Question

At \(\sim 2,900 \mathrm{~km}\), the \(\mathrm{S}\) wave velocity falls to \(0 .\) Why? a. S waves can't travel through solids, and this depth is where the solid inner core exists. b. S waves can't travel through liquids, and this depth is where the liquid outer core exists. c. S waves can't travel through solids, and this depth is where the solid mantle exists. d. S waves entered the shadow zone.

Short answer

S wave velocity falls to zero because they can't travel through the liquid outer core at 2900 km depth.

Step-by-step solution

Understanding S Waves

S waves, or shear waves, are a type of seismic wave that move through the Earth by shearing or cutting the medium sideways. Importantly, S waves can only travel through solid materials since they require the medium to have shear strength.

Analyzing Earth's Layers

The Earth consists of several layers: the crust, mantle, outer core, and inner core. The outer core, which begins at a depth of approximately 2,900 km, is liquid, while the inner core is solid. This composition affects S wave propagation.

Impact of the Outer Core

Since S waves cannot travel through liquids, their velocity drastically falls and essentially stops at the boundary where the solid mantle meets the liquid outer core. This is because the outer core lacks the shear strength needed for the transmission of S waves.

Choosing the Correct Option

Reviewing the options, option b accurately reflects the reason behind the S wave velocity falling to zero at this depth: 'S waves can't travel through liquids, and this depth is where the liquid outer core exists.'

Key concepts

Seismic Waves

Seismic waves are waves of energy that travel through the Earth and are typically generated by earthquakes or artificial explosions. They play a crucial role in our understanding of the Earth’s internal structure. There are mainly two types of seismic waves:

• Body Waves: These move through the interior of the Earth and are further divided into P waves (primary) and S waves (secondary).
• Surface Waves: These travel along the Earth's surface and tend to cause more destruction during an earthquake.

S waves, or shear waves, are particularly interesting because of their inability to move through liquids. Unlike P waves, which can travel through all states of matter, S waves only move through solids. This property gives scientists valuable information about the earth’s inner layers as these waves react in specific ways when they encounter different materials beneath the Earth's crust.

Earth's Layers

Earth is composed of several concentric layers, each with distinct characteristics and compositions. Understanding these layers helps us comprehend how seismic waves interact with our planet.

• Crust: The outermost layer where we live. It is solid and made up of a variety of rocks.
• Mantle: Located beneath the crust, this thick layer is mostly solid but behaves plastically over long timescales. It comprises silicate minerals rich in magnesium and iron.
• Outer Core: This layer is liquid, made mainly of molten iron and nickel, starting at a depth of about 2,900 km.
• Inner Core: Comprising primarily iron and nickel, this core is solid and is found at the Earth's center.

The interface between these layers significantly affects the movement of seismic waves, providing insights into each layer's composition and state.

Outer Core

The outer core of the Earth is a fascinating layer because it is the only fully liquid layer within the planet's structure. Starting at approximately 2,900 kilometers below the Earth's surface, the outer core consists largely of molten iron and nickel.
The liquefied nature of the outer core plays a critical role in the behavior of S waves. When S waves, which require a solid medium to travel through, reach the outer core, they are unable to propagate further. This inability is because liquids cannot support the shearing motions characteristic of S waves. Besides affecting S wave movement, the dynamics of the outer core are also believed to contribute to Earth's magnetic field formation.

S Wave Propagation

S wave propagation is a fascinating study in wave mechanics, as these seismic waves offer vital clues about the internal structure of the Earth. S waves propagate through a medium by causing particles to move perpendicular to the wave's direction, a motion that requires shear strength, exclusively found in solid materials.

• Once S waves encounter liquids, their transmission halts because liquids, unlike solids, cannot sustain the shearing movement necessary for S wave travel.
• This characteristic of S waves means they disappear entirely upon reaching the outer core, a liquid layer beneath the mantle.
• The inability of S waves to pass through the outer core creates a 'shadow zone' on the Earth's surface, where no direct S waves are detected. This shadow zone and the halting of S wave propagation provide compelling evidence of a liquid outer core.

By studying these behaviors, scientists can confirm the layered structure of Earth and deduce the composition and state of these layers.