Question

What kind of earthquake waves stretch and compress rocks? A) surface C) secondary B) primary D) shear

Short answer

Primary (P) waves.

Step-by-step solution

Identify the Question

The question is asking for the type of earthquake waves that stretch and compress rocks. We need to identify which of the given options fits this description.

Understand Wave Types

Earthquake waves can be classified into body waves and surface waves. Body waves travel through the Earth's interior and include Primary (P) waves, which compress and stretch materials, and Secondary (S) waves, which move material perpendicular to the wave direction. Surface waves move along the Earth's surface.

Analyze Each Option

Let's evaluate each option: - (A) Surface waves primarily travel along the Earth's surface and tend to cause the most damage, but they do not compress and stretch rocks. - (B) Primary (P) waves are compressional waves, meaning they cause material to compress and stretch in the direction of wave travel. - (C) Secondary (S) waves cause shear, moving materials perpendicular to the wave direction, rather than compress and stretch. - (D) Shear refers to the motion caused by S-waves, not compression and stretching.

Determine the Correct Answer

Based on the analysis, Primary (P) waves are the type of waves that stretch and compress rocks, making option B the correct answer. They are the fastest traveling seismic waves and can move through both solid and liquid layers of the Earth.

Key concepts

Primary waves

Primary waves, also known as P-waves, are a type of body wave that travels through the interior of the Earth. They are the fastest seismic waves, which means they arrive first at a seismic station after an earthquake occurs. This is where they get their name "Primary" waves.

These waves have a unique motion that involves compressing and stretching the materials they pass through, much like how a slinky compresses and expands when pushed or pulled. This type of motion is called compressional or longitudinal:

• They compress and expand the rocks in the direction of travel.
• They can travel through both solid and liquid layers due to this motion.

P-waves cause minimal damage compared to other seismic waves because of their rapid movement, which often results in a shaking sensation.

Secondary waves

Secondary waves, often referred to as S-waves, are another type of body wave. They arrive after P-waves at a seismic station, hence the name "Secondary." Their journey through the Earth is slower than that of Primary waves.

The motion of S-waves is quite different from P-waves. Instead of compressing and stretching, S-waves move the ground perpendicular to the direction of wave travel. This perpendicular movement is known as shearing motion:

• They move materials up and down or side to side.
• Only travel through solid materials, because liquids can’t support shear stress.

S-waves tend to cause more damage than P-waves, as this shearing motion can be quite violent.

Surface waves

Surface waves differ from body waves because they travel along the Earth's surface rather than through its interior. They are typically the last to be detected, but they often result in the most damage during an earthquake.

There are two main types of surface waves, each characterized by its own pattern of motion:

• Rayleigh waves: Move in an elliptical motion, much like ocean waves, causing both vertical and horizontal ground movement.
• Love waves: Cause horizontal shearing of the ground and travel faster than Rayleigh waves, but slower than body waves.

Because of their destructive nature, surface waves play a major role in the damage inflicted during an earthquake. Their slow speed compared to body waves allows them to build up more energy, amplifying their impact.

Body waves

Body waves encompass both Primary and Secondary waves. They are called body waves because they move through the Earth's "body," or its internal layers, reaching seismic stations before surface waves.

Key characteristics shared by body waves include:

• Traveling through the Earth's inner layers.
• Providing vital information about the Earth's interior structure.
• Being used to locate the epicenter of earthquakes.

While body waves usually cause less damage than surface waves due to their deep travel path, they are crucial for scientists to understand the Earth's inner composition.