Even though the technology does not exist to travel into all of Earth’s layers, scientists can still learn a great deal about Earth’s structure through seismic waves. Seismic waves are vibrations in the earth that transmit energy and occur during seismic activity such as earthquakes, volcanic eruptions, and even man-made explosions. There are two types of seismic waves, primary waves and secondary waves. Primary waves, also known as P waves or pressure waves, are longitudinal compression waves similar to the motion of a slinky (SF Fig. 7.1 A). Secondary waves, or S waves, are slower than P waves. The motion of secondary waves is perpendicular to the direction of the wave travel, similar to the motion of vigorously shaking a rope (SF Fig. 7.1 B).

SF Fig. 7.1 C shows primary or P waves (on top) and secondary or S waves (on bottom) in motion.

Scientists use seismometers (Fig. 7.2) to measure seismic waves. Seismometers measure the vibrations of the ground, relative to a stationary instrument. Data from a seismometer, also called a seismogram, shows velocity on the y axis and time on the x axis (Fig. 7.3). Note in SF Fig. 7.3 that the P wave occurs first, because they travel at a higher velocity.

SF Table 7.1 shows that P waves have a higher velocity than S waves when traveling through several mineral types. The speed at which seismic waves travel depends on the properties of the material that they are passing through. For example, the denser a material is, the faster a seismic wave travels (SF Table 7.1). P waves can travel through liquid and solids and gases, while S waves only travel through solids. Scientists use this information to help them determine the structure of Earth. For example, if an earthquake occurs on one side of Earth, seismometers around the globe can measure the resulting S and P waves.

SF Fig. 7.4 shows wave propagation through Earth. Note that P waves pass through all layers of the earth, while S waves cannot pass through the solid core of the earth, resulting in an S wave shadow on the opposite side of the earthquake.