining data to analyze velocity variations with depth, resulting in a reference Earth model.

  2. Three-Dimensional (3D) Variations: Examines lateral variations in Earth's properties.

    • 3D variations are relatively small, allowing for a linear relationship assumption, simplifying the problem.

    • Uses the 1D model as a reference and analyzes deviations from it.

This discussion will cover the one-dimensional structure and its application to Earth, Mars and the moon, and then transition to three dimensional structures.

Evolution of Understanding

  • 1906: Basic understanding of the core.

  • 1970s-80s: Identification of different layers.

  • Early 1980s: Investigation of three-dimensional structures.

  • Present: Detailed three-dimensional models with hot and cold structures.

One-Dimensional (1D) Earth Structure

Seismic waves interact with boundaries inside the Earth, and we analyze their arrival times at different stations.

Arrival Time vs. Distance

Mamorok Puget analyzed an earthquake in Gratia in 1909 and plotted arrival time versus distance. Direct waves arrive at zero distance and time, with increasing time with distance. Later, waves arriving faster than direct waves were observed at greater distances aligning on what appeared to be a straight line, indicating head waves or refracted waves. The slope of the line is 1/v, where v is the velocity of the layer.

Head Waves/Refracted Waves

These waves travel down to an interface, travel along it, and then return to the surface (Huygens' principle). The interface identified was the crust-mantle boundary or the Moho.

The slope of the line represents the velocity at the top of the mantle.

Crustal Thickness

Recent studies using data from the US array (seismic stations across the US) have imaged crustal thickness. There are strong variations across the US, from 25 km to 45-50 km. The Western US is tectonically active, while the Eastern US is more stable. Mountainous regions like the Rocky Mountains have thicker crust due to isostatic root formation.

Global models show constant crustal thickness in oceans, average thickness of ~30 km in continents, and up to 70 km in mountainous regions.

Velocity Discontinuities and Triplications

Data from 10 to 40 degrees central distance is used. To enhance differences, arrival time minus distance times a reference velocity is plotted. Triplications occur when there is a sudden increase in velocity with depth.

Waves traveling deeper are sped up and reflect upwards earlier. This leads to multiple arrivals at short distances.

The Earth exhibits triplications due to increases in velocity at 410 km, 520 km, and 660 km depths which indicate mantle transition zone. These depths correspond to mineralogical changes:

  • 410 km: Olivine to Wadsleyite.

  • 520 km: Ringwoodite.

  • 660 km: Perovskite (Bridgmanite).

These transitions cause increases in seismic velocity, leading to triplications.

Evidence on Mars

Data from the InSight mission on Mars also shows triplications, suggesting a mantle transition zone around 800-1000 km depth with similar transitions and mineralogy.

Core Detection and Shadow Zones

Direct P waves disappear from 98 degrees to 145 degrees distance due to refraction. Waves slow down and refract towards the normal when entering the liquid outer core, creating a shadow zone.

Direct S waves disappear from 104 degrees onwards. This is because the waves are unable to pass through the core, making the furthest travelling S waves the ones which reflect off the core

P wave and S wave shadow zones differ due to the fact that S waves are unable to travel through the core and that S and P waves travel at different velocities.

Discovery of the Core

Richard Oldham in 1906 identified the core by observing the shadow zone.

Martian Core

In 2021, the InSight mission detected the Martian core by observing a shadow zone between Tharsis and the landing site. The core radius was estimated to be 1830 km, larger than expected.

Inner Core Discovery

Inge Lehmann discovered the inner core in 1936 by observing small peaks in the shadow zone (105-137 degrees). She phrased her discovery carefully. These arrivals indicated energy where there shouldn't have any existed, she then carefully posited based on observable seismic data the existence of a inner core, which led to a triplication within the outer core's shadow zone. This careful postulation ended up being the discovery of the inner core.

One-Dimensional Earth Models

Birch in 1952 divided the Earth into layers (A-G) with depths, mass, and seismic velocities.

The Preliminary Reference Earth Model (PREM) from 1981 is still widely used. It describes density and seismic velocities, serving as a reference for analyzing three-dimensional variations.

Number of Layers Revisited

Main discontinuities include the Moho, mantle transitions, core-mantle boundary, and inner core boundary. Other potential layers: thin outer core layer, layer at the base of the outer core, D'' layer at the base of the mantle, and opportunities in the upper mantle.

Lunar and Martian 1D Structure

Apollo missions provided data for lunar 1D models. The moon has a large mantle, a small solid inner core, and a potentially fluid outer core and partially molten layer. These models were based on four seismic stations from the 1970s, primarily on the near side.

Since 2021, we have Martian 1D structure data from InSight. Mars has a thick lithosphere (~500 km). Whether Mars has a sharp core or a mushy basal magma layer is still being investigated. Partial molten layers influence magnetic fields due to high conductivity.

Three-Dimensional (3D) Imaging

Anomalies affect seismic waves, causing them to arrive later. The differences in arrival times between actual arrivals and the 1D model are measured. Faster or slower arrivals indicate velocity anomalies. Faster anomaly = wave arrives earlier. The turning point of the wave is most sensitive. To localize these anomalies, seismic stations in various locations are required.

Seismic Tomography Analogy

Similar to CT scans or X-rays, seismic tomography uses sources and detectors to image the Earth's interior. However, earthquakes are confined to plate boundaries and seismic stations are mainly on land.

Images/models are created showing seismic velocities in blue (fast) and red (slow).

Implications of Velocity Variations

Fast velocities: lower temperature, increased rigidity (shear modulus, bulk modulus), and increased density.

Slow velocities: higher temperature and compositional variations (presence of fluids).

Imaging Structures in the Earth

S-wave velocity variations are shown as percentage differences from the reference model.

  • 50 km Depth: Slow velocities in oceans (especially along ridges) and fast velocities in continents (old continental lithosphere).

    • Measuring tectonic features in the shallow mantle

  • 200 km Depth: Similar ocean-continent division, with regions of thick lithosphere (e.g., Africa) showing high velocities and areas around Afar showing slow velocities.

    • Imagine continental lithosphere

  • 600 km Depth: Regions of slow velocities under South Asia and South America, indicating deep subduction.

    • Imaging deep subduction

  • 1500 km Depth: Weaker variations, suggesting a well-mixed mid-mantle dominated by convection.

    • Imaging a well mixed mantle with little anomaly

  • Core-Mantle Boundary (2800 km Depth): Two large regions of slow velocities. These structures can be thousands of kilometers wide and a few hundred kilometers high.

Interpretation of Features

Continuous features of fast velocities from the surface to the mantle are interpreted as subducting slabs (cold downwellings). Regions of slow velocities are interpreted as hot mantle plumes (upwellings).

Analyzing the hot and cold spots helps understand material and heat transfer within the Earth. These structures are compared to geodynamic models of the mantle convection to better explain dynamic processes.
Slowness is associated with heat so scientists know that materials bringing heat upwards from the surface contribute to the cooling of the Earth. Colder slabs coming down into the Earth cause the upper lakes to traverse upwards to replace them.

Common Focus Areas

  • Subducting slabs: Analyzed via cross-sections. Complications include flattening and breaking off. Areas of higher anomaly because rigid slab in a hot plastic mantle

  • Hotspots: Imaged beneath oceanic islands (Hawaii, Samoa, Tahiti). Areas of low anomaly because of hot plume in less hot mantle

  • Large Low-Shear-Velocity Provinces (LLSVPs): Huge structures at the base of the mantle under Africa and the Pacific Ocean.

These analyses contribute to understanding dynamic processes within the Earth, such as mantle convection and heat transfer. Comparing seismic observations with geodynamic models helps scientists understand the Earth's dynamic processes.