Monday, April 18, 2016

Inge Lehmann, Seismologist

Ingh, 1932.
Inge Lehmann studied earthquakes and discovered the Earth's solid core, overturning the theory of a molten liquid center.

"You should know how many incompetent men I had to compete with - in vain."

Inge Lehmann was born in Copenhagen, Denmark on May 13th, 1888. Her father, Alfred Georg Ludvik Lehmann, was a psychologist, while her mother, Ida Sophie Tørsleff, stayed at home to manage the house and raise Ingh. Ingh was shy as a child, and would prefer keeping to herself throughout her life.

Inge attended a private coed school that was unusual for its day - it was founded by Hanna Adler, and treated boys and girls as if they were the same. Both engaged in the same subjects, sports, and activities, and were not disciplined in the same strict manner of other schools.

Inge enjoyed attending the school, though she did not always feel sufficiently challenged. She would be surprised later upon finding that not all schools and establishments operated similarly: "No difference between the intellect of boys and girls was recognized," she wrote later, "a fact that brought some disappointment later in life when I had to recognize that this was not the general attitude."

When she was 18, Inge scored a very high mark on the entrance exam for Copenhagen University, and in 1907 she began classes in mathematics, chemistry, and physics. By 1910 she had earned her undergraduate degree. Inge then spent a year at Cambridge University but felt sufficiently exhausted with her studies that she took a break from schooling, and worked for an insurance company until 1918.

She then returned the Copenhagen, and completed her master's in math two years later.


After spending a few more years doing actuarial work as an assistant in Copenhagen University, in 1925 she moved to seismology work with Professor Niels Nørlund, the director of the new Royal Danish Geodic Institute.

Inge travelled to seismic stations in Germany, the Netherlands, Greenland, and France to oversee the installation of seismometers and gather data. Through her observations she learned about different techniques for analyzing the movement of the earth. She taught herself about the field, completed a thesis, and earned another graduate degree in 1928, this time in geodesy: the science of making measurements related to planet Earth.

By now Inge was 40, and was appointed head of the Department of Seismology at the Royal Danish Geodetic Institute. In her job as Director, Inge had much administrative work, but also made time for scientific research and theory. She worked to improve the way data from the different seismic stations in Europe was gathered, coordinated, and analyzed, which would turn out to be a significant step forward.

Theories of the Earth's Core

At the time, seismologists already had an idea of what laid at the center of the Earth, deduced by studying the patterns of vibrations set off by earthquakes. When an earthquake occurs, it sets off two kinds of waves within the earth: P-waves, which result from a wave of compression rolling through the ground; and S-waves, wherein the ground shakes back and forth.

U.S. Geological Society
S-waves cannot travel through liquid, so when an earthquake sent out P and S-waves and only P-waves were measured at certain stations on the other side of the Earth, scientists knew the S-waves were getting blocked by some kind of liquid core.

By analyzing the time it took a shock wave to travel from the epicenter of an earthquake through the earth to different seismic stations around the globe, in 1906 Richard Dixon Oldham concluded that Earth has a large, metallic, molten core and made up the inner 40% of the planet's radius. By modern estimates, he was not far off; we now know that this molten core comprises the inner 3470 km of Earth's 6360 radius.

An Imperfect Theory

Oldham's work explained much of the waves measured at seismic stations, but not all of them. There was a zone where scientists would not expect to find any P-waves or S-waves at all, observing the theory, but some faint P-waves were being recorded.

Scientists wrote this off to faulty seismometers, but Inge had faith in her instruments, having overseen their installation and improved their coordination, and she continued to study what seemed inexplicable.

Inge spent several years studying the data from different stations, using rudimentary models and she pondered. Her nephew Niles Groes recalls going over to her house and finding her in her garden, in front of a large table covered with cardboard oatmeal boxes. On each box was written a different set of data from a different station. Inge would move the boxes around and try to determine how they could get certain readings in different places. Said Groes, "This was before computer processing was available, but the system was the same."

Illustration from Lehman's 1936 paper. 
With this method, Inge was able to deduce that the very center of the Earth was solid, and that while the molten core proposed by Oldman did exist, it was an outer core, surrounding a smaller, inner core. This explained the existence of P-waves in areas where there should not be readings.

Inge's calculations were published in 1936 in a paper called "P," for P-waves. The solution was correct and quickly adopted. Data gathered since that time has confirmed Lehman's theory, and given us additional information about this core (for example, that it is solid due to the enormous amount of pressure pushing down on it from the outer layers of the Earth, and that it has a radius of 1220 km, making it somewhat smaller than the moon - you're welcome).

From Nuclear Planet
Retirement, or Not

In 1953 at the age of 65 Inge retired from the Geodetic Institute, but she hardly stopped working - in fact, she now had more time free from administrative duties to devote to her studies. She took extended trips to Canada and the U.S. conducting research, and became one of the world's foremost experts in the Earth's upper mantle.

In 1958 Inge used seismic data from underground nuclear explosions to discover another discontinuity within the planet's layers, roughly 136 miles below the surface. This has not been fully explained even today, and is known as the Lehmann discontinuity.

Inge wrote her last scientific article at the age of 99, titled Seismology in the Days of Old. The following year, she attended a party held at the Geodic Institute on her behalf, marking her 100th birthday. Inge died at the age of 104 on February 21st, 1993, and left all her possessions to the Danish Academy.

Honored with numerous awards and medals during her lifetime, in 1971 Inge received the William Bowie medal, the highest honor of the American Geophysical Union. In her introduction, she was described as "the master of a black art for which no amount of computerizing is likely to be a complete substitute."

For someone who was able to interpret the data that others dismissed and determine the nature of matter more than 3,200 miles beneath our feet, this was a fitting tribute.


Inge Lehmann, Famous Scientists. 29 May 2015.
Inge Lehmann, the woman who discovered the Earth's inner core. The Telegraph, May 13th, 2015.
How Inge Lehmann used earthquakes to discover the Earth's inner core. Joseph Stromberg, May 13th, 2015.

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