Seismograph | Research & Encyclopedia Articles

Seismograph

Seismographs are instruments used to measure and record ground motion, or seismic waves, caused by explosions and earthquakes. In the late 1800s, John Milne, an English mining engineer, developed the first precise seismometer, the sensor in a seismograph that detects and measures motion. Since then, seismograms, the data recorded by a seismograph, have helped seismologists predict much more than ground movement. These devices have also led to discoveries about the nature of the Earth's interior.

The process of using a tool to detect ground motion dates back to the ancient Han Dynasty when Chang Heng (78-139 A.D.), a Chinese astronomer and mathematician, invented the first seismometer in 132 A.D. Although the internal mechanics of this device are unknown, it probably used a pendulum that, when moved during an earthquake, would dislodge a bronze ball from the mouth of one of eight ornamental dragon's heads that adorned the exterior. While this did not provide clues of an earthquake's force, it gave Heng an idea of the direction of the shock waves and their source. Since then Heng's concept has since been refined considerably.

Later seismographs employed a heavy pendulum with a stylus, or needle, suspended above a revolving drum. The drum was covered with paper or film on which the etchings from the needle were recorded. During an earthquake, the pendulum and needle remained stationary while the drum on the base moved, recording the ground motion. As much as these later pendulum seismographs improved upon the ancient Chinese method, they still failed to solve many of the problems that arose with more precise readings. For example, once a strong motion set off a seismograph's pendulum, the pendulum would swing indefinitely, failing to record aftershocks that followed the initial disturbance. Also, the seismographs of the late 1800s recorded only a limited range of wave sizes and numbers.

The inverted pendulum, invented by German seismologist Emil Weichert in 1899, helped overcome many of these limitations. Weichert employed a system of mechanical levers that linked the pendulum movement more closely to the ground motion. In 1906 Boris Golitsyn, a Soviet physicist and seismologist, devised the first electromagnetic seismograph; for the first time, a seismograph could be operated without mechanical levers. To record the magnitude of an earthquake, electromagnetic seismograms convert ground motion to electrical currents that are then read by a voltmeter. They can therefore register very small waves with great precision. During the 1930s Hugo Benioff designed a vertical component seismometer capable of providing much higher amplification of vertical ground motion and therefore much more precise records.

Although many of the modern seismographs are complicated technical devices, all such instruments contain five basic parts. The clock records the exact time that the event takes place and marks the arrival time of each specific wave. The support structure, which is always securely attached to the bedrock, transfers the ground motion to the recording device during the earthquake or explosion. The inertial mass is the component that remains stationary as the ground and the support structure oscillate around it. The pivot holds the inertial mass in place, that is, it allows the inertial mass to remain stationary in spite of the ground motion. The vibrations are registered through the recording device; in simple systems, this is a pen attached to the inertial mass and a roll of paper. The paper moves along with the support structure while the pen remains stationary. Most modern units now use a digital recorder connected to a computer. In either case, this records the pattern of seismic waves as a set of wavy lines of varying amplitude and period, revealing the strength of the various waves as well as the frequency with which they occurred.

After the first modern seismograph was installed in the United States at the University of California at Berkeley, it recorded the 1906 earthquake that devastated San Francisco. The same year, Weichert and fellow scientist Richard Oldham (1858-1936) determined the existence of the Earth's core through precise recordings of seismic waves. In 1909 the use of a seismograph helped Yugoslavian seismologist and meteorologist Andrija Mohorovicic (1857-1936) discover the depth at which the Earth's crust meets the upper mantle. That discovery was followed in 1914 by German-American seismologist Beno Gutenberg's (1889-1960), determination of the depth to the core-mantle boundary, now known as the Gutenberg discontinuity. Then in 1936, Danish seismologist Inge Lehmann's work suggested the existence of an inner core. These important findings revealed the existence of all of the Earth's major layers: the inner core, the outer core, the mantle, and the crust.

By the late 1950s, the range of motions detectable with seismographs made possible for the first time very precise determinations of earthquake magnitude and depth, rock density and heterogeneity, etc. However, as the sensitivity of seismographs has increased, the need for filtering--removal of background vibrations from local activities--has had to keep pace. Without computers, recent improvements in sensitivity and filtering would not be possible. Likewise, the techniques for artificially inducing seismic waves, by explosions or surface concussion, along with greater instrument sensitivity, enable seismologists to greatly expand the abilities and utility of seismic studies. For example, seismology was critical in the development, and in the continued refinement, of the theory of plate tectonics. It provides evidence for the character of many important features of plates and plate boundaries, including mid oceanic ridge and rift systems, deep ocean trenches, transform faults, and volcanic island arcs.

In addition to helping scientists understand the phenomena of natural ground movement, seismology also includes the study of manmade earth motion. Seismographs provide miners with tools to effectively monitor how much dynamite should be used for quarry blasts and other explosions. They are also widely used today to detect variations in the speed of seismic waves traveling in near-surface rocks, clues which may help locate economic deposits, such as oil and coal. Since the development of the atomic bomb and the heightened threat of nuclear war, seismologists are in greater demand. The need to quickly and accurately decipher the difference between natural events and manmade explosions has brought about the establishment of cooperating seismic stations in sixty countries, called the World-Wide Standardized Seismograph Network (WWSSN). Seismologists can now routinely establish the time, location, depth, and explosive yield of nuclear detonations with great accuracy. For this reason, non-WWSSN member countries such as Russia, France, and Canada have modernized their earthquake observation systems as well.

Since the early 1980s, major advances have been made in linking seismometers to computer networks, enabling seismologists to work with increasingly large sets of data and allowing new types of analyses and graphical presentations. For example, seismic tomography provides two- or three-dimensional views of the Earth's interior by computer-synthesizing hundreds or thousands of seismic records into an image. Seismic tomography works somewhat like a computerized axial tomography or CAT scan. CAT scans are interior views of a person's body produced by computers that combine hundreds of digital x-ray cross sections of the body into a coherent picture. The density of the different tissues provides the contrast that allows recognition of medical conditions. In seismic tomography, differences in density, primarily due to temperature differences, reveal details about the structure of the Earth's interior. This technology only became possible in the mid to late 1990s as computers grew capable of processing huge data sets at extremely rapid speeds. Seismic tomography has shown that the base of the mantle has "mountains" and "valleys" much larger than anything at the Earth's surface. At these locations, low areas probably result from the cooler (denser) mantle sinking downward somewhat into the core and highs result as molten core material convects upwards into the mantle. Seismic tomography also reveals convection currents within the mantle itself, which may be the driving force behind plate tectonics.