Accretionary Wedge #24 : Heroes

This month’s accretionary wedge being hosted by Callan at Mountain Beltway is on the topic of heroes. There were several people that I could have written about, but in the end I felt had to go with a seismologist, so I have chosen Inge Lehmann.

There are many reasons why she is a hero, the discovery of the inner-outer core boundary and the 220km mantle discontinuity, her commitment to detailed, observational seismology, the difficulties she had to combat during the course of her career, being a woman who strived to established a scientific reputation in the first half of the twentieth century, but the thing that has always impressed me the most was the pure simplicity of the title of the paper in which the inner core boundary was described. These days there would be a long-winded title and about seventeen authors and a front cover of Nature. Her paper was simply entitled P’. That’s it, one letter and a punctuation mark (pronounced P-Prime). P’ is now better known as PKIKP, a P-wave traversing the mantle (P), outer core (K), inner core (I) and back out through outer core (K) and mantle (P).

Inge Lehmann
Inge Lehmann was born in Østerbro, Denmark on May 13, 1888. Her early education was at a school where boys and girls all did the same subjects, including rugby and needlepoint. She went to the University of Copenhagen to study mathematics and after passing her initial examinations went to University of Cambridge for a year. However, she burnt out from the pressures put on her to qualify for the mathematics degree course, and she returned to Denmark. She couldn’t face going back to university for a while and worked in an actuarial office for several years, gaining computational experience. Eventually, she returned to the University of Copenhagen in 1918 and attained a master’s degree in 1920.

Her seismology career started in 1925 helping to establishing seismic networks in Denmark and Greenland. In 1928 she was appointed as the first chief of seismology in the Royal Danish Geodetic Institute, a post she held until her retirement in 1953. She analysed and catalogued seismograms from Denmark and Greenland. She realised that the determination of epicentral data was not reliable and she correlated waveforms by eye to produce more robust interpretations. It was from this careful observation of seismic waveforms that she recognised that a distinct inner core was required to explain the interpretation on certain phases recorded at large epicentral distances.

Figure from Lehmann I, P', Bureau Central Seismologique International, Series A, Travaux Scientifiques, 14, 88, 1936.
The P’ paper was published in 1936, describing the inner-outer core boundary, now known as the Lehmann Discontinuity, and how the observed seismic waves were not the consequence of a diffraction, which was the accepted wisdom at the time.

In her later career she spent much of her time as a visiting scientist at Lamont-Doherty, Dominion Observatory, Caltech and Berkeley. She became an acknowledged authority on the structure of the upper mantle. Again by carefully studying the arrival times of certain earthquake phases, she proposed a sharp increase in seismic velocity at 220km and discovered a regional variability in the 400 km discontinuity. In fact, the 220km boundary was informally named the Lehmann discontinuity soon after its discovery, which almost precluded the formal naming of the more significant inner-outer core boundary after her in her centennial year.

Inge Lehmann died on February 21, 1993 at an age of almost 105. Shortly before her death she said that she had looked back on her life and ‘she was content. It had been a long and rich life full of victories and good memories’.

Sources:

Bolt, B. http://www.physics.ucla.edu/~cwp/articles/bolt.html

Carlowicz, M. Inge Lehmann (1888-1993) http://www.agu.org/about/honors/union/lehmann/lehmann_bio.shtml

Lehmann I, P’, Bureau Central Seismologique International, Series A, Travaux Scientifiques, 14, 88, 1936.

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