Seismic Structure of Earth's Layers and Key Boundaries

Seismic Portrait of Earth's Layers

Earth's layers are detectable as changes in seismic wave velocity with depth.

The boundary between the upper and lower mantle is visible at 660 km as a sudden change from rapidly increasing P- and S-wave velocities to slow or no change in P-wave and S-wave velocities.

The core-mantle boundary is apparent as a sudden drop in P-wave velocities, where seismic waves move from solid mantle to liquid outer core.

The boundary between the outer core and inner core is marked by a sudden increase in P-wave velocity after 5000 km, where seismic waves move from a liquid back into a solid again.

Crust–Mantle Boundary (Moho discontinuity)

Depth: Its depth is ~10 km under the ocean floor and ~35 km under the continents.

Discovery: Sometimes seismic waves arrived at seismic stations farther from an earthquake before they arrived at closer ones. They get faster deeper within Earth because higher pressures make layers more rigid.

Core–Mantle Boundary (Gutenberg discontinuity)

Depth: Almost 2900 km below the surface.

Discovery: When an earthquake occurs, there are zones on the opposite side of Earth where S-waves and P-waves are not measured.

The S-wave shadow zone occurs because S-waves cannot travel through the liquid outer core.

The P-wave shadow zone occurs because seismic velocities are much lower in the liquid outer core than in the overlying mantle.

Major Layers

• Crust: made up of less dense rocks, rich in silicon and aluminium, like granite (common in continental crust) and basalt (in oceanic crust).
• Mantle: made up of denser rocks rich in iron and magnesium, like peridotite. It is divided into upper and lower mantle.
• Core: iron and nickel. It is divided into:

Tectonic and Rheological Layers

• Lithosphere: rigid surface of the Earth. It is not a continuous layer, but is broken into pieces called lithospheric/tectonic plates.
• Asthenosphere (sublithospheric upper mantle): plastic layer made of semi-molten rocks on which the tectonic plates move.
• Mesosphere: lower mantle.
• D'' layer: accumulates heat from the outer core and releases it through thermal plumes that move tectonic plates and create hotspots.