page_title: Inge Lehmann
section_title: Career
hierarchical_section_title: Inge Lehmann / Career
caption_attribution_description: English: S-wave velocity near Earth's surface for three tectonic provinces: TNA = Tectonic North America SNA= Shield North America ATL = North Atlantic Français&#160;: Vitesse de propagation des ondes S dans le manteau supérieur, dans trois régions typiques&#160;: TNA = province tectonique nord-américaine, SNA = bouclier nord-américain, ATL = Atlantique Nord
context_page_description: Inge Lehmann ForMemRS was a Danish seismologist and geophysicist. In 1936, she discovered that the Earth has a solid inner core inside a molten outer core. Before that, seismologists believed Earth's core to be a single molten sphere, being unable, however, to explain careful measurements of seismic waves from earthquakes, which were inconsistent with this idea. Lehmann analysed the seismic wave measurements and concluded that Earth must have a solid inner core and a molten outer core to produce seismic waves that matched the measurements. Other seismologists tested and then accepted Lehmann's explanation. Lehmann was also one of the longest-lived woman scientists, having lived for over 104 years.
context_section_description: In 1925 Lehmann's seismology career began as she became an assistant to the geodesist Niels Erik Nørlund. She was paired with three other assistants who had never so much as seen a seismograph before. She began the task of setting up seismological observatories in Denmark and Greenland. In the meantime, she studied seismology on her own. She went abroad for three months to study seismology with leading experts in the field such as Beno Gutenberg, who had determined the distance to the core-mantle boundary within 15 km of the presently accepted value. 
  Based on her studies in seismology, in 1928 she earned the magister scientiarum degree (equivalent to an MA) in geodesy and accepted a position as state geodesist and head of the department of seismology at the Geodetical Institute of Denmark led by Nørlund. Lehmann looked into improving the co-ordination and analysis of measurements from Europe's seismographic observatories, as well as many other scientific endeavours. These improvements lay at the heart of her later discoveries. 
  In a paper titled P' (1936), Lehmann was 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, for example from the strong 1929 Murchison earthquake. Other leading seismologists of the time, such as Beno Gutenberg, Charles Richter, and Harold Jeffreys, adopted this interpretation within two or three years, but it took until 1971 for the interpretation to be shown correct by computer calculations. Lehmann was significantly hampered in her work and maintaining international contacts during the German occupation of Denmark in World War II. She served as the Chair of the Danish Geophysical Society in 1940 and 1944 respectively.
  In 1952, Lehmann was considered for a professorship in geophysics at Copenhagen University, but was not appointed. In 1953, she retired from her position at the Geodetic Institute. She moved to the US for several years and collaborated with Maurice Ewing and Frank Press on investigations of Earth's crust and upper mantle. During this work, she discovered another seismic discontinuity, which are a step-change increase in the speeds of seismic waves at depths between 190 and 250 km. This discontinuity was named after her, being coined as the Lehmann discontinuity. Francis Birch noted that the "Lehmann discontinuity was discovered through exacting scrutiny of seismic records by a master of a black art for which no amount of computerization is likely to be a complete substitute."
caption: A modern understanding of the Lehmann discontinuity: velocity of seismic S-waves in the Earth near the surface in three tectonic provinces: TNA = Tectonic North America SNA = Shield North America and ATL = North Atlantic.[10]
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