Inge Lehmann


by Bruce A. Bolt from Physics Today, 1994

Inge_Lehmann_1932Inge Lehmann died in Copenhagen on 21 February 1993, three months short of age 105. Born in 0sterbro, Denmark, Lehmann graduated in 1920 from the University of Copenhagen with a degree in mathematics and physical science. In 1928, she was appointed chief of the newly formed seismological department of the Danish Geodetic Institute, a post she held until her retirement in 1953.
In 1936 she discovered the Earth’s inner core. Her proposal, published in the Bureau Central Seismoloque International, Travaux Scientifique (14, 87, 1936), was intended to avoid difficulties that arose in interpreting the observed arrival times of seismic core waves when one assumed only a single terrestrial core.
A two-shell Earth with a liquid core gives rise to refraction phenomena analogous to the optical caustic
formed by a spherical lens. It was known by the middle 1930s that the seismic-wave caustic came at approximately 145° epicentral distance, that is, the angle between the earthquake center and the recording seismograph, subtended at the Earth’s center, yet core waves could be clearly observed on seismograms from the antipodes back to distances smaller than 145°. Lehmann was in an excellent situation to observe those waves, because the seismographs of her far-flung Danish network were located at such epicentral distances from large earthquake sources in the South Pacific. Her observations with the network enabled her to make the necessary imaginative jump.
In effect, Lehmann proved an ex- istence theorem: namely, that one can find a plausible tripartite structure for the Earth that explains the main features of the observed core waves. The inverse problem was to use the observed travel times to estimate statistically the inner-core parameters that satisfied them within the measurement uncertainties. This final step was done in 1938 by Beno Gutenberg and Charles F. Richter and, independently, in 1939 by Harold Jeffreys, who had shown already in 1926 that the (outer) core is liquid. In the next few years, arguments by Frances Birch and Keith Bullen established that the increase in P-wave velocity at the Lehmann inner-core boundary was such that the inner core had to be solid if the pressure-induced gradient in elastic incompressibility was to be realistic.
Lehmann’s retirement opened up a new active period of research. One of the research centers she enjoyed visiting during this period was Lamont Geological Observatory (now Lamont-Doherty Earth Observatory), in Palisades, New York. She explored in particular the structure of the upper mantle, using both P and S waves from underground nu- clear explosions and earthquakes. Her work with nuclear explosions was important at a time of much associated political controversy over the validity of monitoring criteria for
a comprehensive test ban, not so much because she found something new but because her stature and integrity provided high respectability and credibility to the independent studies she supported.
The pioneering work of Inge Lehmann was a product of several qualities of a remarkable woman. Those of us who were fortunate to know her as friends and colleagues will always think of her as a strong and independent person, with great physical energy (exemplified by many mountaineering trips to the Alps) and personal warmth.

Bolt, B. (1994). Inge Lehmann Physics Today, 47 (1) DOI: 10.1063/1.2808386


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