Journey to the Center of the Earth

*Special thanks to Liz Boatman for the inspiration for this Photos of the Month from the article “September 1936: Seismologist Inge Lehmann Concludes That Earth Has an Inner Core .”

Many things happen in September. It’s a time to go back to school, to look forward to a change in the weather, to do fall gardening, to plan Halloween costumes, and it’s a time to remember Inge Lehmann’s groundbreaking paper that concludes that the Earth has a solid inner core.

The year was 1936. Thoughts of what might be under the Earth’s crust were present in the public cultural imagination, thanks in part to Jules Verne’s classic and other works of late 19th century subterranean fiction (Will Harben’s Land of the Changing Sun (1894), Gabriel Tarde’s Underground Man (1896), Charles Beale’s The Secret of the Earth from 1899, to name a few).The late 19th century and turn of the 20th was also an important time in the history of seismology. According to Ari Ben-Menahem , seismology “aims simultaneously to obtain the infrastructure of the Earth’s interior with the aid of seismic wave phenomena, and to study the nature of earthquake sources with the ultimate goal of mitigating and eventually controlling the phenomenon.” In other words, seismologists concern themselves with the study of earthquakes and what is under the Earth’s crust. The first seismometer (an instrument with a pendulum or spring that can record earthquakes, volcanic eruptions, and explosions) was designed by James Forbes in 1841. In 1894, the term seismograph appeared, which is now often interchangeable with seismometer, to describe the physical recording of ground displacements. By the early 20th century, scientists were able to tell through seismographs that all was not solid under the Earth’s crust.

Inge Lehmann (1888-1993) was a Danish mathematician based at the Danish geodetic institute, the Den Danske Gradmaaling. She and other scientists in the 1920s and 1930s were studying seismographic data and, though the general consensus was that there was liquid under the Earth’s crust, not all of the data from these seismographs made sense. Lehmann came up with the idea that there could be a solid core inside the liquid core that accounted for the discrepancies in the data. She published a paper titled “P” explaining the idea in September of 1936. For a more detailed explanation, please read Liz Boatman’s excellent article “September 1936: Seismologist Inge Lehmann Concludes That Earth Has an Inner Core ” in APSNews on this subject.

For this September Photos of the Month, please enjoy this exploration of images about Inge Lehmann and seismology in the 20th and 21st centuries from the Emilio Segrè Visual Archives.

Here is the lady herself! Inge Lehmann was born in Copenhagen in 1888 and attended a primary school where boys and girls were treated the same, which was unusual at the time. As a seismologist - she referred to herself as “the only Danish seismologist,” which tells you something about how small and new the field was - she became an expert on the Earth’s mantle and conducted research across the world. She was awarded the prestigious William Bowie medal from the American Geophysical Union in 1971, and she lived to be 105 years old.

Harold Jeffreys was the British seismologist who concluded that the Earth has a liquid core in 1926, prior to Lehmann’s discovery. When Lehmann published her 1936 paper, Jeffreys originally was not persuaded by her inner solid core model, but, by 1939, with more detailed information from better seismographs, he eventually supported the conclusion. Jeffreys also made contributions to astronomy; he calculated that the temperatures of several planets of Jupiter were much cooler than previously thought.

As you can see from this image, he also wore a lei on at least one occasion.

Niels Bohr (1885-1962) was not a seismologist. However, he crossed paths with Inge Lehmann in at least three ways:

1. He was Danish and he lived and worked in Copenhagen around the same time as Inge Lehmann. Niels Bohr’s aunt headed Inge Lehmann’s primary school.
2. Both scientists had connections to Copenhagen University. They both studied at the university as undergraduates around the same time and both ended up working there later in life.
3. In 1962, Harold Jeffreys wrote to Niels Bohr about Inge Lehmann’s discovery of the solid inner core, and said that she deserved to be honored in her native Denmark. Perhaps because of this support, the 1965 Gold Medal of the Danish Academy of Sciences and Letters went to Lehmann.

Speaking of awards! Inge Lehmann was honored by the creation of the Inge Lehmann Medal award. The first award was presented in 1997 and is awarded on an annual basis to “a senior scientist in recognition of outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core. Recipients of this award typically conduct research in the following disciplines: geomagnetism, paleomagnetism and electromagnetism, mineral rock physics, seismology, study of the Earth’s deep interior, and tectonophysics.”

This is an undated photograph of 2007 awardee Ho-Kwang (Dave) Mao, a Chinese-American geologist whose work specializes in high pressure geosciences. He has two minerals named after him: Davemaoite and Maohokite.

In addition to people, the Emilio Segrè Visual Archives also has images of equipment and graphs! This is an image of a seismogram from one of my personal favorite image collections within the ESVA: the USNS Kane Collection. Check out Joanna Behrman’s December 2021 Photos of the Month for more on this collection.

In addition to geology, seismology has been important to the field of oceanography. Aboard the USNS Kane, Marie Tharp and Bruce Heezen used a seismograph for research on plate tectonics and the ocean floor. As Joanna states , “together they did groundbreaking work confirming the theory of continental drift and leading to the overall theory of plate tectonics.” They created the first scientific maps of the ocean floor, which were published in the 1950s, 1960s, and 1970s.

During his career, Carl Kisslinger (left) did significant research on earthquake prediction, remote triggering of earthquakes, earthquake hazard analysis, earthquake aftershocks, and earthquake fault zones processes. Learn more in his oral history interview .

Otto Nutli (right) was a distinguished seismologist. According to his memorial page in the Bulletin of the Seismological Society of America, “he and his colleagues made special efforts to develop strong motion ground and scaling models for eastern North America, and he was called upon to serve as a spokesman on the earthquake risk problem in the Eastern United States by numerous government panels and committees.”

Here we get to see another representation of a seismogram - this time in color! Brian Kennett is known for his contributions to research on the Earth’s internal structure and his work on the Earth’s mantle in Australia.

References and Further Reading

Behrman, Joanna. “Photos of the Month: The USNS Kane.” Ex Libris Universum. (10 December 2021). /history-programs/niels-bohr-library/ex-libris-universum/photos-month-usns-kane

Ben-Menahem, Ari. “A Concise History of Mainstream Seismology: Origins, Legacy, and Perspectives.” Bulletin of the Seismological Society of America 85, no. 4, pp. 1202-1225, (August 1995). https://engineering.purdue.edu/~ce597m/Handouts/ConciseHistory_BenMenahem.pdf

Boatman, Liz. “September 1936: Seismologist Inge Lehmann Concludes That Earth Has an Inner Core.” APSNews 32, no. 9. https://www.aps.org/publications/apsnews/202309/history.cfm

“Brian Kennett.” Wikipedia, The Free Encyclopedia, Wikimedia Foundation. 30 July 2023 last edited. https://en.wikipedia.org/wiki/Brian_Kennett

“Carl Kisslinger.” SEG Wiki.5 August 2016 last edited. https://wiki.seg.org/wiki/Carl_Kisslinger

“Ho-Kwang Mao.” Wikipedia, The Free Encyclopedia, Wikimedia Foundation. 28 May 2023 last updated, https://en.wikipedia.org/wiki/Ho-Kwang_Mao

Interview of Carl Kisslinger by Kai-Henrik Barth on 1998 June 17, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/5907

Lehmann, Inge. Publ. Bur. Cent. Seismol. Int. A 14, 87 (1936)

“Marie Tharp.” Wikipedia, The Free Encyclopedia, Wikimedia Foundation. 9 September 2023 last updated, https://en.wikipedia.org/wiki/Marie_Tharp

Mitchell, Brian. “Memorial to Otto W. Nuttli.” Bulletin of the Seismological Society of America 78 no. 3, p. 1387–1389. (1988) https://doi.org/10.1785/BSSA0780031387

Richardson, Eliza.”Sir Harold Jeffreys: Biographical Information.” PennState College of Earth and Mineral Sciences website. https://www.e-education.psu.edu/earth520/content/l2_p12.html