Emil Wiechert introduced a damping mechanism that restrained the seismograph pendulum and greatly increased its accuracy. The evolution of the modern seismograph, led by Wiechert’s invention, led to an explosion of knowledge about not only earthquakes but also Earth’s crust and interior.
Prior to the development of the science of seismology, the study of earthquakes and measurement of the elastic properties of the earth, the natural philosophers of the late nineteenth century held varying views on the composition inside the earth. One view was that if the inside were liquid, then the surface of the earth would rise and fall almost like the tides of the oceans. Another view was that the geological hypothesis of a fluid interior is untenable, and the overall rigidity of the earth’s interior is considerable.
The invention of the seismograph, an instrument for recording the phenomena of earthquakes, at the end of the nineteenth century was to open the twentieth century with an explosion of discoveries about the inner earth. The first seismographs were delicate horizontal pendulums that registered singular waves. Such an instrument was used by the German seismologist E. von Rebeur Paschwitz on April 18, 1889, when he correlated horizontal pendulum recordings at Potsdam and Wilhelmshaven with a great earthquake in Tokyo.
An important fact in the study of earthquakes is that there are two types of waves that can be transmitted through a homogeneous, isotropic, elastic solid such as the earth. Isotropic means having the same properties in like degree in all directions. These waves are dilatational (compressional) waves, such as sound waves, which involve particle motions parallel to the direction of the energy; and transverse (shear) waves, which involve motion at right angles to this direction. Emil Wiechert, in 1899, independent of similar work by Richard D. Oldham
The Milne seismograph and others developed at the end of the nineteenth century fell short of meeting the demands of a science that was asking ever more complex questions. These early machines measured only a portion of the broad band of wave size and frequencies. Another shortcoming was the tendency of the seismograph’s pendulum to keep swinging indefinitely once a strong motion had started it. Without a way to control the pendulum’s motion, the seismograph was unable to record accurately the other kinds of waves that arrived later. Wiechert, a German seismologist, started developing a damping mechanism around 1898 that restrained the seismograph pendulum, greatly increasing the seismograph’s accuracy. He perfected his early model in 1900. It had one major shortcoming, however, in that it was bulky. It depended upon weights large enough to remain at rest despite the energy transmitted both by the shaking of the instrument’s frame and by the mechanical linkage that inscribed the seismic waves on paper.
The Wiechert seismograph, an inverted pendulum, permitted the detection of the vertical component of long earthquake waves. The amplification of the boom movement was achieved by using a system of mechanical levers and recorded by a stylus scratching on smoked paper. It had a mass of seventeen tons supported on a vertical column, which acted as the boom. Its period was about a second, and its magnification was two thousand.
In his experimentation, Wiechert found that the only way to make a seismometer (an instrument for measuring the direction, intensity, and duration of earthquakes) react to the vertical component above was to counteract the force of gravity by a spring. In the inverted pendulum, the mass is held on the right and left by spiral springs. It rests and oscillates upon a sharp horizontal edge turned at right angles to the direction of the movements.
Prior to inventing the inverted pendulum seismograph, Wiechert had suggested the existence of a central core within the earth. He speculated that this central core was appreciably different from the outer shell. In 1901, a year after he invented his seismograph, Wiechert founded the Geophysical Institute in Gottingen, Germany. The institute quickly became a center for the study and compilation of earthquake data from observatories worldwide. One of the important seismologists affiliated with the Geophysical Institute was Beno Gutenberg, a German seismologist who, in 1914, after studying seismic data collected by the institute from worldwide sources, estimated the depth of the boundary between the inner and outer cores at 2,900 kilometers from the earth’s surface. Later, Harold Jeffreys
Wiechert observed that seismograms at many observatories revealed the presence of additional small earth movements, called microseisms. These small movements complicated the problem of the accurate recording of ordinary earthquakes; their form is likely to be related to features that are similar on records traced at observatories distributed over a wide area. One of the features is the approximately simultaneous occurrence of maximum amplitudes at all the observatories involved. These microseisms may persist for many hours at a time and may have more or less regular periods of from 2 to 10 or more seconds. Wiechert suggested that microseisms are generated by the action of rough surf against an extended steep coast; this hypothesis was followed up by Gutenberg. Norwegian and Japanese seismologists have concluded a similar relationship, as suggested by Wiechert.
Shortly after Wiechert’s invention, in 1906, Boris Golitsyn
With the development of the seismograph during the late nineteenth century came an explosion of knowledge about the inner earth. Very early in the twentieth century, Oldham and Wiechert independently postulated the presence in the center of the earth of a large, dense, and at least partially molten core. In 1909, Andrija Mohorovičić, a Yugoslav meteorologist and seismologist, discovered the discontinuity between seismograph recordings of earthquake waves, which led to his discovery of the boundary between the earth’s crust and upper mantle. This boundary, called the Mohorovičić Discontinuity, varies from about 10 kilometers beneath the basaltic ocean floor to between 32 and 64 kilometers beneath the granitic continents.
In 1914, Gutenberg, later affiliated with the California Institute of Technology, discovered the boundary between the mantle and the outer core. In 1936, Inge Lehmann
Around 1924, portable seismographs were being introduced to record seismic waves from quarry blasts and other relatively small explosions. Seismic monitoring of rock blasting in quarries and other methods of mining
A worldwide network of recording stations in the 1960’s, improved seismographs, and computers have all contributed to a greater understanding of the earth in recent times. Some of these discoveries include that the earth’s core is not a single molten mass, but consists of an inner and outer core with a transition zone dividing a solid inner core from its surrounding molten rock. The mantle has been found to consist of an upper and lower section. There are concentric envelopes of elusive discontinuities with new ones being detected frequently. It is the region of the upper mantle that commands the special attention of seismologists because it is at the juncture of the mantle and the crust that most earthquakes have their source. Understanding this region is also important for understanding the movement of continental plates.
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Bolt, Bruce A. Inside the Earth: Evidence from Earthquakes. New York: W. H. Freeman, 1982. An introductory text of a nonmathematical nature for earth science. The principal focus is on earthquake waves, main shells of the earth, structural detail, earth vibrations, and physical properties of the earth. Illustrated with simplified diagrams and boxed excerpts of major discoveries in seismicity. -
Eiby, G. A. Earthquakes. Exeter, N.H.: Heinemann, 1980. This book is intended for general readers who want elemental knowledge about earthquakes; it is a good reference for high school students and college undergraduates. Of particular interest is the chapter on recording an earthquake. -
Halacy, D. S., Jr. Earthquakes: A Natural History. Indianapolis, Ind.: Bobbs-Merrill, 1974. This is a well-written elementary book designed for high school and lower-level college students as well as general readers. Illustrations and photographs complement the text and are well prepared. A short book covering the basics on earthquakes and is highly recommended. -
Stacey, Frank D. Physics of the Earth. 3d ed. Brisbane, Australia: Brookfield Press, 1992. The principal audience is the graduate and advanced undergraduate student of physics. Topics covered include the solar system, rotation and figure of the earth, the gravity field, seismology and the internal structure of the earth, and the geomagnetic field. -
Tazieff, Haroun. When the Earth Trembles. Translated by Patrick O’Brian. New York: Harcourt, Brace & World, 1964. An excellent book for the general reader and high school student. The book has three sections: The first covers the great Chilean earthquake of May, 1960; the second describes the geography of earthquakes in the Mediterranean Belt, Asiatic Belt, Pacific Belt, and Midoceanic Ridge; and the third is on instruments that record earthquakes. The text is well written, illustrated, and contains a brief bibliography. -
Walker, Bryce. Earthquake. Rev. ed. Alexandria, Va.: Time-Life Books, 1984. This book is structured for the average reader, high school student, and undergraduate college student. It has excellent photographs, many in color and full page, and the text is written in narrative style. Areas covered include the Alaska earthquake of 1964, the history of earthquakes, and seismology from earliest times to 1982. -
Wood, Robert Muir. Earthquakes and Volcanoes. New York: Weidenfeld & Nicolson, 1987. This is an excellent book for the nontechnical reader and is copiously illustrated with color photographs. It is written in a very understandable format on the causes and prediction of earthquakes. The second part of the book discusses the effects of volcanoes.
Tambora Volcano Begins Violent Eruption
Ampère Reveals Magnetism’s Relationship to Electricity
Lyell Publishes Principles of Geology
Krakatoa Volcano Erupts
Bjerknes Founds Scientific Weather Forecasting
Stratosphere and Troposphere Are Discovered
Georges Cuvier; Sir Charles Lyell.