(b. Copenhagen, Denmark, 13 May 1888; d. Copenhagen, 21 February 1993),
Lehmann is first and foremost known as the discoverer of Earth’s inner core in 1936, but she is also highly regarded for her studies of Earth’s mantle, carried out during many visits to the United States in the 1950s and the 1960s. In these studies she identified a low velocity layer between 130 and 220 kilometers (80 to 140 mi) below Earth’s surface. The bottom of this layer, at which the velocity of seismic waves rises abruptly, is now called the Lehmann discontinuity.
Childhood and Education . Inge Lehmann grew up and lived almost all her life in Copenhagen. She came from an influential family of academic traditions. Her paternal grandfather laid down the first Danish telegraph cable in 1854, and her father, Alfred Lehmann, became the first professor of experimental psychology at Copenhagen University in 1919. The family also included several prominent women. Lehmann’s mother, Ida Sophie Tørsleff, had a sister who was an active proponent of women’s rights, and her daughter, Lis Groes, became Danish minister of commerce in the 1950s. Lehmann had a younger sister, Harriet, who became an actress and who had family and children in contrast to Lehmann, who lived by herself all her life.
A strong influence on the young Inge Lehmann was her schooling at the coeducational Fællesskolen, a school run by Hanna Adler, an aunt of Niels Bohr. At Adler’s school, no differentiation based on sex or social status was accepted, and both girls and boys were taught needlework and played soccer. In an obituary of Adler, Lehmann wrote in 1947 that “there was no unnecessary discipline, and we were not burdened by the prejudice [regarding gender, race, or social status], which makes life difficult for so many people.” In 1906 Lehmann left Adler’s school after passing the university entrance examination.
In 1907, Lehmann studied mathematics at Copenhagen University, and she continued her studies at Newnham College in Cambridge in 1910–1911. Here the relationship between men and women was very different from what she had experienced at Adler’s school. Even though the many restrictions on young women’s behavior dissatisfied Lehmann, she enjoyed her stay in Cambridge. However, in 1911 Lehmann returned from Cambridge overworked, and so she put her university studies on standby. For a few years she worked at an actuary’s office, where she acquired good computational skills, before resuming her studies at Copenhagen University in 1918. Two years later she completed the candidates magisterii degree in mathematics and physical science. In the autumn of 1922 she studied mathematics with Professor Wilhelm Blaschke in Hamburg, Germany, and when she returned to Denmark in 1923, she accepted a position as assistant to the professor in actuarial science at Copenhagen University, J. F. Steffensen.
Introduction to Seismology . A turning point in Lehmann’s career came in 1925, when she was appointed assistant to Niels Erik Nørlund, the newly appointed director of Gradmaalingen, a Danish geodetic institution. Nørlund had plans to establish seismological stations in Denmark and Greenland, and Lehmann’s job was to run the stations and interpret and publish the observations. Research work was not a part of her job description, but she was free to take it on. Lehmann supervised the establishment of one seismological station in Copenhagen, and she helped prepare instruments for two stations in Greenland. The Copenhagen station was in the old fortress that encircled the old part of the city, while the Greenland stations were placed in the mining city Ivittuut on the west coast and at Ittoqqortoormiit on the east coast. The station on the east coast turned out to be a particular challenge to run, because the only contact with the station was by boat once per year.
Why Lehmann was named to the position at Grad-maalingen is unknown, but one might speculate that Hanna Adler played a role through Niels Bohr, who was married to Nørlund’s sister. In any case Lehmann quickly engaged in the new field, and in the summer of 1927 she visited seismological stations around Europe and had an extended stay in Darmstadt with Professor Beno Gutenberg, who in 1914 had determined the depth of Earth’s core. Based on her studies of seismology, Lehmann in 1928 acquired the magister scientiarum degree (equivalent to an MA) in geodesy. The same year she was appointed state geodesist at the Danish Geodetic Institute, which had been established with the merging of Gradmaalingen and the general staff ’s Topographic Department in early 1928.
In 1927 Lehmann participated in the meeting of the International Union of Geodesy and Geophysics in Prague, even though “it was not customary for a person in my position,” as Lehmann wrote in “Seismology in the Days of Old” (1987, p. 33). Both Gutenberg and the renowned British seismologist Harold Jeffreys participated, and an important topic at the conference was the determination of travel times of seismic waves through the interior of Earth. Many attempts had been made to construct travel-time curves that described the travel time of seismic waves as a function of epicentral distance. At the heart of these attempts were problems with the accuracy of seismographic measurements. The network of seismological stations was uneven, and the instrumentation and methods of reading seismograms were very heterogeneous. Lehmann realized that an assessment of the accuracy of different stations and a consistent interpretation of seismograms was decisive in the construction of travel-time curves. She took on the task of evaluating the European stations, concluding that Copenhagen’s was among five particularly accurate stations on the continent. She also made the thorough and consistent analysis of seismograms from several stations the trademark of her own research, and within a few years her hard work would result in an important discovery.
The Inner Core . Following the conference in Prague, Lehmann had a lively correspondence with Harold Jeffreys on the problem of travel-time curves. Lehmann was interested in how the observations of seismic waves varied with epicentral distance, a relationship that is also reflected in the travel-time curves. After an earthquake, two main types of seismic waves that have traversed Earth’s interior are observed: compressional P-waves or pressure waves, and transverse S-waves or shear waves. Because P-waves have higher velocity than S-waves in Earth’s mantle, P- waves arrive ahead of S-waves. Both P- and S-waves are observed up to an epicentral distance around 103°. Above this epicentral distance no waves are observed until about 143°, and after this point only P- waves are seen. (These waves are called PKP or P’ indicating that they have passed through Earth’s core). The reason for this is that seismic waves have a lower velocity inside the core, and therefore the core acts as a converging lens, thus making a shadow zone between 103° and 143°, where no direct P-waves are observed. The absence of S- waves also above an epicentral distance of 143° was taken as evidence of a fluid core, since transverse waves cannot penetrate a fluid medium.
All this had been known since 1914, when Gutenberg had used these observations to estimate the depth of Earth’s core at 2,900 kilometers (1,800 mi). But in the 1920s many observations of weaker phases of P’ between 103° and 143° were made. Lehmann discussed these observations in a letter to Jeffreys in May 1932: “But it remains to explain P’ at smaller distances [than 143°]. I suppose they could be explained by the assumption of a discontinuity surface within the core at which the velocity increases. There is hardly anything to disprove the existence of such a surface in present observational data.” (Hjortenberg & Larsen, 2004). Despite this early hint of a possible inner core, Jeffreys indicated in the Jeffreys-Bullen travel-time tables that were published in 1935 that the P’-waves between 103° and 143° were caused by a diffraction phenomenon on the surface of the core. Gutenberg had in his travel-time tables made a similar interpretation of the unexpected P’. He designated them gebeugte Wellen (bent waves) with no further explanation of their origin.
The problem of the unexplained P’-waves had not been considered very seriously, because the amplitudes of the observed phases had been rather small. Lehmann, though, in her meticulous examination of seismograms from all over Europe of a New Zealand earthquake in 1929, realized that the reason for the weak P’-phase was that at many stations, only the horizontal component had been measured. The vertical component, by contrast, turned out to be significant, and Lehmann rejected the possibility that it could be caused by diffraction. She instead hypothesized that inside the core there is an inner core, in which the velocity of seismic waves is higher than in the outer core. The P’-waves observed in the shadow zone would then be caused by P’-waves being refracted at the inner core. Lehmann supported her hypothesis by thorough analysis of seismograms from four different stations, and in her 1936 article with the conspicuously short title “P’,” she concluded: “It cannot be maintained that the interpretation here given is correct since the data are quite insufficient.… However, the interpretation seems possible, and the assumption of the existence of an inner core is, at least, not contradicted by the observations; these are, perhaps, more easily explained on this assumption” (p. 115).
Gutenberg quickly recognized Lehmann’s discovery of an inner core, while Jeffreys was more reluctant. Within a few years, though, Jeffreys showed that the diffraction theory could not explain the observations and accepted the inner core. In 1938 Beno Gutenberg and Charles F. Richter determined the radius of the inner core at 1,200 kilometers (750 mi) as well as the velocity of P- waves in the inner core at 11.2 kilometers per second (6.9 miles per second). Lehmann’s hypothesis was thus accepted broadly in the seismological community within a few years.
Earth’s Upper Mantle . Inge Lehmann frequently participated in international meetings, and she engaged actively in several societies. She regularly attended meetings of the International Union of Geodesy and Geophysics, beginning with the Prague meeting in 1927. In 1936 she was one of the founders of the Danish Geophysical Society, and she chaired the organization in 1941 and 1944. She also participated in the establishment of the European Seismological Federation (ESF) in 1950 and was elected its first president. The European Seismological Commission, of which she was a member, succeeded ESF in 1951.
In 1951 Maurice Ewing of Lamont Geological Observatory in Palisades, New York, visited the seismological station in Copenhagen. Ewing was a close friend of Lehmann and valued her special skills in reading seismograms. He invited her to come to Lamont to do research on a newly discovered surface wave, Lg (a transverse surface wave in Earth’s crust. Lehmann went to Lamont for several months in 1952, bringing European seismograms to compare with the American observations of Lg. She succeeded in estimating travel times for Lg and demonstrated significant differences in the European and American records, which reflected different structures of the upper mantle under each continent. The same year Lehmann was considered for a professorship in geophysics at Copenhagen University. She was judged fully qualified for the position by the evaluation committee, but Niels Bohr, who was on the committee, had his own candidate for the position. Other members of the committee did not find Bohr’s candidate nearly as qualified as Lehmann, and the result was that neither got the position, which was not filled until a decade later. This must have been a disappointment to Lehmann, and might have been part of the reason she retired from her position at the Geodetic Institute in 1953, five years before the mandatory retirement age of seventy.
Retirement for Lehmann was not retirement from research. Instead, it opened new opportunities for research and international collaboration. As Francis Birch said when Lehmann was awarded the American Geophysical Union’s Bowie Medal in 1971: “Since her retirement from the Geodetic Institute, Dr. Lehmann has increased her rate of publication, which is understandable, since she no longer has to worry about keeping someone on the job at Scoresbysund [Ittoqqortoormiit]!” (Bolt and Hjortenberg, 1994, p. 231). In the 1950s and 1960s Lehmann had many extended stays in the United States at the Lamont Observatory in New York State, at the Seismographic Stations at the University of California at Berkeley, and with Gutenberg at the Seismological Laboratory in Pasadena, California. She also spent time at the Dominion Observatory in Ottawa, Canada.
Lehmann’s stays in North America coincided with a period when seismology began to receive much attention after decades of neglect. As a response to the shift from atmospheric to underground testing of nuclear bombs, a research program named Vela Uniform was established, to develop improved and standardized seismographs to detect underground explosions. These standardized instruments were installed at two hundred seismological stations around the world, constituting the Worldwide Standardized Seismographic Network. The combination of more accurate measurements and Lehmann’s unique analytical skills made possible more detailed analysis of Earth’s upper mantle. Also, Lehmann benefited from the measurements of the seismic waves caused by underground nuclear explosions. Because their time and place of origin were well defined, travel times could be determined very accurately. Lehmann paid particular attention to measurements of S and P-waves at small epicentral distances and found evidence of a low velocity layer beginning a little below 100 kilometers’s (62 mi’s) depth and extending down to a depth of around 220 kilometers. At the bottom of this low velocity layer Lehmann found indications of an abrupt velocity increase. Jeffreys had already found indications of a change in the velocity gradient at a depth of 220 kilometers (136 mi), but Lehmann showed that there was actually a discontinuity in the velocity profile. This discontinuity, as well as the one at the surface of the inner core, have become known as Lehmann discontinuities.
Skills and Personal Qualities . As a seismologist Lehmann stood out with her exceptional analytical skills and her ability to identify and compare phases in seismograms from stations all over the world. Perhaps her aunt’s grandson, Niels Groes, has described these qualities the best:
I remember Inge one Sunday in her beloved garden on Søbakkevej; it was in the summer, and she sat on the lawn at a big table, filled with cardboard oatmeal boxes. In the boxes were cardboard cards with information on earthquakes and the times for their registration all over the world. I remember Inge With her cardboard cards and her oatmeal boxes, Inge registered the velocity of propagation of the earthquakes to all parts of the globe. By means of this information, she deduced new theories of the inner parts of the Earth. (Bolt, 1997)
Lehmann was also a very shy person and disliked being the center of attention. When in Denmark she enjoyed spending time at her quiet summer cottage, where many colleagues visited her over the years. Despite her shyness she maintained an extensive network of colleagues, and many considered her a close friend. She was a very active person, loved to ski in the Alps or Norway, and to climb mountains in the summertime.
As a woman in a male-dominated science, Lehmann’s career was often an uphill struggle. Groes reports that she said, “You should know how many incompetent men I had to compete with—in vain” (Bolt, 1997).
Lehmann wrote her last article, “Seismology in the Days of Old,” in 1987 at the impressive age of ninety-nine. The following year she celebrated her 100th birthday at a reception at the Geodetic Institute, attended by several internationally renowned geophysicists. In February 1993, Lehmann died at the age of 104.
Honors and Awards . During her career Lehmann received great international recognition. From 1936 to 1948 she was a member of the executive committee of the International Seismological Association and in two periods, 1951–1954 and 1957–1960, was a member of the executive committee of the International Association of Seismology and Physics of Earth’s Interior (IASPEI).
From 1963 to 1967 she was vice president of the executive committee of IASPEI. She was elected associate of the Royal Astronomical Society, London, in 1957 and Honorary Fellow of the Royal Society, Edinburgh, in 1959.
Lehmann received several travel awards, and in an unusual honor, she received the Danish Tagea Brandt travel award twice, in 1938 and 1967. In 1964 she received the Deutsche Geophysikalische Gesellschaft’s (German Geophysical Society’s) Emil-Wiechert Medal and in 1965 was awarded the Gold Medal from the Royal Danish Academy of Sciences and Letters. In 1971 she received the American Geophysical Union’s Bowie Medal for “outstanding contributions to fundamental geophysics and unselfish cooperation in research” in 1971. Finally, in 1977 she received the Medal of the Seismological Society of America. Though her recognition in Denmark came late, she was especially pleased when she received an honorary doctor of philosophy degree at Copenhagen University in 1968. In 1964 she received the honorary degree of doctor of science at Columbia University.
Lehmann’s name lives on in two awards. One is the Inge Lehmann Medal, which was established by the American Geophysical Union in 1997 and is awarded every other year for “outstanding contributions toward the understanding of the structure, composition, and/or dynamics of Earth’s mantle and core.” The other is a travel award, which was instituted by Lehmann herself and given in alternate years to a psychologist and a geophysicist.
Inge Lehmann bequeathed her personal papers to her colleague Erik Hjortenberg, who has systematized and scanned thedocuments. The material is available through the Danish state archives and through Storia Geofisica Ambiente in Bologna, Italy. A complete list of Inge Lehmann’s publications is given in Bolt (1997).
WORKS BY LEHMANN
“P.’” Publications du Bureau Central Seismologique International, series A 14 (1936): 87–115.
“Rektor Hanna Adler in Memoriam.” Kvinden og Samfundet 63 (1947): 29.
“S and the Structure of the Upper Mantle.” Geophysical Journal of the Royal Astronomical Society 4 (1961): 124–138.
“Recent Studies of Body Waves in the Mantle of Earth.” Quarterly Journal of the Royal Astronomical Society 3 (1962): 288–298.
“Seismology in the Days of Old.” Eos 68, no. 3 (1987): 33–35.
Bolt, Bruce A. “Inge Lehmann. 13 May 1888–21 February 1993.” Biographical Memoirs of Fellows of the Royal Society 43 (1997): 286–301.
Bolt, Bruce A., and Erik Hjortenberg. “Memorial Essay, Inge Lehmann (1888–1993).” Bulletin of the Seismological Society of America 84, no. 1 (1994): 229–233.
Brush, Stephen G. “Discovery of the Earth’s Core.” American Journal of Physics 48, no. 9 (1980): 705–724.
Hjortenberg, Erik, and Tine B. Larsen. “The Scientific Correspondence between Inge Lehmann and Harold Jeffreys.” Poster at the European Seismological Commission, XXIX General Assembly, Potsdam, Germany. September 2004.
Kölbl-Ebert, Martina. “Inge Lehmann’s Paper: ‘P’” (1936).” Episodes 24, no. 4 (2001): 262–267.
Lehmann, Inge (1888-1993)
Lehmann, Inge (1888-1993)
Trained as a mathematician and an actuary, Danish geophysicist Inge Lehmann used painstaking analyses, measurements and observations of shock waves generated by earthquakes to propose in 1936 that the earth had a solid inner core. Throughout her long career, which extended far beyond her official retirement in 1953, Lehmann conducted research in Europe and North America and was active in international scientific organizations including serving as the first president and a founder of the European Seismological Federation.
Lehmann was one of two daughters born to Alfred Georg Ludvig Lehmann, a University of Copenhagen professor of psychology, and Ida Sophie Torsleff. As a child, she attended and graduated from the first coeducational school in Denmark, an institution founded and run by Hanna Adler, the aunt of future Nobel Prize winning physicist Niels Bohr . She began her university education by studying mathematics at the University of Copenhagen from 1907 to 1910. She continued her mathematical studies the following year at Cambridge University in England before returning to Denmark, where she worked as an actuary from 1912 to 1918. She also continued her formal education. In 1920, Lehmann earned her masters degree in mathematics from the University of Copenhagen and later studied mathematics at the University of Hamburg. In 1925, Lehmann began her career in seismology as a member of the Royal Danish Geodetic Institute and helped install the first seismographs at her Copenhagen office. "I was thrilled by the idea that these instruments could help us to explore the interior of the earth, and I began to read about it," she was quoted in a 1982 article published in the Journal of Geological Education. Lehmann later helped establish seismograph stations in Denmark and Greenland.
After further study with seismologists in France, Germany, Belgium, and the Netherlands, and after earning a M.S. degree in geodesy from the University of Copenhagen in 1928, Lehmann was named chief of the Royal Danish Geodetic Institute. In that position, held until her retirement in 1953, Lehmann was Denmark's only seismologist for more than two decades. She was responsible for supervising the Denmark's seismology program, overseeing the operation of the seismograph stations in Denmark and Greenland, and preparing the institute's bulletins.
Despite this heavy workload, Lehmann still found time to explore scientific research. In 1936, she published her most significant finding, the discovery of the earth's inner core, under the simple title of "P." The letter P stood for three types of waves generated by Pacific earthquakes that Lehmann had been carefully observing through the planet for ten years. By studying the shock waves generated by earthquakes, recorded on seismographs as travel-time curves, she theorized that the earth has a smaller solid inner core. Within a few years, work by other scientists, including Harold Jeffreys and Beno Gutenberg, substantiated her findings.
Lehmann continued her research well after her retirement in 1953, exploring the nature of the planet's interior in Denmark, in Canada at the Dominion Observatory in Ottawa and in the United States at the University of California at Berkeley, the California Institute of Technology, and the Lamont Doherty Earth Observatory at Columbia University. She was a named a fellow of both the Royal Society of London and Edinburgh and was named to the Royal Danish Academy of Science and Letters and the Deutsche Geophysikalische Gesellschaft. In 1971, she was awarded the William Bowie Medal of the American Geophysical Union in recognition of her "outstanding contributions to fundamental geophysics and unselfish cooperation in research." She was also awarded honorary doctorates by the University of Copenhagen and Columbia University.
Lehmann remained single throughout her long and productive life. Her interests were not restricted to science. She was concerned with the poor in her native Denmark and the plight of European refugees. Travel in conjunction with her work also afforded her frequent opportunities to pursue two of her hobbies—visiting art galleries throughout Europe and the United States, and the outdoors. Lehmann enjoyed hiking, mountain climbing, and skiing. She died at the age of 105.