The study of earthquake waves, seismology, dates back to attempts made in China almost 2000 years ago to determine the direction from which these waves originated. The earliest known instrument, invented by mathematician and scientist Zhang Heng, was a large hollow jar containing a weight suspended from the top ▼. The suspended weight (similar to a clock pendulum) was connected to the jaws of several large dragon figurines that encircled the container. The jaws of each dragon held a metal ball. When earthquake waves reached the instrument, the relative motion between the suspended mass and the jar would dislodge some of the metal balls into the waiting mouths of figurine frogs directly below.

In principle, seismographs, devices used to record seismic waves, are similar to the instruments used in ancient China. Early versions of seismographs had a seismometer, a sensing device composed of a weight attached to a spring, freely suspended from a support securely attached to bedrock ▼. When vibrations from an earthquake reached the a seismograph, the inertia of the seismometer kept it relatively stationary, while Earth and the support moved. Inertia can be simply described by this statement: Objects at rest tend to stay at rest, and objects in motion tend to remain in motion, unless acted upon by an outside force. You have experienced inertia when you have tried to stop your automobile quickly and your body continued to move forward. This movement of the seismometer as a result of the seismic waves was recorded on rotating drums of paper that continuously recorded seismic activity.

Modern seismographs ▲ are systems of sensors (seismometers), recorders, and transmitters. Here, the seismometer is composed of magnets that act as the suspended weight and coils of wire that record movement in the form of electric signals. These sensitive instruments measure motion in three perpendicular motions—north-south, east-west, and vertically (up and down). They are also designed to amplify ground motion in order to detect very weak earthquakes or a great earthquake that has occurred in another part of the world. Instruments used in earthquake-prone areas are designed to withstand the violent shaking that can occur near a quake’s epicenter.

Seismic Waves

The records of ground shaking obtained from seismographs, called seismograms, provide useful information about the nature of seismic waves. Seismograms reveal that two main types of seismic waves are generated by the slippage of a rock mass. One of these wave types, called body waves, travel through Earth’s interior. The other type, called surface waves, travel in the rock layers just below Earth’s surface ▼.

Body Waves

Body waves are further divided into two types—primary waves, or P waves, and secondary waves, or S waves—and are identified by their mode of travel through intervening materials. P waves are “push/pull” waves; they momentarily push (compress) and pull (stretch) rocks in the direction the waves are traveling ▼. This wave motion is similar to that generated by striking a drum, which moves air back and forth to create sound. Solids, liquids, and gases resist stresses that change their volume when compressed and, therefore, elastically spring back once the stress is removed. Therefore,  waves can travel through all these materials.

By contrast, S waves “shake” the particles at right angles to their direction of travel. This can be illustrated by fastening one end of a rope and shaking the other end, as shown in the figure above ▲. Unlike  waves, which temporarily change the volume of intervening material by alternately squeezing and stretching it, S waves change the shape of the material that transmits them. Because fluids (gases and liquids) do not resist stresses that cause changes in shape—meaning fluids do not return to their original shape once the stress is removed—liquids and gases do not transmit  waves.

Surface Waves

There are two types of surface waves. One type, the Rayleigh wave, named after physicist Lord Rayleigh, causes Earth’s surface and anything resting on it to move up and down, much like waves in the ocean ▼.

The Love wave, named after mathematician A. E. H Love, produces side-to-side motion at Earth’s surface ▼.

Comparing the Speed and Size of Seismic Waves

By examining the seismogram shown in the figure below ▼ you can see that another major difference among the types of seismic waves is their speed of travel. P waves are the first to arrive at a recording station, then S waves, and finally surface waves. Generally, in any solid Earth material, P waves travel about 70 percent faster than S waves, and S waves are roughly 10 percent faster than surface waves.

In addition to the velocity differences in the waves, notice in the figure above ▲ that their seismograph recordings differ in height, or amplitude, which reflects the amplitude of shaking they cause. S waves have slightly greater amplitudes than P waves, and surface waves exhibit even greater amplitudes. Surface waves also retain their maximum amplitude longer than P and S waves. As a result, surface waves tend to cause greater ground shaking and, hence, greater property damage, than either P or S waves.

• Seismology is the study of seismic waves. A seismograph measures these waves, using the principle of inertia. While the body of the instrument moves with the waves, the inertia of a suspended weight keeps a sensor stationary to record the displacement between the two waves.

• A seismogram, a record of seismic waves, reveals two main categories of earthquake waves: body waves (P waves and S waves), which are capable of moving through Earth’s interior, and surface waves, which travel only along the upper layers of the crust. P waves are the fastest, S waves are intermediate in speed, and surface waves are the slowest. However, surface waves tend to have the greatest amplitude, S waves are intermediate, and P waves have the lowest amplitude. Large-amplitude waves produce the most shaking, so surface waves usually account for most damage during earthquakes.

• P waves and S waves exhibit different kinds of motion. P waves momentarily push (compress) and pull (stretch) rocks as the waves travel through a rock body, thereby changing the volume of the rock. S waves impart a shaking motion as they pass through rock, changing the rock’s shape but not its volume. Because fluids do not resist forces that change their shape, S waves cannot travel through fluids, whereas P waves can.

• body waves: Seismic waves that travel through Earth’s interior.

• inertia: A property of matter that resists a change in its motion.

• P waves (primary waves): A type of seismic wave that involves alternating compression and expansion of the material through which it passes. Also called primary waves.

• S waves (secondary waves): Seismic waves that involve oscillation perpendicular to the direction of propagation. Also called secondary waves.

• seismographs: Devices used to record and measure seismic waves. Often referred to synonymously with the term seismometer.

• seismogram: The graphical record made by a seismograph showing ground motion as a function of time.

• seismology: The scientific study of earthquakes and seismic waves.

• seismometer: A sensor within a seismograph that records the ground movements associated with seismic waves. Often referred to synonymously with seismograph.

• surface waves: Seismic waves that travel along the outer layer of Earth.