Inge Lehmann - Seismologist, Family and Family

Inge Lehmann was born on May 13, 1888 in Osterbro, Copenhangen, to Alfred Georg Ludvik Lehmann and Ida Sophie Tørsleff. Her father was an experimental psychologist by profession.

Lehmann was educated at pedagogically progressive high school under Hanna Adler. The school was one-of-its-kind for despite being a co-educational, no differentiation based on sex was made. Girls and boys enjoyed equal status. Adler cast an important influence on the mind of young Lehmann, as did her father.

Completing her preliminary education, in 1907, Lehmann gained admission at the University of Copenhagen, studying mathematics. However, her frail health partially interrupted her studies. In 1910, she gained admission at the Newnham College, at the University of Cambridge continuing her study of mathematics.

In 1911, Lehmann returned from Cambridge. Her frail health did not allow her to study any further, as she suffered from fatigue and lassitude. She temporarily took up work in an actuary’s office where she developed good computational skills.

In 1918, Lehmann resumed her studies at the Copenhagen University. After two years of study, she finally completed candidatus magisterii degree in physical science and mathematics

In the autumn of 1922, Lehmann studied mathematics with Professor Wilhelm Blaschke, in Hamburg, Germany only to return to Denmark a year later. She accepted the position of assistant J.F. Steffensen, the professor of actuarial science, at Copenhagen University.

Year 1925 marked the turning point in Lehmann’s career. She was appointed as an assistant to Niels Erik Nørlund, the newly appointed director of Gradmaalingen, a Danish geodetic institution. Nørlund planned to set up seismological observatories in Denmark and Greenland and wanted Lehmann to run the stations and interpret and publish the observations.

In her new profile as Nørlund’s assistant, Lehmann supervised the establishment of one seismological station in Copenhagen. She also helped prepare instruments for two other stations in Greenland. Though Lehmann’s job description did not include research work, she was free to indulge in it.

In 1927, Lehmann visited the seismological stations across Europe. Same year, she attended an International Union of Geodesy and Geophysics in Prague. The conference was attended by the who’s who of seismology, including Gutenberg and Harold Jeffreys. From the conference, Lehmann comprehended that to construct a time travel curve, an assessment of the accuracy of different stations and a consistent interpretation of seismograms was decisive.

In 1928, based on her studies, she gained the magister scientiarum degree (equivalent to an MA) in geodesy. Same year, she was appointed state geodesist head of the department of seismology at the Geodetical Institute of Denmark.

Over time, scientists had realized the fact that seismographic data from Earthquakes could be used to deduce what sort of stuff the interior of the Earth was made up of. Lehmann was inquisitive about how observations of seismic waves varied with epicentral distance, a relationship that was also reflected in the travel-time curves.

Through research, scientists found out that there were two types of seismic waves that transverse the Earth, P waves or pressure waves and S waves or shear waves. Like other scientists, Lehmann too was puzzled by the fact that the P waves were not traveling through Earth in the way they were expected to. In fact, they were appearing in locations they weren’t supposed to.

In an attempt to come out of the P wave confusion, Lehmann developed mathematical models of the Earth in which she featured a solid inner core. She concluded that P-waves were appearing in unexpected locations because they were being refracted and reflected to these locations by the boundary between the Earth’s solid inner core and liquid outer core. The inner core, she calculated, had a radius of about 1400 km. She became the first to interpret P wave arrivals which inexplicably appeared in the P wave shadow of the Earth's core as reflections from an inner core. All these findings were published in a paper entitled, ‘P’, in 1936.

After the broad acceptance of Lehmann’s hypothesis that the Earth’s core was solid and surrounded by molten core, she became a popular figure in the seismological meets. She frequently participated in international meetings, and engaged herself actively in several societies.

In 1936, Lehmann served as one of the founders of the Danish Geophysical Society. She chaired the organization twice in 1941 and 1944. She also participated in the establishment of the European Seismological Federation (ESF) in 1950 and was elected its first president..

In 1952, Lehmann was considered for professorship in geophysics at Copenhagen University, but was not appointed. The following year, she retired from her position at the Geodetic Institute.

The retirement from Geodetic Institute did not mean end of career for Lehmann. Instead it only freed her from administrative work so as to fully concentrate on scientific research. She moved to the US where she collaborated with Maurice Ewing and Frank Press, investigating Earth's crust and upper mantle. It was during this time that she discovered another seismic discontinuity, which lay at depths between 190 and 250 km. This seismic discontinuity was eventually named after her.

In 1987, at the age of 99, Lehmann penned her last scientific article titled ‘Seismology in the Days of Old’. Following year, she celebrated her 100th birthday at her former workplace, the Geodetic Institute, amongst internationally renowned geophysicists.

Lehmann’s most important contribution to the world of science has been in the field of seismology. Through her extensive research and investigation, she brought forth the fact that the inner core of the Earth is solid, rather than the presumed belief of its being molten. She further researched and determined that the solid inner core has a liquid outer core that in turn explained the transits of seismic waves. Using mathematical concepts, she analysed the way energy was released by Earthquakes and travelled through the Earth. Other than being discoverer of the Earth’s core, Lehmann also figured out a low velocity layer between 190 and 250 km below the Earth’s surface. This seismic discontinuity was later renamed Lehmann discontinuity.

Lehmann was honoured with numerous awards and honors for her outstanding scientific achievements. She received the Tagea Brandt Award in 1938, the Gordon Wood Award in 1960, the Emil Wiechert Medal from the German Geophysical Society in 1964, Gold Medal from the Danish Royal Society in 1965, the William Bowie Medal in 1971 and Medal of the Seismological Society of America in 1977.

She twice chaired the Danish Geophysical Society in her scientific career, once in 1941 and later in 1944.

In 1950, she served as the President of the European Seismological Federation. In 1957, she was elected associate of the Royal Astronomical Society, London. In 1959, she became Honorary Fellow of the Royal Society, Edinburgh.

From 1963 to 1967, she served as the vice president of the executive committee of IASPEI.

In her lifetime, she held numerous honorific memberships of various scientific societies across the world. From 1936 to 1948 she was a member of the executive committee of the International Seismological Association and in two periods, from 1951 to 1954 and later from 1957 to 1960, she was a member of the executive committee of the International Association of Seismology and Physics of Earth’s Interior (IASPEI).

In 1969, she was elected as the Fellow of the British Royal Society.

Inge Lehmann never married nor had any children.

She breathed her last on February 21, 1993 at the age of 104. She left all her possessions to the Danish Academy.

Posthumously, to mark the contribution made by Lehmann in the field of seismology, the American Geophysical Union established the Inge Lehmann Medal in 1997. The medal is awarded for outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core, every year.