Question

From which of the following do primary and secondary waves move outward? A) epicenter C) Moho B) focus D) tsunami

Short answer

Primary and secondary waves move outward from the focus.

Step-by-step solution

Understanding Seismic Waves

In an earthquake, seismic waves are the energy that travels through the Earth. These waves are categorized into primary (P) waves and secondary (S) waves, which are both types of body waves.

Identifying the Origin Points

The origin point of these seismic waves is crucial to understanding their movement. Seismic waves do not start from an epicenter, Moho, or a tsunami as none of these are starting points for seismic waves originating from an earthquake.

Clarifying Terminologies

The focus is the point inside the Earth's crust where the energy release occurs. It is from the focus that the primary and secondary waves move outward. The epicenter is the point on the surface directly above the focus.

Selecting the Correct Answer

Now that we understand the focus is the origin for these waves, we can deduce that option B, focus, is the correct choice regarding where primary and secondary waves move outward from.

Key concepts

Seismic Waves

Seismic waves are crucial in understanding earthquakes, as they carry the energy released by the Earth's movement. Imagine throwing a stone into a pond—just like the ripples that spread out from the point where the stone hits the water, seismic waves spread out from the focus of an earthquake.
These waves travel through the Earth and can be felt on the surface, which is why we experience the shaking during an earthquake. There are two main types of seismic waves that travel through the Earth's interior—body waves and surface waves.

• Body waves include primary and secondary waves (P and S waves). They move through the Earth's interior and are essential for understanding the planet's layers.
• Surface waves, on the other hand, travel along the Earth's surface and are typically responsible for the shaking felt during larger earthquakes.

Both types of waves help seismologists locate an earthquake's focus and epicenter.

Primary Waves (P Waves)

Primary waves, also known as P waves, are the fastest type of seismic waves. They travel through the Earth's interior at high speeds and are the first waves to be detected by seismic instruments after an earthquake.
Understanding their characteristics is key to interpreting seismic data.
P waves move in a "push-pull" fashion, meaning they compress and expand the ground as they move through it. This is similar to the motion of sound waves undulating through air. Because of this motion, P waves can travel through all types of media—solids, liquids, and gases.

• Their speed and the ability to move through different materials make them crucial for early-warning systems, as they provide the first indication of an earthquake's occurrence.
• Despite their speed, P waves are generally less destructive than other types, as their ground movement is less pronounced.

By analyzing P waves, scientists can learn a lot about the inner properties of the Earth.

Secondary Waves (S Waves)

Secondary waves, referred to as S waves, are slower than primary waves but are often more destructive due to their stronger ground-shaking characteristics. S waves travel through the Earth's interior and arrive after the P waves.
Understanding their movement is important for evaluating earthquake impacts and risks.
S waves move in a side-to-side fashion. Unlike P waves, they can only travel through solids, not liquids or gases. This limitation helps seismologists gather information about the Earth's internal layers, especially the liquid outer core.

• Their ground motion is perpendicular to the direction of wave travel, which creates a noticeable shaking effect.
• This side-to-side motion makes S waves particularly damaging to structures, as the Earth moves horizontally beneath buildings and other infrastructure.

By studying S waves, scientists can better understand the Earth's composition and the behavior of different materials during seismic events.