Core Principles: The Life and Work of Seismologist Inge Lehmann.

At 10:17 in the morning on June 17, 1929, a 7.8 magnitude earthquake shook New Zealand’s Murchison region, causing landslides that claimed seventeen lives, and sending seismic P-waves throughout the Earth’s interior, to be picked up by seismology stations scattered across the globe, including to a handful of outposts that, according to everything everybody knew about the inner structure of the Earth, should not have been able to detect the quake. For some, these results were waved away as products of mechanical or user error - after all, what was more likely, that the most advanced existing model of the Earth was wrong, or that something went awry with the machinery over in Azerbaijan?

One person, however, saw the results coming in from Baku and Siberia, and put not only her years of experience in daily direct interpretation of seismographic data, but her deep abilities in mathematical modeling, to work to re-imagine the Earth’s deep interior to create a configuration that would not only allow Azerbaijan to pick up P-waves from Murchison, but require it. She was Inge Lehmann (1888-1993), a woman one year into her directorship of the seismology department at Denmark’s Geodætisk Institut who had spent most of her twenties working in the insurance business as an actuarial assistant before changing her life’s path to the study of the mathematics behind seismic motion within the Earth.

She had been working directly with seismographs and the regular interpretation of their readings since she began working with Niels Erik Nørlund in 1925, and was quickly recognized by some of the leading lights in the seismological field as an expert in the computation of seismic epicenters through data gathered by different instruments around the world. One of her most regular correspondents was Harold Jeffreys, who in 1926 hypothesized that Earth has a liquid core which bends P-waves inwards, creating shadow zones, regions on Earth where a quake’s tremors could not be detected. This theory perfectly explained the mystery of why certain stations could not pick up certain quakes, regardless of how fine their machinery was, and was the last word on the Earth’s inner structure, until Siberia and Azerbaijan, which lay firmly in the hypothetical shadow region of the Murchison quake, detected it on their instruments.

Inge, right, pictured with her sister.

Lehmann, through her personal and direct work on the seismographs in Denmark and the nation’s seismological outpost stations in Greenland which it was her job to run, knew that the readings weren’t simple errors, but that they contained within them the seeds of a new approach to the Earth, that within Jeffreys’s liquid core there must lie another solid core, which was able to re-direct the waves refracted by the molten core and disperse them outwards again into the shadow zone.

Running her models and comparing them minutely against the results from outposts the world over, Lehmann knew that she was right, but the trick was going to be convincing the seismological community of the merit of her ideas. She had not been raised to assert herself. She was the daughter of a philosophically and psychologically inclined father who kept himself distant from his children so he could pursue his life’s work, and a mother who read her own tendency towards nervous exhaustion onto her daughters and was ever encouraging them to Do Less. In this atmosphere of neglect and anxiety, Inge grew up a deeply shy individual, who was as sure of her own intellectual abilities as she was suspicious of her body’s ability to withstand the rigors of her ambitions. 

Her parents were progressive for their age, and signed Inge up for a new co-educational school led by educational innovator Hanna Adler, who became a lifelong mentor to Lehmann. Inge’s aptitude for mathematics was apparent from a young age, and in a decided twist on the story we are used to hearing in tales of Women in Mathematics, her father positively encouraged her in this direction, seeing the practical careers that a training in mathematics would open for her. That approval very nearly produced the opposite of its intended result, as Lehmann considered moving towards a humanities track, perhaps to kick against expectations, but ultimately she realized that, to have a life of maximum impact, she should lean into her gifts rather than fleeing from them, and so she settled into her mathematical future.

In 1907, Lehmann matriculated at the University of Copenhagen, but the continued proximity to her family and the emotional wear and tear resulting therefrom filled her with a desire to continue her studies abroad, and she soon set her sights on Newnham College, that great glimmering tower of women’s mathematical education. Though the cost was not trivial to the family, she was permitted to go in 1910, and discovered there a second version of herself. She was able to shed, at least within the circle of her friends, her shyness, and experience what it was like working with a group of people who were devoted to stretching their minds to their utmost limit.

Ultimately, as many do who find themselves free of familial constraints and surrounded by stimulating ideas, she overdid it, throwing herself into an accelerated track to take the rigorous Tripos mathematical exam while working simultaneously on the English fluency exam that was required for her degree. It was too much all at once, and her mother’s persistent warnings that she was constitutionally too frail for such work could not have helped, and in 1911 Lehmann’s body broke down. She was able to pass her English fluency exam, but the idea of taking the Tripos had to be put on hold, as she made her way back home to heal, reassured by the belief that she would return to pick up where she left off.

In the end, for whatever configuration of practical and personal reasons, that did not happen, and she remained in Copenhagen, where she took her father’s practical advice at last to use her math skills in a sensible and stable profession, working for the insurance industry from 1911 to 1917, when the combination of a failed engagement and the realization that she would never, as a woman, be in line for significant promotions at her company, caused her to take the no doubt painful step of moving back in with her parents, and enrolling at the University of Copenhagen to complete her education in mathematics, physics, and chemistry.

She received her degree in 1920 and, after a brief span of study in Hamburg, she took up a post in the University of Copenhagen’s Actuarial Mathematics Laboratory, an extension of the work she had done in the 1910s which did not pay particularly well, but which at least got her foot in the door of the university system. One of Lehmann’s colleagues at the mathematical department was Niels Erik Nørlund, a mathematician who was the director of the Institute for the Measurement of Degrees. Nørlund heard Lehmann’s woes about low pay and less than stimulating work, and brought her into the Institute, putting her to learning the ropes of seismological measurement and touring Europe to consult with other leaders in the field about their methods and theories. In 1928, she earned her Master’s Degree in seismology, and was given charge of the Geodætisk Institut’s seismological activities in Denmark and Greenland. 

This was the position she was in when the Murchison data came in, and though that marked the beginning of the work that would carry her name onwards through eternity, professionally she was anything but happy. Nørlund distanced himself from her, and stopped actively promoting her advancement, while all around her, her colleagues were biding their time to undermine her, a chance they received when she broke several ribs in 1932 on the way back from doing fieldwork in Greenland and had to take time off to recover. Upon returning, she found her projects had been given to others, who not only pushed her out of leading them, but even participating in them. 

Meanwhile, she continued ironing out her theory of the Earth’s solid inner core, collecting more data and using it to determine the characteristics of the boundary between the molten and hypothesized solid core of the Earth, publishing her results at last in an iconic paper titled simply P’ in 1936. It was a masterpiece of seismological interpretation and mathematical modeling which compelled respect abroad, even if at the Institute Lehmann’s life continued along its previous grey track. By 1939, her results were independently confirmed, and the model of the Earth as we now know it, with its crust, mantle, outer liquid core, and inner solid core, was complete.

The years after the publication of P’ were largely difficult and unsatisfactory. Following the Nazi occupation of Denmark from 1940 to 1945, which Lehmann saw through making what contributions she could to protect those targeted by the German invaders, she experienced the slow administrative gnarl of watching the University of Copenhagen consider the creation of a geophysics department, while going out of its way not to consider her, perhaps the most famous Danish geophysicist of her age, to head it, instead persistently opting for a man with good connections who, by his own admission, knew virtually nothing about geophysics.

It was thus no surprise when Lehmann, upon turning 65, retired from the Institute, walking through its doors after twenty five years of generally frustrating service without a look backwards. She had in fact built up a strong connection to the American geological community through her visit to the United States in 1952, and in particular through her growing friendship with Maurice Ewing, the eccentric leader of the Lamont Geological Observatory who longtime readers of the Archive will recognize from the story of Marie Tharp. After leaving the University of Copenhagen and theoretically heading towards retirement, Lehmann strengthened her connection with Lamont, and also with UC Berkeley, carrying out work related to seismological detection of atomic detonations and also discovering the layer of the Earth that now bears her name, the Lehmann Discontinuity, which exists between 190 to 250 km beneath the Earth’s surface and features a distinctive change of wave velocity that Lehmann plumbed. 

Inge Lehmann lived over a century upon this planet whose interiors her mind regularly probed with a skill colleagues described as a form of “dark magic.” She lived to see organizations she had joined in their infancy become global centers of scientific study, but also to see all the friends and colleagues of her youth grow old and pass away. There was no shortage of honors in her “retirement” years - election to the Royal Society (1969), honorary doctorates (1964, 1968), the Seismological Society of America Medal (1977) - but perhaps the greatest tribute was the pile of memorials and well wishes that poured in upon the occasion of her 100th birthday, some from organizations she built, and many from colleagues she had mentored when they were students and now were major figures in their fields, all testifying to the impact she had made on her chosen field, our conception of the planet we live on, and the lives of those she came in contact with. 

Inge Lehmann passed away on February 21, 1993, three months shy of her 105th birthday.

FURTHER READING:

I was so excited when I saw the announcement of Hanne Strager’s I Am Right, and I Know I Am, a full biographical treatment of Lehmann. I pre-ordered it and shunted it to the top of the read pile when it arrived. It’s a very fine book when it’s discussing Lehmann, but it is also highly padded. There are long stretches of material that are only tangentially related to Lehmann which are tangibly there to get the page count over 250, the most substantial of which is a long section on Marie Tharp which tells the standard story of her discovery, though she and Lehmann had no particular relationship to each other, and they worked on completely different things, on the justification that, since they worked in the same building, they might have talked from time to time? It’s kind of the only game in town at the moment, so you should get it, but just manage your expectations ahead of time so you don’t end up like me.