Seismograph

A seismograph is an instrument used to measure seismic waves. Seismic waves are waves of energy that passes through Earth’s crust, or top layer. Seismic waves occur for many different reasons—volcanoes, bombs, explosions, wind, vehicles, and people—but are most often measured during earthquakes. Recording seismic waves during an earthquake can help scientists understand the earthquake’s strength and origin. Scientists also hope they can use this information for hazard assessment and early warning and to possibly predict future earthquakes.

The first rudimentary seismograph was created in the late 1800s. This simple and early seismograph used paper and a pen to record Earth’s movement. It had a base on which the equipment rested. A roll of paper sat on the base. A pen was above the paper. Its point rested on a drum. A weight was usually on top of the pen. The pen recorded a line on the paper that showed the energy measured by the seismograph. This paper was referred to as a seismogram.

Background

Seismographs measure seismic waves during earthquakes. Earthquakes often occur because two tectonic plates, which are huge chunks of Earth’s crust, do not slip past each other because their rough edges stick together. Over time, stress builds as the plates continue trying to move. When the stress becomes greater than the friction holding the rocks in place, the rocks suddenly break or slip along a fault. This sudden movement releases a large amount of energy that travels outward from the source of the earthquake in the form of seismic waves.

Different types of seismic waves exist. They are classified by how they move through the ground. The two main types are body waves and surface waves. Body waves can travel through Earth’s crust and inner layers. The body waves include P waves (compressional or primary) and S waves (transverse or secondary). P waves are the fastest type of seismic wave and compress and expand the ground in the direction in which they travel. P waves can travel through solid rock and liquids such as water. S waves are slower than P waves and move the ground perpendicular to the direction in which S waves travels. S waves travel through rock but cannot travel through liquids. Surface waves travel only through Earth’s crust. They include Love and Rayleigh waves and act somewhat like ripples on the surface of water.

Overview

Seismographs work based on the physics principle of inertia, which states that an object at rest tends to stay at rest and an object in motion tends to stay in motion. When earthquakes occur, seismic waves travel through Earth’s crust and inner layer. Their energy moves the seismograph. This movement is what records the seismic waves.

The words seismograph and seismometer are often used interchangeably but have different meanings. A seismograph is the entire instrument used to record the seismic waves. A seismometer is the weight, mass, or pendulum inside a seismograph that is on top of the pen. The base of the seismograph is the part that moves. The weighted seismometer tends not to move.

As the base moves, the pen stays stationary. At the same time, the drum turns the paper. As the paper turns, the pen records a line on it. When no seismic waves are present, the pen records a straight line. However, during a time when seismic waves are present, the pen draws a zigzag or wavy line as the seismograph moves. Sometimes the pen records small waves that are not earthquakes. These waves may be created by large vehicles, people, wind, and more. The most sensitive seismographs can measure movements of roughly 1/10,000,000 centimeters. During earthquakes, the pen records large wavy lines. The more the seismograph moves, the larger the printed zigzags and waves will be. The waves in the line will be larger and closer together, with more powerful seismic waves.

Horizontal seismographs have the recording pen and weight suspended above the horizontal paper roll. A horizontal seismograph measures side to side ground motion. Vertical seismographs have the recording pen and weight suspended by a spring and resting on the side of a vertical paper roll. This type of seismograph measures up and down ground motion. Today, nearly all seismographs are digital and do not use paper. In addition to traditional seismographs, scientists can use fiber-optic cables to detect seismic waves. These cables can act as long sensors and provide measurements over large areas, including locations where conventional seismic instruments are difficult to deploy.

The seismograph produces a seismogram, the imprint created by the seismograph. Scientists study seismograms to learn about earthquakes. On a seismogram, the horizontal axis shows the time that elapsed, and the vertical axis shows the ground displacement (or how much the ground moved). Scientists measure the waves on the x- and y-axes on the seismogram. After an earthquake, scientists look at seismograms from many different locations. By comparing the data from numerous seismograms, scientists can determine the origin of the earthquake and its strength.

Scientists commonly use the moment magnitude scale to describe earthquake size, while the Richter magnitude scale, often just called the Richter scale, to measure the amount of energy released by small, and nearby earthquakes. The Richter scale is often measured by numerals 1 through 10. Each one-unit increase in magnitude represents about ten times greater wave amplitude and about thirty-two times more energy release. The largest earthquake that was ever recorded measured a 9.5 on the Richter scale. Earthquakes of this power are very rare and extremely destructive.

Some seismographs are buried in underground seismic stations, which often use solar power. The equipment is inside a vault to protect it. Since these seismographs are underground, they can pick up body waves that might be harder to measure on Earth’s surface. So scientists use networks of seismographs and compare data from many different instruments to provide the most accurate description of an earthquake.

Bibliography

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