Mechanical Waves and Energy

In this lesson, students use springs to model longitudinal and transverse mechanical wave phenomena in an investigation into how the amplitude and frequency of seismic wave phenomena relate to how much energy the waves carry.

• Waves are disturbances that travel through space and/or matter, transferring energy as they move.
• Mechanical waves are caused by vibrations in matter. As mechanical waves travel through a medium, they cause molecules within the matter to bump into each other. As this happens, energy transfers from one molecule to the next.
• There are two categories of seismic waves that transfer energy: body waves, which travel within Earth’s interior, and surface waves, which travel at or near Earth’s outer surface. There are two main types of body waves: primary waves (P-waves) and secondary waves (S-waves).
• All waves represent repeating patterns, which can be understood by thinking about the anatomy of a simple wave. All waves have certain properties in common. All waves have a wavelength, frequency, and amplitude.

• How do waves transfer energy from one place to another?
• When water waves transfer energy, the water moves up and down. Does this mean the water wave transferred matter?
• What has to happen for a seismic wave to occur?
• Where does the energy come from in seismic waves?
• How does an earthquake release energy?
• How are seismic waves similar to ripples in a pond if a pebble is dropped in the water?
• How are seismic P-waves different from seismic S-waves?
• How is the wavelength measured in longitudinal waves compared to transverse waves?
• What makes high-frequency water waves different from low-frequency water waves?

Misconception: Waves transfer matter as they pass through a medium.

Fact: Waves transfer energy, not matter, from one place to another.

Amplitude : a measure of a wave’s displacement from its resting position

Earthquake : the shaking of the ground caused by movements within Earth’s crust that result in a sudden release of energy

Frequency : the number of waves that pass a set point in a given amount of time

Seismic waves : the waves of energy caused by the sudden breaking of rock within Earth or an explosion

Transmit : to pass on

Wave : a disturbance that travels through space and/or matter, transferring energy as it moves

Wavelength : the distance spanned by one cycle of the motion of a wave

A Surprise in Earthquake Research

A group of scientists in Southern California spent six months in 2011 listening to vibrations deep within the planet. They focused on a region in Southern California, around the Newport-Inglewood Fault. This fault stretches nearly 80 kilometers (50 miles) from Culver City to Newport Beach.

The scientists made a surprising discovery. They learned that small earthquakes occur much deeper in Earth’s interior than they thought possible. Their findings will require additional research to help them understand how this is possible. “This is a wide-open question that we don’t have a good answer to,” study co-author Asaf Inbal told Science News in 2016.

Earth’s Structure

The scientists were surprised because of the properties of Earth’s four internal layers. The inner core is Earth’s hottest layer, and is made up of a mixture of solid iron and nickel. It is surrounded by an outer core that is made up of a less dense mixture of liquid nickel and iron.

Surrounding the inner and outer core is the mantle, which is mostly molten, semi-solid rock called magma. The mantle is in constant motion, powered by the transfer of energy between Earth’s warmer and cooler materials. Earth’s crust is the hard rock layer of the planet that supports the continents and oceans. The crust is Earth’s thinnest and coolest layer.

Tectonic Plates and Earthquakes

Earth’s surface is fragmented into drifting slabs of solid rock, called tectonic plates. As magma moves beneath the crust, it pushes the tectonic plates toward or away from each other. A fault is a break in the rock where rocks on either side have moved past one another. Faults often occur where Earth’s tectonic plates meet.

When two tectonic plates suddenly slip past one another, there is a sudden release of energy that causes the ground to shake, producing earthquakes. According to some estimates, 1.3 million earthquakes occur every year. Most earthquakes are so small that we do not even feel them. Occasionally, however, the energy released by an earthquake can dramatically shake the ground. This can cause buildings to collapse, mudslides to happen, and tsunamis to form.

Energy of an Earthquake

All earthquakes occur as energy transforms from potential to kinetic energy. As the plates move, they release kinetic energy. The edges around the tectonic plates are jagged, causing them to get stuck on other plates as they move. Because magma is constantly moving below them, the rest of the plate tries to keep moving. As a result, the energy that would normally cause the plates to slide past one another transforms into stored elastic energy. When the force of the moving plates finally causes the jagged areas of the plates to separate, huge amounts of kinetic energy are released.

The earthquakes recorded by Asaf and his team were more than 30 kilometers (18 miles) deep, within Earth’s upper mantle. This was surprising to the scientists because the rocks here are so hot they should flow like hard honey when under stress, rather than accumulate stored energy.

Waves Transfer Energy

Asaf is a seismologist. Seismology is the study of earthquakes and seismic waves that move through and around Earth. Seismic waves are the waves of energy caused by the sudden breaking of rock within Earth or an explosion. All waves are disturbances that travel through space and/or matter, transferring energy as they move.

Seismic waves are similar to sound waves and water waves because they are mechanical waves, which means they are caused by vibrations in matter. As mechanical waves travel through a medium, they cause molecules within the matter to bump into each other. As this happens, energy transfers from one molecule to the next. It is important to note that waves don’t carry matter. After the energy moves through, each molecule will return to its original position.

Seismic Waves from an Earthquake

The point within Earth where the rocks start to fracture or break in an earthquake is called the focus. This is the source of the initial disturbance. The energy released travels out in all directions through seismic waves, much like ripples traveling outward when you drop a pebble in a pond. The epicenter is the location on Earth’s surface where the earthquake starts.

In this lesson students use springs to model longitudinal and transverse mechanical wave phenomena to model how the amplitude and frequency of seismic wave phenomena relate to how much energy the waves carry. Students collect and analyze investigational data on the difference between a high-amplitude wave and a low-amplitude wave, as well as between a high-frequency wave and a low-frequency wave.