Seismometers

A seismograph is a device that detects, measures, and records the ground motion at a point. The sensor that detects and, in part, measures the motion is called a seismometer. The recorded ground motion, a seismogram, is the output of a seismograph.

History and Development

According to the historical records, the first seismometer was invented by Chinese astronomer and mathematician Zhang Heng (78-139 c.e.), during the Han Dynasty. The records indicate that it was a bronze vessel containing a suspended pendulum. The pendulum was connected to an eight-spoked wheel, and each spoke terminated in the mouth of one of eight externally mounted dragon heads with movable jaws. Each mouth contained a bronze ball, and eight open-mouthed frogs were located around the base. During an earthquake, the ground motion would displace the pendulum laterally, ejecting one of the balls into a frog's mouth. The ejected balls would give some idea of the direction of the traveling waves and the source of the earthquake.

The first truly precise seismometer was developed by an English mining engineer, John Milne, in the late 1800's. This instrument was improved by J. J. Shaw in the early 1900's, when the Milne-Shaw seismograph was introduced. In the United States, the first seismographs were installed in 1887 at the University of California, Berkeley. The 1906 San Francisco earthquake was the first large event in the United States to be recorded.

In the 1960's, largely as a result of the development of the atomic bomb and concern over the Cold War, the improvement and development of seismographs took a large leap forward. There was a national security need to be able to discriminate between an underground nuclear explosion and natural events. Such effort led to the construction and the deployment of 120 seismic stations in sixty countries, called the World Wide Standardized Seismograph Network (WWSSN). This period also marks a turning point in bringing the science of observational seismology to the forefront of the physical sciences. At the same time, countries such as the Soviet Union, France, and Canada modernized their earthquake observation systems.

Mechanics

Currently, many types of seismographs are available. Most of them incorporate similar physical principles, utilizing some sort of spring or pendulum to detect and measure the ground motion. The exception is a strain seismograph, invented by Hugo Benioff, which uses a long horizontal bar (20-30 meters long) to measure the ground deformation between two points.

A simple seismometer can be considered as a pendulum attached to a rigid frame, which is anchored to a horizontal ground surface. When an earthquake occurs, seismic waves are radiated from the earthquake source in all directions through the earth. For a sufficiently strong earthquake, the seismometer site experiences ground vibration. Assuming that there is no slippage between the ground and the rigid frame of the seismometer, the frame experiences the same motion as does the ground. If the pendulum could stay motionless, which is the ideal case, the relative motion of the frame with respect to the pendulum would be the true ground motion. Yet, that is not the case; the pendulum undergoes motion, and because of its resistance to motion (inertia), it tends to lag the motion of the frame (ground). This lag results in a complex differential motion between the pendulum and the ground.

Pendulums and springs have the characteristic that, if they are set in motion, they oscillate with period independent of the amplitude of motion as long as the amplitude does not become extreme. This characteristic period or frequency (frequency is the inverse of period) is called the free-period or the natural frequency of a pendulum or spring, respectively. A seismometer pendulum or spring can be designed to have a high- or a low-period sensitivity. The relative frequency content of the seismic waves with respect to the natural frequency of the pendulum or spring-mass determines in part the nature of the recorded seismograms. The deflection of a pendulum that experiences ground motion with frequencies much higher than its natural frequency is proportional to the ground displacement. The deflection, however, is proportional to the ground acceleration if the frequency content of the ground motion is much lower than the natural frequency of the pendulum, and the seismograph is called an accelograph. Finally, if the pendulum has a natural frequency close to the frequency content of the ground motion, the deflection is proportional to the ground velocity. Nevertheless, displacement, velocity, and acceleration are mathematically related. For example, ground velocity and displacement could both be determined from an accelogram. Early seismometers amplified small motions with mechanical linkages or optical levers with a mirror to reflect light onto recording paper. Modern instruments use electronic amplification.

The relative motion of the pendulum with respect to the ground must be magnified, allowing very small wave amplitudes to be distinguished. The most sensitive seismometers, at a quiet observatory, can detect ground displacement of a thousandth of a millionth of a meter, which is on the order of atomic size. The record of such ground motion might show an amplitude of 1 centimeter, a magnification of 10 million. The ratio of the largest to the smallest amplitudes which are undistorted is called the dynamic range of the seismograph. Conventional visible seismographs (those that produce records directly on paper or film) have a dynamic range of approximately one hundred to three hundred.

Complications and Limitations

Unfortunately, there is an unwanted complication in detecting useful seismic signals. The ground is always in motion. This motion, microseismicity, is caused by natural sources such as wind, storms, and human activities. Seismographs should be designed so that the recorded seismograms are least affected by the unwanted microseismic noise—to get an optimum signal-to-noise ratio. Most microseismic noise has a period in the range of 5-8 seconds. These noises are effectively avoided in the commonly used short-period (1 hertz) seismographs. The moon is the ideal place for readings from advanced seismographs because of the absence of wind, ocean waves, and human-made noise. The lunar seismographs left behind by the Apollo mission astronauts can detect the seismic waves generated by a 1-kilogram meteor striking anywhere on the moon's surface.

Borehole seismographs have been developed which can operate within deep boreholes. The advantage of a downhole recording is that the noise level at depth is much lower than at the surface, and, with certain geometrical arrangements, the recording of seismic waves that travel desired paths can be made. Seismologists have employed such data-acquisition techniques in conjunction with mathematical models similar to those used in medical tomography to construct a three-dimensional picture of the surveyed volume of the earth.

Using mechanical vibration as the only means of measuring and recording motion, although relatively simple, has a few drawbacks. Such instruments have limited magnification and sensitivity, and the recording is mechanical, with inherent friction. In older seismographs, very large pendulum masses were used to reduce the friction—for example, 1,000 kilograms in many Wiechert seismographs. Some Wiechert seismographs built with a pendulum mass of roughly 20 tons were installed in the beginning of the century in several places in central Europe. The older seismographs all use mechanical methods for transferring the motion of the pendulum to the recording pen. The recording pen can have several variations; the most common are a stylus on a rotating smoked paper drum and a reflected light beam onto a photographic paper. The highest measurable frequency of mechanical recording depends upon the speed of the recording medium relative to the stylus pen or light spot. The typical recording speed ranges from 0.1 millimeter per second for frequencies at roughly 0.05 hertz, up to 10 millimeters per second for frequencies as high as 20-50 hertz. The dynamic range of these systems is approximately one hundred to three hundred.

Introduction of Electromagnetics

In 1906, Russian physicist Boris Golitsyn elegantly utilized the principles of electromagnetics to translate mechanical motion into electrical voltage. It is known that if a conducting loop (a coil) crosses lines of magnetic flux, there will be an induced electromotive force that will generate an electric current proportional to the velocity at which the coil crosses the magnetic field. This idea has had a significant effect on the later development of seismographs. A system of this kind consists of a coil of wires and a magnet: One of the components is fixed to the rigid frame and undergoes the same motion as the ground, while the other element is suspended by a spring from the frame. The relative motion of the coil with respect to the magnet will produce electromotive force between them, with voltage proportional to the velocity of motion. The current generated in the coil is sent through a sensitive galvanometer (an apparatus to measure electrical current) that makes a continuous record on photographic paper, a mirror, or a hot stylus. In most cases, the coil is fixed to the frame and the magnet is suspended, but there is a wide variety of possible arrangements. The sensitivity of an electromagnetic seismometer depends upon the magnet's strength, the number of turns of wire in the coil, and the geometry of the crossing of the magnetic flux by the coil. Modern seismometers have become quite small as magnetic materials with greater strengths have become available.

The introduction of the electromagnetic principles in seismic instrumentation seismographs has been a great step forward in the development of seismographs. The virtual elimination of friction also eliminates the need for the large pendulum mass used in the mechanical systems to overcome friction. Also, a much higher recording magnification with respect to the mechanical models is possible.

Damping

When the motion of a pendulum has started, the oscillation will continue at its natural frequency for a time, depending upon the rate of energy dissipation. To measure and detect the arrival of various waveforms separately, the motion of the pendulum must be damped to prevent its free-period swinging. If the damping is small, any seismic impulse will set the suspended spring into motion with an oscillatory or “ringing” output at a period of roughly the pendulum free-period. This problem can be pronounced for long-period seismometers. The damping of a pendulum or a spring-mass can be visualized by pulling a horizontal mass-spring, resting on a smooth oily surface, to one side by 1 centimeter and then releasing it. The distance that the spring overshoots the original rest position determines the damping ratio; for example, an overshoot of 0.1 centimeter corresponds to a damping ratio of 0.1, or 10:1. In older seismographs the damping was achieved in various ways, such as air damping in Wiechert seismographs or oil damping in the early version of the Benioff seismograph. Damping in modern-day seismographs takes advantage of the same electromagnetic principles as discussed previously. As the coil moves with respect to the suspended magnet, an electric current is induced in the coil, which in turn generates a magnetic force opposing the motion. This resisting force acts as the damping mechanism.

Seismograph Design

In designing a seismograph (a seismometer and the recording device), one must address issues such as the frequency content of the seismic waves of interest, the direction of the ground motion (vertical or horizontal), the smallest and the largest amplitudes of motion to be measured accurately, the plausible magnification of the pendulum motion relative to the ground motion, the method to be used for relative or absolute timekeeping, the recording of the measured ground motion (a visual recording on paper or recording on a magnetic tape), and the means of recording (an on-site or a telemetry recording). The nature of a particular application will determine the type of seismometer required, the frequency range of interest, the accuracy, and the resolution of the produced record.

Seismic waves generated by earthquake or explosive sources can have a broad frequency band of about 100-0.00033 hertz. Most of the primary earthquake-generated seismic waves have periods of between 0.05 and 20 seconds (frequencies of 20-0.05 hertz). Large earthquakes, such as the 1960 Chilean earthquake with a magnitude of 8.3, can put the whole earth into an oscillatory motion with a period of roughly one hour. Such an unusually broad frequency band creates a need for seismographs to record waveforms of different frequency bandwidth. A good seismological station such as WWSSN often houses six seismometers for obtaining a complete description of ground motion: one vertical and two horizontal (north-south and east-west) short-period seismographs, which are sensitive to the arrival of waveforms in the 0.05- to 2.0-second period range, and three long-period instruments with wave periods in the 15- to 100-second range. Seismographs are usually designed so that the output has a relatively constant magnification for displacement, velocity, or acceleration over some design frequency range. For example, a Wood-Anderson seismograph—the instrument whose seismograms are used to define the Richter magnitude scale—is a horizontal displacement seismograph with a flat response to ground displacement over the frequency range of greater than approximately 1.25 hertz with a magnification factor of 2,800. An earthquake with such a ground displacement at the distance of 100 kilometers is defined to have magnitude of 3.0 on the Richter scale.

Timing Systems

Without exact time data, a seismic record is not very useful. Seismologists use the Greenwich Mean Time (Universal Time) for timing seismograms. The timing system can be based on either internal or external clocks, or on a combination of the two. The internal timing system is a precise crystal oscillator, with temperature compensation, for keeping the time base. This low-cost and low-power-drain clock has a drift rate as low as 0.1 second per month. The timing system of this type of oscillator can be as simple as a series of minute and hour pulses for visible records, or as complex as a digital time code with day, hour, minute, and second information repeated once per second. The external timing system is used to keep the internal clock correct to 1 millisecond by continuous reception of a radio time signal. The most common radio signal comprises the time signals transmitted by a standard world time service (such as WWV in the United States). These short wave signals can usually be received, with adequate equipment, anywhere in the world. The reception of this signal, however, is not of good quality. Low-frequency time broadcasts in the 15-100 kilohertz range, such as WWVB, can be received reliably enough for continuous recording over a large area.

Recording and Calibration Systems

In the 1970's, the development of analog and digital circuits, coupled with increasing access to computers, resulted in the development and use of magnetic tapes for seismic recording. The recording on magnetic tapes can be either analog or digital. In analog, the motion is directly converted into proportional magnetization on the tape, although the analog data cannot be directly used by computers and have to be digitized. In digital recording, the analog ground motion is magnified and digitized at some selected time intervals, and the measured signals are recorded onto a magnetic tape.

A more recent development in seismic recording systems has been integrated circuit modules, allowing a huge amount of information to be held in memory on a single printed circuit board. That enables the recording system to screen the data regarding their significance and, thus, to make the decision if the data should be recorded. Such “self-editing” systems with the use of low-cost cassette tapes have made digital recording very attractive.

Seismologists are interested in the true amplitude of the ground motion. Therefore, seismographs must be calibrated to construct the true ground motion from the recorded seismograms. It is impossible to do a theoretical calibration because of the difficulty in modeling the physical behavior of each component. A direct method of calibration is to use shaking tables, where a seismometer is placed on a table, which is set into harmonic motion with a known amplitude and period. The record of such motion gives direct information on the magnification at that period, assuming that the period is not contaminated by the free-period of the shaking table. The experiment is repeated for different periods.

Earthquake Hypocenter and Motion Detection

Earthquake prediction and the design of earthquake-resistant structures have important social and economic value. A successful program would save lives and billions of dollars; the key to success is understanding the physical mechanisms of earthquakes, if there is to be hope of predicting the time, the location, and the magnitude of future earthquakes. Considering that the majority of earthquakes are caused by the jerky motion of a volume of earth along a fault plane, their spatial distribution could be used to delineate the fault zone. Seismologists use the relative travel times of the primary seismic waves to determine the location (hypocenter) and the time of occurrence of earthquakes. To accomplish that, seismographs are distributed around the expected seismogenic region to record the earthquake-generated seismic waves. The accuracy of pinpointing the hypocentral locations depends upon the accuracy in the relative timekeeping and the location of seismographs. For example, an error of 0.1 second in the travel time of a P wave (pressure wave) could translate to some 600 meters in error in the hypocentral location (P-wave velocity in upper crust is roughly 5,000-6,000 meters per second).

Seismologists determine the direction of motion along the fault surface by studying the relative direction, upward or downward, of the initial motion of the P wave at sites surrounding an earthquake. The mechanism is similar to moving two blocks in opposite directions in a sandbox. The upward or downward motion of the sand particles on the surface can be directly related to the direction of the motion of the blocks.

Perhaps the most important single factor driving the development of seismographs has been the need to detect and differentiate underground nuclear tests from earthquakes. At the present time, seismographs are the main source of information for monitoring the time, the location, and the size of such tests.

Earthquake-Resistant Structural Design

For the earthquake-resistant design of structures, it is necessary to determine the expected level of ground shaking from future earthquakes. Seismologists use the recording of past earthquakes to obtain relationships between the maximum ground motion, the magnitude of earthquakes, and the source-site distance for earthquakes in various regions (attenuation equations). Such relationships are used to predict the maximum ground acceleration, velocity, or displacement of future earthquakes of given magnitudes and distances for a desired site.

Another aspect of the seismic design of structures is the determination of the structural response to strong ground shaking. For this purpose, accelographs are mounted in various levels of structures to record the vibration. From the study of such records, earthquake engineers determine the frequency-dependent magnification factors of typical structures. Such information, coupled with the knowledge of the natural period of the building and the site (the site period is the period at which ground motion is magnified), is critical for the design of structures safe against future earthquakes.

Seismic waves from large earthquakes travel thousands of kilometers through the center of the earth. These waves carry much information about the physical properties of the earth and have been used to construct a picture of the interior of the earth. Any discontinuity in the earth's material properties that crosses the traveling path of seismic waves reflects part of the seismic energy and transmits the rest into the adjacent region. Examples of major discontinuities are the earth's crust-mantle and mantle-core discontinuities. The reflected waves reach the recording stations as distinct waveforms; seismologists identify these waveforms on seismograms and determine the location of the discontinuity in the earth by modeling the travel times of such waveforms.

Principal Terms

earthquake: a sudden release of strain energy in a fault zone as a result of violent motion of a part of the earth along the fault

natural period of vibration: the period at which structures undergo oscillation if they are set in motion by an impulse

particle motion: the motion of a particle in a material volume when it experiences the passage of seismic waves

seismic waves: the propagation of a disturbance in the form of energy release in a solid medium; the released energy propagates in the solid from one region to another by setting individual particles in motion in a particular direction

seismogram: the recorded output of a seismograph

seismograph: a device that detects, measures, and records the ground motion

wave velocity: the velocity at which a particular seismic wave travels through a medium

Bibliography

Bath, Markus. Introduction to Seismology. New York: John Wiley & Sons, 1981. Chapter 2, “Seismometry,” gives a very good and comprehensive description of seismometers and seismic recording systems. Well written and includes much interesting information.

Berlin, G. Lennis. Earthquakes and Urban Environment. 3 vols. Boca Raton, Fla.: CRC Press, 1980. Chapter 2, “Earthquake Descriptors,” gives very easily read information on the history and the fundamentals of seismic recording systems. A well-written book worth reading.

Bormann, P., ed. New Manual of Seismological Observatory Practice. 2d ed. Potsdam, GFZ. 2002. An excellent resource for anyone studying applied seismology. Chapters 5 and 6 reference seismometer technology and other equipment used in seismological observation. This text also includes useful datasheets, information sheets, and exercises.

Bullen, K. E., and B. A. Bolt. An Introduction to the Theory of Seismology. Cambridge, England: Cambridge University Press, 1985. Chapter 9, “Seismometry,” gives relatively technical details of the principles of seismometers and the recording systems. One may ignore the equations and still get some good information from this chapter.

Carlson, Shawn. “The New Backyard Seismology.” Scientific American 274 (April 1996). This article provides easy-to-follow instructions for building a rudimentary seismograph.

Dobrin, M. B. Introduction to Geophysical Prospecting. 4th ed. New York: McGraw-Hill, 1988. Gives a general description of the principles of the electromagnetic seismometers and the analog and digital recording systems.

Garland, G. D. Introduction to Geophysics: Mantle, Core, and Crust. 2d ed. Philadelphia, Pa.: W. B. Saunders, 1979. Chapter 5, “Seismometry,” gives a general background on seismometry. The text is semitechnical. Also discusses seismic arrays. The whole work is a good introductory book on the subject of geophysics.

Hutt, C. Robert, et al. Albuquerque Seismological Laboratory: 50 Years of Global Seismology. U.S. Geological Survey: FS 2011-3065. 2011. This fact sheet provides a comparison of seismology practices from the 1960's to 2011. There are many excellent images reflecting the change in technology over that time period.

Manukin, A. B., et al. “Compact High-Sensitivity Accelerometer-Seismometer.” Cosmic Research 48 (2010): 346-351. This article presents a design for an accelerometer-seismometer, a description of the instrument's characteristics, and techniques for use. The article is very technical; a background in seismology is recommended.

Plummer, Charles C., and Diane Carlson. Physical Geology. 12th ed. Boston: McGraw-Hill, 2007. A college-level introductory geology textbook that is clearly written and wonderfully illustrated. An excellent sourcebook of basic information on geologic terminology and fundamentals of geologic processes. An excellent glossary.

Reynolds, John M. An Introduction to Applied and Environmental Geophysics. 2d ed. New York: John Wiley, 2011. An excellent introduction to seismology, geophysics, tectonics, and the lithosphere. Appropriate for those with minimal scientific background. Includes maps, illustrations, and a bibliography.

Wiegel, R. L., ed. Earthquake Engineering. Englewood Cliffs, N.J.: Prentice-Hall, 1970. Chapter 6, “Ground Motion Measurements,” was written by D. E. Hudson and concentrates more on the development of accelographs. The whole book is highly recommended, as it contains valuable information on the history of earthquake engineering and seismic hazards.