Seismic observatories

A global network of seismic observatories detects and records seismic waves produced by earthquakes and other energy releases. Their data can be used to locate the sources and magnitudes of earthquakes, to interpret the earth's internal structure, to delineate seismically active zones, to study dynamic processes in the earth's crust, and to monitor nuclear tests.

Monitoring Seismic Waves

Earthquakes are one of nature's most sudden and terrifyingly unexpected phenomena. They are dangerous to human life and cause the destruction of buildings and property. However, they do not occur randomly in time and space. Seismology—the study of earthquakes and the outgoing seismic waves they produce—has yielded much understanding of the earth and its dynamic physical environment, including knowledge, since the 1960's, of plate tectonics. Large tectonic plates in the earth's lithosphere (crust and upper mantle) move at rates of several centimeters per year. It is at the margins of these plates that earthquakes have been most common in the past and are most likely to occur in the future. At their margins, tectonic plates spread apart, slip sideways, or collide in great processes of mountain-building or deep-ocean trench subduction (diving down into the earth's mantle).

The seismic wave vibrations generated by earthquakes travel through the entire earth and, by observing them, scientists can yield information about the earth's interior structure, physical properties, and likely composition. Study of seismic waves can also provide understanding of other sudden energy releases, such as volcanic eruptions, landslides, meteorite impacts, and large subsurface explosions such as nuclear weapons tests.

All this is possible because of the detection, recording, and analysis of seismic waves at seismic observatories. By sharing data with other observatories that have appropriate equipment, scientists can determine a seismic event's time of origin, size (energy release), geographic location, and depth. By considering many such events, scientists can investigate wave travel through the earth and interpret its structure and properties; map the distribution of earthquakes over the earth's surface and over time; delineate seismically active zones, major plate tectonic boundaries, and local faults (where crustal rock masses are slipping and crunching past one another); and assess seismic hazards.

Advances in seismology were made possible by the invention of seismographs in the 1870's; the establishment of progressively larger, more coordinated, and more sensitive global networks of seismic observatories beginning in the early twentieth century; and the introduction of computers in the 1960's. Computers have allowed researchers to deal with massive amounts of data and complicated mathematical analysis procedures, as well as subtle and difficult problems of interpretation.

Earthquakes and Seismic Waves

An earthquake occurs when rock masses in the crust and upper mantle suddenly break and shift along a plane or zone called a fault. This can occur when stress has built up, as from plate tectonic movement in the lithosphere, and exceeds the breaking strength of the rock. The accumulated strain energy is released as frictional heating on the fault zone, plus seismic waves (vibrations) that are transmitted rapidly out from the focus in all directions through the earth. These waves can range from barely perceptible to catastrophically destructive.

There are different types of waves produced by an earthquake. “Body” waves are generated by the faulting, designated as either P (for “primary,” or compressional, having forward-and-back motion like coils moving in a slinky spring) or S (for “secondary,” or shear, having sideways motion like a wave in a rope snapped sideways at one end). P waves always travel faster than S waves and thus arrive at a distant point sooner. When body waves travel up to the earth's surface, they can produce surface waves that travel out across the earth's surface like large ripples from a stone dropped into a pond. Surface waves are termed L waves (for “large”), since they are typically larger in amplitude (or extent of oscillation) than body waves. Because of their amplitude and the nature of ground shaking, they can be quite destructive.

When earthquakes are detected and analyzed by seismic observatories, information on their time of origin, epicenter, magnitude (energy release), and depth of focus can be catalogued and mapped at a central data repository. Two prime sites for global data collection are the U.S. Geological Survey's National Earthquake Information Center (USGS/NEIC) in Golden, Colorado, and the International Seismological Center in Cambridge, England. The data are publicly available on the Internet. One example would be a map that shows seismicity around the Pacific Rim. This margin around the Pacific Ocean basin is defined by the major plate tectonic collision and subduction as the Pacific sea floor spreads apart on the East Pacific Rise and is forced to dive down upon collision with the other plates rimming the Pacific. The pattern of earthquake epicenters effectively outlines these active plate margins.

The amplitude (height, or deflection, of the measured vibration) of seismic waves can be used to calculate a “magnitude” for the earthquake. This is a measure of the energy released by the event. Several slightly differing versions of magnitude exist, depending on the wave type analyzed and the distance to the earthquake epicenter. Richter scale magnitudes vary numerically from about 1 (very minor earthquake) to about 9 (extremely large and rare) in a logarithmically increasing scale. Each year there are several hundred thousand earthquakes that are instrumentally detectable, at least locally. Many of these are too small to be felt by people. A “major” earthquake has a Richter magnitude of at least 7. There are typically several of these events worldwide each year.

When incoming seismic waves arrive at a seismological observatory, they are detected and ultimately recorded for display on a seismogram—a line trace related to the variation of ground motion from the passing wave train as time advances. The seismogram, in conjunction with other seismograms from other observatories, can be analyzed by computers and by human interpreters to give the precise arrival times of various waves. The waves include those that have traveled directly from the focus, with travel paths shaped by the wave velocities and properties of the layers in the earth's interior, and those that have been multiply reflected inside the earth. Major earthquakes produce so many waves that they often continue to arrive with enough energy to be detected for several hours.

The travel velocity of seismic waves in the earth is several kilometers per second, depending on the type of wave, the composition of the material at depth, and the temperature and pressure at depth. For example, a seismic station at the antipode (the point on the earth's far side opposite the epicenter, 12,740 kilometers straight through the earth) would detect the first-arriving P wave about twenty minutes after the earthquake occurred. It would be followed by a wave train of many waves having different travel paths.

Seismograph Equipment

Any collection of data at a seismological observatory is only as useful as the quality of its staff and equipment. A seismograph is a sensitive device designed to detect the waves of energy generated by earthquakes. It consists of a seismometer, which mechanically or electromagnetically detects arriving seismic waves and other ground vibrations by measuring the movement of the ground; an amplifier, which uses electrical or optical systems to magnify the small vibrations being sensed by a factor of several thousand; and a recording system, which makes a record, for storage or display (on a seismogram), of the arriving seismic waves as a function of time advancing. Recording methods include an ink pen on a rotating paper drum, a light spot on photosensitive paper, or analog or digital data stored on magnetic tape or disk. Analog-to-digital converters are also used.

A common seismometer technique is to measure the movement of the ground—at the surface, in a buried vault, or down a borehole or mine—with respect to a suspended mass such as a pendulum. The mass, not being fixed rigidly to the ground, briefly “hangs back” as the ground suddenly moves, because of its inertia. This indicates the relative motion.

The world's first seismometer is attributed to Zhang Heng of China in the year 132 c.e. during the Han dynasty. Its design featured a bronze kettle with eight dragon heads holding balls over eight toads. Inside was probably an inertial pendulum with linkages to the dragons' heads. When earthquake ground motion moved the kettle sideways, the pendulum hung behind and opened one or more dragons' mouths so that those balls dropped into the open mouths of the toads below. This ingenious seismoscope measured the occurrence of a vibration but could not make a record of its duration and behavior. Instead, it indicated the relative intensity of the effects felt and possibly the direction from which the seismic waves came. The principle of earthquake duration was neglected until European geologists again addressed the problem of earthquake detection and measurement during the nineteenth century.

John Milne, an English professor of mining and geology, arrived in Japan to teach in Tokyo in 1875. Interested in the frequent earthquakes there, he began developing seismographs. He, along with John Gray and James Ewing, who were also visiting Tokyo, designed swinging pendulums that detected and recorded wave motion by scratching a trace on a moving smoked glass plate. The instruments measured three components of motion, which, when combined, could give the net ground motion. Milne left Japan for England in 1894. By 1900, his efforts had encouraged the installation of seismographs at a couple of dozen seismic observatories around the world. Thus began the systematic collection of global seismic data.

Other work to develop or refine seismographs was done in Germany by Ernst von Rebeur Paschwitz and Emil Wiechert, who introduced a damped pendulum in 1898 so motions could be more quickly recovered, and in Russia by Boris Galitzin, who introduced moving-coil electromagnetic recording of motion in the early twentieth century.

In practice, seismographs must be designed to accurately sense a wide range of wave amplitudes and periods. The period of a wave is the time (in seconds) between successive oscillation, or vibration, peaks (from crest to crest). The frequency is the inverse—that is, the number of peaks passing per unit time (in cycles per second, or Hertz). A seismograph might be designed, or tuned, to emphasize quarry blasts with wave periods of less than one second (frequency greater than 1 Hertz), nearby earthquakes with body wave periods of one to ten seconds and surface wave periods of ten to sixty seconds. Seismographs might also specialize in measuring distant earthquakes (called teleseisms), with arriving waves having longer periods (the shorter-period energy is progressively absorbed during travel through the earth), or earth oscillations and even earth tides having periods of hours.

A typical seismological station would have six seismographs. Three would be short-period (about a one-second response for nearby earthquakes and vibrations) and would measure the three components (north-south, west-east, and vertical). Three would be long-period (about a twenty- to thirty-second response for teleseisms and surface waves). All would record continuously. Apart from the need to detect and record a wide range of periods, it may also be necessary to have both low- and high-sensitivity instruments for global data collection and interpretation. This is because a high-sensitivity instrument, designed to respond to small local events or moderate distant events, would be deflected or driven “off scale” (exceeding its reading limits) by a large seismic event.

A variety of filtering, damping, and data-analysis techniques can assist in yielding a useful data set. One innovation is to restrain the relative motion of the inertial mass with a “forced feedback” mechanism. The restraining force is measured as a signal. This allows a greater range of sensitivity, and, without large excursions of the inertial element, the device can be more compact. An example is the Wielandt-Streckeisen STS-1 leaf-spring seismometer, developed in 1986, in which the mass is on a flexible strip rather than on a pendulum.

Global Network of Observatories

In the early twentieth century, a worldwide network of seismological stations that used the rudimentary seismograph technology available at the time was established for the purpose of studying earthquakes and the interior structure of the planet. The 1906 earthquake in San Francisco was recorded, for example, at dozens of seismological stations around the world, including in Japan, Italy, and Germany. By 1960, about seven hundred seismic observatories were operating worldwide using various types of seismographs and standards, which led to incomplete data exchange and analysis. Most observatories were operated by government agencies and universities.

A standardized global network of calibrated instruments was needed, with coordinated accurate timing and central data collection. This was not only a vexing organizational problem, but one that required significant funding. Nonetheless, scientists recognized that seismology and the mapping of earthquakes would contribute to the emerging study of global plate tectonics and the dynamic processes at the plate boundaries during the 1960's.

By convenient coincidence, the impetus and funding for these earth-centered interests emerged from the new military and international political need to monitor underground nuclear testing. The 1963 Test Ban Treaty prohibited atmospheric, oceanic, and space testing of nuclear technology—all of which could be monitored fairly directly. Nations with existing or developing nuclear programs now needed to test underground. Military competition between the United States and the Soviet Union necessitated a program of remote surveillance, and the techniques of seismology—particularly the ability to distinguish small, shallow earthquakes from buried nuclear explosions—were needed and quickly applied. In the early 1960's, the United States began a program that deployed a series of stations in the World Wide Standardized Seismograph Network (WWSSN). Over time, the network would grow to about 120 stations in total. The stations used Benioff short-period (a period of one second) seismographs and Sprengnether long-period (a period of fifteen to sixty seconds) seismographs with moving-coil electromagnetic seismometers and galvanometers to record on seismogram drums.

During the 1970's, seismic recording began a conversion from analog recording to digital recording onto magnetic tape. Digital technology samples the seismic signal at short time intervals and stores the data. It can retain greater dynamic range and is convenient for direct computer processing. The combination of new digital seismic observatories and some of the WWSSN stations that were upgraded to digital recording had a major impact on global seismology from the mid-1970's to the mid-1980's.

In 1986, seismographs based on the forced-feedback mechanism to restrain motion and increase range of response, along with digital recording of signals onto magnetic tape, began being used to form the Global Seismic Network (GSN). This is a joint effort of a consortium of universities (the Incorporated Research Institutions for Seismology [IRIS], the U.S. Geological Survey (USGS), and upgraded stations run by European countries, Canada, Australia, and Japan. Funding comes from the U.S. National Science Foundation and the USGS. The GSN's plan was to have 128 seismic stations when the network was complete. The one hundredth station was installed in 1997. The high-quality global network, designed to replace the WWSSN, exemplifies international scientific collaboration. Within hours of an earthquake, data are automatically collected and made available to government and university scientists. Data are also made available to the general public over the Internet.

Regional Networks

In addition to the coordinated global collection and analysis of seismic data, a variety of regional, or more localized, networks exist. These are set up for more detailed and rapid analysis of local earthquakes or other events, such as for detection of patterns of small earthquakes. They also aim to detect foreshocks that can occur prior to a larger event in the hopes of possibly predicting a larger forthcoming earthquake.

These networks are commonly set up by technologically advanced nations in regions at high risk of suffering seismic hazards, such as the United States (particularly California, the Pacific Northwest, and the New Madrid area of southeast Missouri), Japan, and Canada. The U.S. National Seismograph Network (USNSN), under development since 2000, uses sensitive forced-feedback seismometers with satellite telemetry of data back to the USGS/NEIC in Golden, Colorado. Stations are being developed in the continental United States, Alaska, Hawaii, Central America, and the Caribbean region. The network is designed to detect, locate, and analyze earthquakes of magnitudes as small as 2.5 but also has seismographs with high dynamic range to handle large earthquakes. The National Science Foundation's Earthscope project includes the fifteen-year USArray program, begun in 2004, to place a dense network of permanent and portable seismic stations across the United States at 2,000 locations over a ten-year period to better understand the structure and evolution of the crust and lithosphere under North America. The network is designed to detect, locate, and analyze earthquakes of magnitudes as small as 2.5 but will also have seismographs with high dynamic range to handle large earthquakes.

The occurrence of tsunamis (seismic sea waves) generated by some large earthquakes or subsea volcanic eruptions, particularly in the Pacific Ocean, prompted the United States set up the Pacific Tsunami Warning System in 1948. This was in response to a destructive tsunami that hit Hawaii in April 1946. It traveled south from an earthquake off the coast of the Alaskan peninsula, killing 179 people in Alaska and Hawaii with waves that rose to crests of up to 30 meters high on Unimak Island, Alaska, and up to 10 meters high in Hawaii. The system is administered by the National Oceanic and Atmospheric Administration (NOAA), with coordination, data processing, and alerts issued from a warning center located in Honolulu, Hawaii. The system quickly activates when any of its thirty or so participating seismographic stations, located around the Pacific Rim and on Pacific islands, detects an earthquake or other disturbance that could potentially generate a spreading tsunami wave. Another seventy-eight stations have tide gauges for monitoring unusual changes in sea level which can detect a tsunami as it passes by. If a tsunami is detected, an alert can be issued, with prediction of tsunami arrival times at Pacific-bordering nations or islands. Coastal people can be evacuated inland, and ships or fishing boats can be taken out to sea to ride out the much more subdued offshore waves.

Portable seismograph systems are used for rapid and temporary setup of local seismic networks. These are deployed to monitor precursory earthquakes before a suspected major earthquake or aftershocks after a large one has occurred. They can also be used around volcanoes before or after eruptions, or in anticipation of an underground nuclear test or some other energy-releasing disturbance. Local temporary arrays have also been used during the seismic probing of deep geological structures and to study local seismicity patterns.

Principal Terms

epicenter: the spot on the earth's surface directly over the focus of an earthquake

focus: also called the hypocenter, the region in the earth's crust or upper mantle where an earthquake begins; larger earthquakes have a focus several tens of kilometers in size

seismic waves: oscillatory vibrations generated by an earthquake that travel outward in all directions through the earth (as body waves) or along and near the earth's surface (as surface waves)

seismogram: a recording, by ink pen, film, or digital data, that measures the train of seismic waves arriving by the variation in ground motion as time advances

seismograph: a sensitive instrument that mechanically or electromagnetically detects and records arriving seismic waves, usually by measuring the motion of the ground with respect to a relatively fixed mass

time of origin: the time of an earthquake's occurrence in local time or—more conveniently for analysis of worldwide events on a standard time scale—in Coordinated Universal Time (CUT, or Greenwich Mean Time)

Bibliography

Frechet, Julien, Mustapha Meghraoui, and Massimiliano Stucchi, eds. Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes. New York: Springer, 2010. A compilation of articles which provide new approaches and a historical review of seismic observation.

Havskov, Jens, and Lars Ottemoller. Routine Data Processing in Earthquake Seismology. New York: Springer, 2010. This text provides practical application of software and data analysis in the field of seismic observation. The text comes with software and exercises. It has use within an undergraduate course, in observatory processes, and in research.

Lane, N., and G. Eaton. “Seismographic Network Provides Blueprint for Scientific Cooperation.” EOS/Transactions American Geophysical Union 78 (September 1997): 381. The authors discuss attempts to install a global network of modern seismological stations.

Lay, T., and T. C. Wallace. Modern Global Seismology. San Diego: Academic Press, 1995. Chapter 5 discusses the development of seismographs as well as their deployment in regional and global networks for interpreting the earth's internal structure, earthquake characteristics, and nuclear weapons testing. Written at a more technical level, suited for graduate students with a background in mathematics.

Natural Resources Canada/Canadian National Seismology Data Center Website: earthquakescanada.nrcan.gc.ca. Database of earthquakes as recorded in Canada.

Simon, R. B. Earthquake Interpretations: A Manual for Reading Seismograms. Los Altos, Calif.: W. Kaufmann, 1981. Instructions for reading earthquake seismograms, with examples.

U.S. Geological Survey Earthquake Website: earthquake.usgs.gov. Contains useful information about earthquakes and seismicity.

U.S. Geological Survey/National Earthquake Information Center Website: earthquake.usgs.gov/regional/neic. Database of earthquakes in the United States and around the world, as located and assembled by NEIC in Golden, Colorado. Includes data and listings on worldwide seismological stations, earthquake listings by geographical region, and listings of significant historical earthquakes.

Walker, Bryce. Earthquake. Planet Earth Series. Alexandria, Va.: Time-Life, 1982. Chapter 3, “Secrets of the Seismic Waves,” discusses seismology, recording instruments, and historical developments. Map included.