This entry is part 2 of 3 in the series Structure of the Earth Theme Week

The Danish Temblor


In our previous issue, we explored the recent confirmation of a theory that Earth has five layers instead of the previously believed four.

The standard model included the crust, the mantle, the outer core, and the inner core. Before 1936, the models would have indicated the earth had just three layers: crust, mantle, core. It’s obviously a challenge to peel open a planet and take a gander at the core, so geophysicists needed to become creative to infer attributes about the intestines of our world. They employed seismic waves from earthquakes to measure differences between the layers.

Humans believed there were three distinct layers, but the data pointed to a bit of weirdness. Along came the protagonist of our story, a young Danish woman, to sort it all out, fundamentally shaking our view of the innards of the Earth.

Inge Lehmann in 1932 - The National Library of Denmark

Inge Lehmann entered our realm in Copenhagen, Denmark, on 13 May 1888. Her father was an experimental psychologist, likely exposing her to the broader world of science as a child. Inge attended Fællesskolen, a progressive school that treated boys and girls equally.

**SHOCKING ALERT** Give girls the same treatment and access as boys and they can excel in all the fields traditionally left to men **SHOCKING ALERT**

A woman named Hanna Adler, the aunt of famous atomic scientist Niels Bohr, ran the academy. Lehmann flourished at Fællesskolen and credited Adler with a major influence on her scientific advancement. By 18, she received first-rank marks in entrance exams to Copenhagen University. She enrolled there in 1907, studying mathematics, chemistry, and physics. In 1910 and 1911, she impressively continued her education at Cambridge. By 1920, she had acquired master’s degrees in mathematics and physical science. 

Representation of the propagation of a P wave on a 2D grid - image by Christophe Dang Ngoc Chan

Lehmann became the assistant to geodesist Niels Erik Nørlund in 1925 and her career in seismology was off and sprinting. She set up seismological observatories in Denmark and Greenland. She spent significant time improving the coordination and conversation between the rest of Europe’s tremor monitors, which would soon pay dividends.

In 1936 Lehmann published a paper called P’. The “P” stands for primary waves, one of the two main types of seismological propagations. She had studied the data from the earthquake monitoring web she established and realized that primary waves showed up in the “P wave shadow” of the core of the planet. The basic gist of this phenomenon is that waves are slightly refracted when they pass between the mantle and the liquid outer core. The result is a “shadow” where the primary wave will not show up on a seismometer in areas that are between 103° and 142° from the earthquake’s focus on the globe. Lehmann noticed that some primary waves actually were showing up in the shadow, which meant something more was going on inside.

Her breakthrough came when she understood that this seemingly inexplicable enigma was the result of two separate states in the core! The four-layer Earth idea was born.

The behavior of P waves and S waves as they travel through the earth - image by Encyclopedia Brittanica

The leading men of the era adopted her interpretation nearly immediately, but universal acceptance did not arrive until 1971 when computer calculations proved the notion correct.

World War II put a hamper on a lot of scientific progress, especially those fields which required physical information from varied points on the planet. Lehmann continued research in seismology after the war. In 1953, she retired from the Geodetical Institute of Denmark and moved to the United States, where she worked on projects which dealt with the mantle and the crust.

During this period, she discovered a discontinuity that lies between depths of 190 and 250 kilometers below the surface. Lehmann noticed the speed of seismic waves changes at that depth, hinting at a change in the material in the mantle. This discontinuity was named for Inge: the Lehmann discontinuity.

Velocity of seismic S-waves near the Lehmann discontinuity - graphic by Brews Ohare

As we’ve noted a few times, it’s impossible to physically see into and measure the inside of the planet. Lehmann’s discoveries were the result of deep insight and tedious work. A famous geophysicist, Francis Birch, paid Lehmann a high compliment. He noted 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.”

Lehmann’s trophy closet was full. Among others, she nabbed the Gordon Wood Award, the Emil Weichert Medal, the Gold Medal of the Danish Royal Society of Science and Letters, the Tagea Brandt Rejselegat (twice), the William Bowie Medal (the first woman to win), and the Medal of the Seismological Society of America.

She has an asteroid and a species of beetle named in her honor! The American Geophysical Union established a medal in her name to honor “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth’s mantle and core.”

In 2015, on what would have been her 127th birthday, Google honored her with the day’s Google Doodle:

Google Doodle for Lehmann's 127th birthday

Inge Lehmann actually didn’t miss the Google honor by that long. She died in 1993 at the age of 104!

The Dane probably would have cheered the recent affirmation about the innermost inner core. We adore her for her contributions to seismology and geophysics. She is indeed worthy of a spot in the annals of Woman Crush Wednesday!

Inge Lehmann, Beastlord

Further Reading and Exploration

Inge Lehmann – Famous Scientists Biography

Inge Lehmann – Brittanica

Inge Lehmann: Discoverer of the Earth’s Inner Core – American Museum of Natural History

Inge LEHMANN – History of Scientific Women

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