Inge Lehmann

Inge Lehmann (born May 13, 1888, Copenhagen, Denmark—died February 21, 1993, Copenhagen) was a Danish seismologist best known for her discovery of the inner core of Earth in 1936 by using seismic wave data. Two boundary regions, or discontinuities, are named for her: one Lehmann discontinuity occurs between Earth’s inner and outer core at a depth of roughly 5,100 km (about 3,200 miles), and the other occurs at a depth of approximately 200 km (about 120 miles) beneath Earth’s surface in the upper mantle.

Inge Lehmann was born to Alfred Lehmann, a psychology professor, and Ida Tørsleff. Growing up in Copenhagen, she attended a high school that treated girls and boys equally—a progressive idea at the time. In 1907 she began her study of mathematics at the University of Copenhagen, intending to obtain a candidata magisterii (cand.mag.; comparable to a master’s degree). From the autumn of 1910 to December 1911, Lehmann attended Newnham College, Cambridge, but fatigue and overwork forced her return to Copenhagen.

Lehmann did not attend school between 1911 and 1918, instead serving as an actuarial assistant. She returned to the University of Copenhagen in 1918 and graduated with a cand.mag. in mathematics in 1920. She continued studying mathematics at the University of Hamburg during the fall of 1922, before taking another position as an actuarial assistant in 1923, this time working with a professor in the actuarial science department at the University of Copenhagen. In 1925 she became an assistant to the head of the Royal Danish Geodetic Institute, and part of her work involved setting up Denmark’s first seismic stations near Copenhagen, as well as in Ivigtut and Scoresbysund (now Ittoqqortoomiit), Greenland. Because of her growing interest in that topic, she again enrolled in the University of Copenhagen and studied seismology during the summer of 1927, later graduating with a magister scientiarum (master of science) in 1928. That same year Lehmann was appointed as the state geodesist and was made the head of the Seismological Department of the Royal Danish Geodetic Institute. She held the latter post until her retirement in 1953.

The bulk of Lehmann’s work at the Seismological Department invlved managing the seismological stations both in Denmark and in Greenland, as well as collecting seismograph information and creating the bulletins associated with the stations. She became interested in determining the location of earthquake epicentres more accurately from the data her seismographs provided. She did so by correlating the primary seismic wave forms collected.. She was also interested in calculating the travel times of various types of seismic waves through the planet.

In 1929, while examining seismograph data collected after a large earthquake in New Zealand, Lehmann noticed that seismographs stationed in the Russian cities of Swerdlowsk (Yekaterinburg) and Irkutsk collected seismic waves with amplitudes that were higher than she had expected. She also discovered that some waves traveling away from the earthquake’s focus appeared to have been “bent.” It was known at the time that Earth’s core deflected secondary (S) waves and some primary (P) waves—thereby creating shadow zones behind the core—as those waves traveled outward from an earthquake’s focus to its antipode on the other side of the planet. In 1936, Lehmann published her findings in a paper that posited a three-shelled model of Earth’s interior (which was made up of the mantle, outer core, and inner core), with seismic waves traveling through each shell at different but constant velocities. The model included Earth’s core but also postulated the existence of an inner core. It was not until 1970 that advances in seismographs provided unequivocal evidence of the inner core’s existence. The boundary between the inner and outer core, which occurs at a depth of roughly 5,100 km (about 3,200 miles), is known as the Lehmann discontinuity.

Lehmann is also known for researching Earth’s mantle. Working with American seismologist Beno Gutenberg in 1954, she noticed the existence of a region in Earth’s upper mantle in which seismic waves travel faster. That region, which spans perhaps 50 km (about 31 miles) and is also known as the Lehmann discontinuity, occurs about 200 km (120 miles) below Earth’s surface.

In addition to her discoveries, Lehmann cofounded the Danish Geophysical Society (1936) and chaired the organization in 1941 and 1944. She was awarded the William Bowie Medal of the American Geophysical Union in 1971 for her contributions in the field of geophysics and received the Medal of the Seismological Society of America in 1977. The American Geophysical Union created the Inge Lehmann Medal in her honour in 1995, and, starting in 1997, it was awarded to researchers displaying “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.”