There’s a ball of solid metal at the center of the Earth, more than 1,500 miles (2,400 km) in diameter and made up primarily of a nickel-iron alloy. Buried beneath the solid mantle and the liquid outer core, this inner core can only be “seen” indirectly, through its influence on seismic waves that pass through the body of the Earth, reflecting and refracting at the boundaries between its major layers. In 1936, seismologist Inge Lehmann realized that an extra interface could explain some mysterious earthquake data she was seeing at her seismic stations in Denmark and Greenland. By piecing together the scattered seismic wave arrivals and fitting them to a new model of Earth’s structure, she reimagined our planet from the inside out and revealed its deepest secret: the inner core.
The 1929 Murchison earthquake in New Zealand (magnitude 7.8 on the Richter scale) sent body and surface waves careening through the planet and around its circumference, reaching Lehmann’s Scandinavian seismic stations in a matter of minutes. Seismographs recorded the wave phases coming in on an unwinding spool of paper jostled under a weighted and inked pendulum bob. There was something puzzling about some of these these wave arrivals, however: they shouldn’t exist.
Lehmann started analyzing data from the 1929 quake, focusing on seismic stations in the P-wave “shadow zone,” roughly 103° to 143° from the epicenter in New Zealand. The fluidity of the outer core results in a characteristic bending of seismic waves coming through the solid mantle. When a P-wave (the first phase to arrive) hits the core-mantle boundary, it’s either reflected, if the angle is steep enough, or refracted through the outer core. This bending is such that there are certain distances from the earthquake epicenter that just cannot be reached–unless there’s something else to bounce off of in the middle of the outer core. The existence of an inner core could explain the mysterious P’ phase that Lehmann saw in her earthquake data, and it didn’t take long for the rest of the seismology community to embrace her suggestion and revise the accepted model for the structure of the Earth. Subsequent investigations by Birch in 1940 and Bullen in 1946 went a long way toward establishing the solid phase of the inner core.
Born in 1888 and raised in the Østerbro district of Copenhagen, Inge Lehmann learned from a very early age that girls and boys have equal ability when it comes to math and science. She attended a progressive co-educational school run by Hannah Adler (an aunt of Niels Bohr, incidentally), where “[n]o difference between the intellect of boys and girls was recognized, a fact that brought some disappointments later in life when I had to recognize that this was not the general attitude.” After three years at the University of Copenhagen and another at Newnham College, Cambridge, she worked in an actuary office before returning to her studies in 1918 and completing her degree in physical science and mathematics.
Lehmann’s seismology career began in 1925 when she became an assistant to geodesist Niels Erik Nørlund. With three other assistants “who had never seen a seismograph before,” she set about installing seismographs at the Greenland sites in Ivigtut and Scoresby-Sund and preparing the seismographic station near Copenhagen. After studying seismology on her own in the meantime, she was sent abroad for three months to work with several leading European experts in the field, including Beno Gutenberg in Darmstadt, who had determined the distance to the core-mantle boundary in 1914 to within 15 km of the present value. In 1928, Lehmann accepted a post as state geodesist and head of the department of seismology at the Danish Geodetical Institute, a position she held until her retirement in 1953. She co-founded the Danish Geophysical Society in 1936 and served as its chair in 1941 and 1944. In 1950, she was elected the first president of the European Seismological Federation.
Rather than retreating from scientific research following her retirement, Lehmann used her new-found freedom to travel and work closely with seismologists all around the world. Collaborating in the 1950s with Frank Press and Maurice (“Doc”) Ewing at the Lamont-Doherty Earth Observatory, she discovered another seismic interface in the upper mantle between 190 and 250 km depth. She received the honorary degrees of Doctor of Science from Columbia University in 1964 and D.Phil. from the University of Copenhagen in 1968, and served as Vice-President of the International Association of Seismology and Physics of the Earth’s Interior from 1963 to 1967.
Throughout her long career in seismology, Inge Lehmann maintained a keen physical intuition and unwavering support for international collaboration. She didn’t suffer fools (her nephew described her as “probably not always very diplomatic”) and she never took lightly the unique opportunities offered her.
In 1996, the American Geophysical Union established the Inge Lehmann Medal to acknowledge “outstanding contributions toward the understanding of the structure, composition and/or dynamics of the Earth’s mantle and core.”
Written by Meg Rosenburg @trueanomalies
Edited & posted by Suzie @suzie_birch
Check out Meg’s recent blog post about the Earth’s core, including some bits about Inge Lehmann, here.
Lehmann, Inge (1936): P’. Publications du Bureau Central Séismologique International A14(3), S.87-115
Lehmann, “Seismology in the Days of Old”
Bruce Bolt, “Inge Lehmann”
Brush, 1980 “Discovery of Earth’s Core.” Am. J. Phys. 48 (9).
For more information on Earth’s structure: