Seismic Waves & Earth's Interior

You can feel how earthquake waves move using a simple Slinky. When an earthquake happens, it sends out waves of energy that travel through the Earth. Two important kinds are P waves and S waves. A P wave is a push wave, also called a compression wave. If you hold one end of a Slinky and shove it forward, the coils squeeze together and then spread apart, and that squeeze travels down the spring. That is how a P wave moves rock: it pushes and pulls the ground in the same direction the wave is going. An S wave is a shake wave. If you move the end of the Slinky side to side, a wiggle travels along it while the coils move across the direction of the wave. S waves shake the ground up and down or side to side. Both waves carry energy through solid rock, and because they move differently, they travel at different speeds. Scientists use those differences to learn what is deep inside the Earth.

Nobody has ever dug to the center of the Earth, so how do we know what is down there? The answer is earthquakes. When a big earthquake strikes, it sends seismic waves traveling through the whole planet. Seismometers, which are instruments that record shaking, pick up those waves all over the world. In this lab activity, students look at real data from an actual earthquake and compare it with a model of the Earth. Waves slow down, speed up, or bend when they pass through different materials, so the pattern of where waves show up, and where they do not, tells us what the inside is like. By matching the model to the recorded data, students can even work out how wide Earth's core is. It is a bit like a doctor using an ultrasound: you cannot see inside directly, but the waves that come back paint the picture.

Earth helps us understand rocky planets. To investigate its interior, scientists track waves produced by earthquakes. The materials these seismic waves cross affect their speed and direction. Some cannot travel through liquids. Records of arrivals and missing waves reveal a metallic core surrounded by a deep layer of rock, the mantle. Above this lies a thin crust. Ocean crust consists mainly of dark basalt; continental crust includes lighter granite. Movement of liquid metal within the outer core helps create Earth's magnetic field. This field turns away many charged particles arriving from space.

You can turn your whole class into an earthquake wave. Have everyone line up shoulder to shoulder. You and your classmates are now the material the wave travels through, just like rock deep inside the Earth. To make a P wave, the person at one end gives a gentle push down the line. Each person bumps the next, and the squeeze travels along the line in the same direction as the push. That is exactly how a compression wave moves through the ground. To make an S wave, the person at the end steps to the side and back, and each person copies the sideways motion one after another. Now the wiggle travels down the line while each person only moves side to side. That is how a shear wave shakes rock.

This interactive activity lets you play detective with the inside of the Earth. The planet is shown as four nested layers, like the skin of an onion: crust, mantle, outer core, and inner core. P waves travel through solid and liquid layers. S waves cannot cross the liquid outer core. Wave speeds and paths change at layer boundaries. Your job is to change how fast each kind of wave travels in each layer until the waves in the model arrive at detectors at the same times as real earthquake waves did. When your model matches the real data, you have done what seismologists do: you have figured out the hidden structure of a planet without ever seeing inside it. It shows why the liquid outer core creates a shadow zone where direct S waves do not arrive, because shear waves cannot travel through liquid.