seismograph
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What is a seismograph?
A seismograph is an instrument used to detect and record seismic waves produced by earthquakes, explosions, and other ground-shaking phenomena. Equipped with electromagnetic sensors, seismographs convert ground motions into electrical changes, which are then processed and recorded either through analog or digital circuits. While the terms seismograph and seismometer are often used interchangeably, a seismograph has the capacity to record seismic activity, with the recording shown on a display screen or paper printout called a seismogram. Though originally designed to locate earthquakes, seismographs have expanded utility in areas such as oil exploration, studying Earth's structure, and monitoring volcanoes.
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What does a seismograph measure?
A seismograph is an instrument used to detect and record seismic waves produced by earthquakes, explosions, and other ground-shaking phenomena. Equipped with electromagnetic sensors, seismographs convert ground motions into electrical changes, which are then processed and recorded as seismograms.
Seismographs measure ground motion using different types such as short-period, long-period, and broadband instruments. These instruments record seismic waves and measure ground motion in both earthquakes and microseisms. By analyzing the data captured by seismographs, scientists can determine the location, magnitude, and faulting characteristics of earthquakes.
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How does a seismograph detect earthquakes?
A seismograph detects earthquakes by recording ground oscillations caused by seismic waves. These waves travel through the Earth or along its surface from the earthquake's origin. The seismograph uses electromagnetic sensors to translate ground motions into electrical changes, which are then processed and recorded.
The instrument works on the principle of inertia, where a pendulum tends to stay in place as the ground moves. The relative motion between the pendulum and the ground is recorded, and this movement is often amplified electrically. Modern seismographs use three pendulums to record movement in three directions: two horizontal and one vertical. The data is displayed on a screen or paper as a seismogram.
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What are the main parts of a seismograph?
A seismograph detects and records seismic waves from earthquakes or explosions. The main components of a modern seismograph include:
- Pendulum: A pendulum that remains relatively still due to inertia when the ground moves.
- Electromagnetic sensor: Sensors translate ground motions into electrical changes.
- Damper: A damper minimizes free oscillation of the pendulum, often using electromagnetic methods.
- Recording system: Modern seismographs use electronic circuits to amplify and digitize ground motion for precise readings, with digital recording directly onto magnetic tape or memory chips.
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Why is it important to record seismic waves?
Seismic waves are vibrations generated by earthquakes, explosions, or other energetic sources that travel within the Earth or along its surface. Recording seismic waves is important because seismographs, instruments equipped with electromagnetic sensors, translate ground motions into electrical changes, which are processed and recorded. The data obtained from seismographs provide valuable insights into Earth's subsurface structure, assist in oil and gas prospecting, and contribute to engineering studies. Analysis of seismic waves is crucial for understanding Earth's internal constitution and detecting various geological features.
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How do seismographs help scientists learn about Earth's interior?
Seismographs measure ground oscillations from seismic waves caused by earthquakes, explosions, or other sources. Different waves (P, S, and surface waves) travel through or along the Earth. Analyzing the arrival times and patterns of these waves on seismograms helps scientists determine the distance to the earthquake's origin.
By observing how seismic waves refract and reflect within the Earth, scientists can infer the internal structure, including the boundaries between the crust, mantle, and core. The absence of S waves in the outer core, for example, indicates that it is liquid. This data is crucial for understanding Earth's deep interior.
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seismograph, instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomenon. Seismographs are equipped with electromagnetic sensors that translate ground motions into electrical changes, which are processed and recorded by the instruments’ analog or digital circuits. The terms seismograph and seismometer are often used interchangeably; however, whereas both devices may detect and measure seismic waves, only a seismograph possesses the capacity to record the phenomena. A record produced by a seismograph on a display screen or paper printout is called a seismogram.
Although originally designed to locate natural earthquakes, seismographs have many other uses, such as petroleum exploration, investigation of Earth’s crust and lower layers, and monitoring of volcanic activity.
Development of the first seismographs
An early seismic instrument called the seismoscope made no time record of ground oscillations but simply indicated that shaking had occurred. A Chinese scholar, Zhang Heng, invented such an instrument as early as 132 ce. It was cylindrical in shape with eight dragon heads arranged around its upper circumference, each with a ball in its mouth. Around the lower circumference were eight frogs, each directly under a dragon head. When an earthquake occurred, balls were released from a dragon’s mouth, probably by an internal pendulum that moved back and forth according to the direction of vibration, and were caught by a frog’s mouth, which produced noise.
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In 1855 Italian scientist Luigi Palmieri designed a seismograph that consisted of several U-shaped tubes filled with mercury and oriented toward the different points of the compass. When the ground shook, the motion of the mercury made an electrical contact that stopped a clock and simultaneously started a recording drum on which the motion of a float on the surface of mercury was registered. This device thus indicated time of occurrence and the relative intensity and duration of the ground motion.
The basic problem in measuring ground motions is to attain a steady point that remains fixed when the ground moves. Various types of pendulums have been used for that purpose. The simplest type is a common pendulum in which a heavy mass is suspended by a wire or rod from a fixed point (as in a clock). Other forms are the inverted pendulum, in which a heavy mass is fixed to the upper end of a vertical rod pointed at its lower end, and the horizontal pendulum, in which a rod with a mass on its end is suspended at two points so as to swing in a nearly horizontal plane instead of a vertical plane. After a series of earthquakes struck near Perthshire, Scotland, in 1839, a seismometer with an inverted pendulum was installed near Comrie in 1840.
The first true seismograph, according to Italian seismologists, was created in 1875 by Italian physicist Filippo Cecchi. The Cecchi seismograph also used pendulums, but it was the first to record the relative motion of the pendulums with respect to Earth’s ground motions as a function of time. The motions produced by seismic waves would activate a clock, and the recording surface (which tracked ground motion) advanced 1 cm (0.04 inch) per second, which would allow a reader to establish the timing of an earthquake’s onset as well as its duration.
Seismograph developments occurred rapidly in 1880 when Scottish physicist Sir James Alfred Ewing, Scottish engineer Thomas Gray, and English geologist John Milne, who were working in Japan at the time, began to study earthquakes. Following a severe earthquake that occurred at Yokohama near Tokyo in that year, they organized the Seismological Society of Japan. Under its auspices various devices, forerunners of today’s seismograph, were invented. Among the instruments constructed in that period was Milne’s famous horizontal pendulum seismograph. Milne successfully used that seismograph to record several earthquakes in Japan. Then, after returning to England, he established a small worldwide seismographic network using such instruments.
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The horizontal pendulum seismograph was improved greatly after World War II. The Press-Ewing seismograph, developed in the United States for recording long-period waves, was widely used throughout the world. That device employed a Milne-type pendulum, but the pivot supporting the pendulum was replaced by an elastic wire to avoid friction.