Dr Inge Lehmann was born on 13 May 1888
at Osterbro by the Lakes in Copenhagen. She grew up and lived
much of her long life there, and for over 50 years, on Kastelvej,
Copenhagen. The Lehmann family had its roots in Bohemia; the Danish
branch had many influential members of high standing, including
barristers, politicians and engineers. Inge Lehmann's paternal
grandfather laid out the first Danish telegraph line (opened in
1854) and her great grandfather was Governor of the National Bank.
Her mother's father, Hans Jakob Tørsleff, belonged to an
old Danish family with a priest in every generation. A granddaughter,
Anne Groes, was for a while Minister of Commerce. Inge's childhood
was a happy one in the peaceful atmosphere of the 1890s. Her father,
Alfred Lehmann, was a professor of psychology at the University
of Copenhagen who pioneered the study of experimental psychology
in Denmark. He was rarely seen except at mealtimes, though sometime
on Sunday he took the family for a walk. (A fund endowed by her
estate makes a travel award each year alternatively to a psychologist
and a geophysicist.) She had a sister, Harriet.
Inge was sent by her parents to an enlightened
co-educational school run by Hannah Adler, an aunt of Niels Bohr.
She learnt there that boys and girls could be treated alike for
work and play: 'No difference between the intellect of boys and
girls was recognized, a fact that brought some disappointment
later in life when I had to recognize that this was not the general
attitude' (quoted in Brush 1980). The teacher of mathematics stood
out, sometimes encouraging Inge's interest by giving her special
problems to solve. Her parents, however, objected to this, because
at that time she did not seem strong enough for extra work. Lehmann
later remarked (autobiographical notes), 'They could not be expected
to understand, I suppose, that I should have been stronger if
I had not been so bored with school work.' She left this splendid
Faellesskole in July 1906 after having passed the university entrance
examination with the distinction of first class.
She entered the University of Copenhagen
in the autumn of 1907 and studied mathematics, aiming at cand.
mag. (candidata magisterii), for which she also needed physics,
chemistry and astronomy. She passed the first part of the examination
in 1910 and, in the autumn, was admitted to Newnham College, Cambridge,
for a one-year stay. Harold Jeffreys (1891-1989) was at St John'
College, Cambridge, at the same time, but the two evidently did
not know each other at that time (Lady Jeffreys, personal communication).
She enjoyed her stay 'in spite of the severe restrictions inflicted
on the conduct of young girls, restrictions completely foreign
to a girl who had moved freely amongst boys and young men at home'.
Her stay was prolonged because of the possibility of entering
for the Mathematical Tripos. However, serious overwork from accelerated
study to pass the entrance examinations led to her return home
in December 1911. Her recovery was slow, and for a long time she
could not consider resuming her university studies.
For some years she worked in an actuary'
office where she acquired considerable training in computations.
In the autumn of 1918 she re-entered the University of Copenhagen
and graduated in the summer of 1920. In February 1923, she became
assistant to the professor in actuarial science at the University
for three years and during that time acquainted herself with the
theory of observations.
In 1925, she was appointed assistant to
Professor N.E. Norlund, who in the capacity of Director of 'Gradmaalingen',
was planning to have seismographic stations installed near Copenhagen
and in Ivigtut and Scoresbysund in Greenland. In this fortuitou
way she entered the discipline of seismology, a rare profession
in aseismic Denmark.
I began to do seismic work and had
some extremely interesting years in which I and three young men
who had never seen a seismograph before were active installing
Wiechert, Galitzin-Wilip and Milne-Shaw seismographs in Copenhagen
and also helping to prepare the Greenland installations. I studied
seismology at the same time unaided, but in the summer of 1927
I was sent abroad for three months. I spent one month with Professor
Beno Gutenberg in Darmstadt. He gave me a great deal of his time
and invaluable help. I paid short visits to Professor E. Tam
in Hamburg, to Professor E. Rothe in Strasbourg, Dr van Dijk in
De Bilt and Dr Somville in Uccle. Everywhere I was very kindly
received and given much information.
These men were, at the time, the leading
practitioners of seismology in Europe. Gutenberg was especially
notable, already in 1914 having used recordings of distant earthquake
to infer an average depth of 2900 km to the boundary of the Earth'
central core (Gutenberg 1914). (The present estimate is 2885
3 km.)
In the summer of 1928, Lehmann passed an
examination in geodesy at the University of Copenhagen, her thesi
having a seismological subject, and obtained the degree mag. scient.
(magister scientiarum). In 1928 she was appointed chief of the
seismological department of the newly established Royal Danish
Geodetic Institute, a post she held until her retirement in 1953.
Her responsibilities included maintaining what turned out to be
the very reliable and internationally important seismographic
observatories of Copenhagen, Ivigtut and Scoresbysund. Each of
them had a caretaker whom she had to instruct. The caretaker
of the Scoresbysund station were a special concern. They did not
stay for many years running, and the connection with the station
in those days was by a boat calling in once a year.
At the Copenhagen observatory Lehmann had
to keep the instruments properly adjusted (3-*Numbers in thi
form refer to the bibliography at the end of the text.) and to
see to the maintenance and repairs of the establishment, including
the buildings. The key to her unequalled knowledge of the seismic
wave patterns recorded from very distant earthquakes by the seismograph
of the Danish network was established early, because it fell on
her to interpret the seismograms and to publish the station bulletin
of the measurements. She was free to do scientific work, but it
was not considered a duty and it was not encouraged. Sometime
she had assistants, but more often not, not even for office work.
When Inge Lehmann began her research, she
realized that the determination of earthquake epicentre parameter
was not reliable. To minimize the reading errors in reported arrival
times of seismic waves, she correlated by eye similar wave form
between different seismograms (1, 2, 5). In this way, phases could
be matched clearly and robust interpretations obtained. Characteristically
modest, Lehmann wrote in her biographical notes, 'The most important
result arrived at was that the presence of a distinct inner core
was required for the interpretation of some phases recorded at
great epicentral distances.' This jewel in her observational crown
is discussed below, but Lehmann's important work was not confined
to her discovery of the inner core. Her publications list from
her Danish Institute years contains 35 papers, many in Danish
publications and reports of the Geodetic Institute. Her first
paper appeared in 1926, on the accuracy of interpolation.
In the last half of the 19th century, routine
readings of seismograms were made for each substantial earthquake
at observatories around the world and collected by mail for publication
in the International Seismological Summary (ISS) at Kew, England.
It was crucial to know the relative accuracy of measurements from
each station before geological inferences could be based on them.
In reaction to this need Lehmann made an early determination (4,
11) of the reliability of European seismological stations; she
became interested in the various manners in which records were
interpreted and discussed how the most useful readings could be
obtained. Northern Europe is not earthquake country, so like many
of her seismological contemporaries in Europe, she turned to the
study of small local earthquakes and explosions (5, 10), and the
striking microseismic wave motions (12, 13) generated by Arctic
and North Sea storms.
In her last publication (28), Lehmann give
valuable reminiscences of her professional work at the Copenhagen
Observatory. She points out the difficulty of seismologists at
that time determining the epicentres of teleseisms. She comments,
'As a result of many considerations it was found that while a
group of stations did not allow travel times to be accurately
determined, it made it possible to determine the slope of the
travel time. In modern technology, it might be said the European
stations were used as an array.' She entered into a 'lively' correspondence
with Harold Jeffreys during the period in which he and K.E. Bullen
were calculating their famous seismological travel-time curve
(Jeffreys & Bullen 1935). Like Jeffreys, she also had an early
and substantial lifetime interest in the seminal problem of determining
the travel-time curves of various types of seismic waves through
the Earth (6, 7, 9). In sharp contrast with the theoretical power
of these two mathematicians, who reduced the published ISS set
of arrival-time readings of others, Lehmann, like Gutenberg, brought
to bear the sharp observational insight provided by the seismographic
patterns themselves.
Students entering seismology now may find it hard to appreciate the early difficulty in bootstrapping from the crudest values to the highly precise and robust modern travel-time tables. Nowadays the concern in this research field has shifted to making second order, regional and frequency-dependent adjustment to established curves.
The dark years of the Second World War stifled
the traditional international exchange of earthquake observation
from the largely serendipitous global network of seismographic
stations. Yet at many observatories, such as Copenhagen, routine
maintenance and reading of earthquake waves continued.
As the postwar burden lifted, a fertile
new turn in Lehmann's research occurred when Professor Maurice
Ewing, Director of the Lamont Geological Observatory (L.G.O.)
of Columbia University, came to visit the Copenhagen seismological
station in 1951. He was enormously impressed by her expertise
and he invited her to come to L.G.O. to study a newly defined
seismic phase called 'Lg' that he and Professor Frank Press had
found in American records. Her consequent visit to L.G.O. for
several months in 1952 (see below) was to be followed after her
retirement by many more to Lamont and other earthquake observatorie
in the USA and Canada.
She investigated observationally existence
criteria for the Lg surface wave phase (14, 17) and became greatly
impressed by the various surface wave studies and wave-guide approach
of Ewing and Press. She did not, however, have the background
for the theoretical side of wave-guide propagation and so continued
her reliance on careful measurements of body-wave amplitudes and
travel times. This part of observatory seismology is less tractable
than surface wave analysis. Moreover, like all seismologists up
till about 1960, she was severely handicapped by the heterogeneity
and lack of coordination of seismographs in the first five decade
of the century.
By its nature, the science of seismology
demands intra- and international cooperation. Lehmann soon became
one of the central pillars of international seismological activities.
In 1931 she attended the Centenary Meeting of the British Association
in London and, in 1938, the Association meeting in Cambridge.
In September 1936 she participated in the Assembly of the International
Union of Geodesy and Geophysics (I.U.G.G.) in Edinburgh and afterward
returned to Cambridge for about 3 months' work. In 1947 the deliberate
large-size chemical explosion at Heligoland was recorded by the
Danish seismographs, and she was invited to present her measurement
at a November meeting of the Royal Society and at a subsequent
meeting on the same experiment in Cambridge. She stayed on in
Cambridge working for about 2 months.
She became a regular Danish delegate and
contributor at the I.U.G.G. Assemblies (Prague 1927, Stockholm
1930, Edinburgh 1936, Oslo 1948, Brussels 1951, Rome 1954, Toronto
1957, Berkeley 1963 and Zurich 1967). The Instituto Nazionale de
Geofisica invited her to an important seismological meeting in
Verona in 1950, at which the delegates formed the European Seismological
Federation. Its form was not approved, however, by the I.U.G.G.,
and in 1951 it was replaced by the still-flourishing European
Seismological Commission (E.S.C.) under the auspices of the International
Association of Seismology and Physics of the Earth's Interior
(I.A.S.P.E.I.). She presented papers at many subsequent meeting
of the E.S.C.
In 1952 she spent several months at the
L.G.O. and went on to short visits to the observatories at Tucson,
Pasadena, Berkeley, Saint Louis and Washington. She was at the
Dominion Observatory, Ottawa, working for 3 weeks in August 1954
and for 2 months in the summer of 1957. She was at the L.G.O.
working for about 6 months in 1957-58 and for a month and a half
in the spring of 1960. In 1962-64 she returned there in three
periods, in all for about 17 months. By this time, Lehmann's scientific
reputation began to be appropriately recognized, and in 1964 she
was awarded the prestigious Emil Wiechert Medal of the Deutsche
Geophysikalische Gesellschaft. She went back to North America
in 1964-65, spending 4 and a half months at Ottawa, and to L.G.O.
again in December 1968, where she stayed for 5 months. After three
visits to the Seismographic Stations of the University of California
at Berkeley in 1952, 1954 and 1965, she undertook more sustained
research there for 3 and a half months in the winter and spring
of 1968, when she became a favourite of graduate students. She
had written in 1965, saying that when she was at Berkeley for
the I.U.G.G. in 1963, she had been assigned noisy accommodation.
For this reason, a room was secured at the Women's Faculty Club,
a quiet lovely site at the centre of campus. She made many friend
at this institution.
In the early 1960s a critical discussion
emerged on upgrading the ISS, at that time many years in arrear
and underfunded. She was active in the establishment of its successor,
the International Seismological Centre (I.S.C.) in Edinburgh.
She took part in the 1961 Paris Meeting of the ISS Committee and
attended ISS Advisory Committee meetings until the 1967 I.U.G.G.
Assembly at which she resigned.
Lehmann retired from her post at the Geodetic
Institute in 1953, 5 years before the mandatory retirement age
of 70. She states in her autobiographical notes that afterward
she planned to use the freedom from routine for a continuation
of her seismological studies. In his personal obituary, delivered
at the Lehmann home on Kastelvej, her cousin's son Nils Groe
comments:
This was probably the first time
that she felt really free ... she was first recognized abroad
where she received several academic distinctions. But equally,
Inge was pleased by the international academic friendships which
she nursed up to her death. The Danish recognition did not arrive
until late. However, it was appreciated.
The award of its Bowie Medal to Lehmann
by the American Geophysical Union in 1971 was a milestone in the
establishment of her distinction. Professor Francis Birch gave
an acute and gracious citation:
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... Since her retirement from the Geodetic
Institute, Dr Lehmann has increased her rate of publication, which
is understandable since she no longer has to worry about keeping
someone on the job at Scoresbysund!
It happened that her 'retirement' from formal
observatory responsibilities at Copenhagen coincided with a dramatic
upswing in the quantity and quality of seismological activitie
worldwide. At the time Lehmann began planning her retirement studies,
global stations had declined in number and often quality. The
Depression and the Second World War had seen only the barest financial
support, and seismographic recording became an unimportant sideline
at astronomical observatories. There was a hodgepodge of seismograph
recording on smoked or photographic paper, responding to wave
in different frequency ranges and recording at various rates.
Few instruments were calibrated so that the actual motion of the
ground could not be measured, let alone frequency-domain analyse
of the wave spectra.
Every empirical science requires for it
progress adequate observational equipment, and by the 1950s, seismologist
looked with envy at the postwar growth of high-energy physic
laboratories and of sophisticated radio-astronomy observatories.
Rather suddenly and unexpectedly, a major stimulus for seismology
came, not from demands for reduction of earthquake risk, but from
the problem of surveillance of clandestine underground nuclear
explosions (Bolt 1976). A high-level conference of experts from
the nuclear powers at Geneva in 1958 produced a discouraging evaluation
of seismological reading capability. President Eisenhower wa
prompted to set up a Panel of Seismic Improvement, called after
its Chairman, Lloyd Berkner. It was charged with reviewing the
feasibility of improving seismic surveillance of secret explosion
and of promoting research in seismology, particularly in the United
States, related to the detection and identification of underground
nuclear explosions. One critical consequence in the United State
was a jump in expenditure in a research programme called VELA
UNIFORM. This programme and parallel efforts in other countrie
aimed at the development of improved, standardized seismographs.
These new recorders were installed in the early 1960s at about
200 existing seismographic stations around the world as the Worldwide
Standardized Seismographic Network (WWSSN). (Copenhagen was one
of these.)
Instrumentation for the WWSSN consisted
of six calibrated, optically recording seismographs: one set of
three short-period seismographs and one set of three long-period
seismographs. The frequency-response curves for all of these instrument
were carefully measured. The all-important time accuracy was obtained
by means of standard crystal clocks, precise to one part in lO6.
By 1960, records from the WWSSN were becoming
available to the seismological community with no publication restriction
and were easily obtained from the US Coast and Geodetic Survey
data centre in Washington, D.C. The new seismograms permitted
a keen observer like Lehmann to undertake much more precise measurement
and analysis of earthquake phases than had been possible before.
It is important therefore to remember that after 1960 her research
on correlating readings and wave forms was based on this upgraded
WWSSN network. Lehmann's unparalleled experience in reading seismogram
was looked to immediately in the United States as a great resource
in attempts to achieve the goals of the VELA UNIFORM programme.
Perhaps Lehmann's first important research
abroad was a continuation of her studies of seismic body wave
during 4 months in 1954 working with one of her early seismological
mentors, Professor Beno Gutenberg (by this time Director of the
Seismological Laboratory at the California Institute of Technology
at Pasadena). She made a careful study of travel times for small
epicentral distances. However, travel times differ regionally,
and she demonstrated that a low-velocity layer for P in the upper
mantle, postulated by Gutenberg elsewhere, was not in evidence
for Europe and northeastern America (16). She endeavoured to find
a velocity structure that would fit revised travel times of Jeffrey
by assuming the existence of a strong, possibly abrupt, velocity
increase at a depth somewhat greater than 200 km (18). This model
was designed initially to replace the 400 km depth discontinuity
derived from the Jeffreys-Bullen tables. Her '220 km discontinuity'
came to play an important role in subsequent modelling of the
upper mantle by others (Anderson 1981). From assumed velocity
structures that included it, Lehmann calculated travel times in
close agreement with various sets of seismograph readings. The
travel times of some well-recorded European earthquakes were first
considered, but later she made extensive use of explosion record
(20, 22) to demonstrate clear evidence of it in some regions.
However, even at the end of her career, the uneven geographical
nature of seismological measurements precluded any conclusion
on the full angular extent of the 220 km and 400 km discontinuitie
(27).
She extended her studies (19) to those
of S waves as recorded in Europe. Because S waves always arrive
as superposed and converted signals on the P wave trains, their
discrimination is much more difficult. Nevertheless, she succeeded
in explaining the presence of two S phases in European record
at small epicentral distances. A comprehensive account of all
these body-wave studies on low-velocity layers and discontinuitie
in the upper mantle was published (21). Subsequently, she continued
such body-wave studies by examining the new evidence for the 400
km discontinuity (24, 26). She found it to be very clear in some
regions, but poorly recorded or absent in others.
One of the research centres she enjoyed
most was Lamont Geological Observatory (now Lamont-Doherty Earth
Observatory). It was then in its heyday of innovation in seismology
under the directorship of Maurice Ewing (1906-1974). Ewing wa
a close friend and admirer of Inge Lehmann, and he made possible
several of her visits to Lamont through Harry Oscar Wood award
of the Carnegie Institution of Washington. In a personal letter
to Lady Jeffreys (12 June 1974) from Denmark soon after Ewing'
death, Lehmann expressed her debt and affection:
I was profoundly shaken by the death
of Maurice Ewing. It is very sad that his activities should be
cut off so early. I admired him immensely, but to me he was not
only the great scientist, he was also a wonderful friend. He invited
me to come to Lamont in the first place, and it was due to hi
encouragement and support that I could continue to work for many
years under better conditions than I had ever known before. The
last chapter of my life became interesting and very pleasant.
Reviewing now the events of those 20 years, I feel more keenly
than ever what I owe to him. In Galveston [Ewing transferred in
1972 to the University of Texas] he was far away but his death
evoked in me a curious feeling of loneliness. No one here know
much about him, no one has any idea of what he has done for me-so
there is no one to talk to about him. It was very nice indeed
to get your letter.
Initially, her interest in the Lg phase
drew her to Lamont. She brought earthquake records from Europe
and determined travel times and other features of Lg. It was at
Lamont in 1960 that I first had an opportunity to discuss seismology
with Inge Lehmann, and I was at once struck by the deep seismological
knowledge that she had regarding seismograms and their wave patterns.
By now over 70 years of age, Lehmann became
greatly stimulated by the seismological challenges and opportunitie
of the VELA UNIFORM programme. With financial support from thi
source, she turned to probe the structure of the upper mantle,
using the seismic body waves recorded from underground nuclear
explosions (23, 25). These studies involved interpretations of
seismograms containing complicated reflected seismic phases. She
advocated the reading of such records by one person, ' [who] paid
attention to the shape of the curves so that it might be possible
to trace a phase from one station to another one, and in thi
way determine a time curve which was not otherwise observable'.
Her critical approach to science was exemplified by her pointing
out the dangers of reading phases where they are Bullen tables.
Her '220 km discontinuity' came to play an important role in subsequent
modelling of the upper mantle by others (Anderson 1981). From
assumed velocity structures that included it, Lehmann calculated
travel times in close agreement with various sets of seismograph
readings. The travel times of some well-recorded European earthquake
were first considered, but later she made extensive use of explosion
records (20, 22) to demonstrate clear evidence of it in some regions.
However, even at the end of her career, the uneven geographical
nature of seismological measurements precluded any conclusion
on the full angular extent of the 220 km and 400 km discontinuitie
(27).
She extended her studies (19) to those
of S waves as recorded in Europe. Because S waves always arrive
as superposed and converted signals on the P wave trains, their
discrimination is much more difficult. Nevertheless, she succeeded
in explaining the presence of two S phases in European record
at small epicentral distances. A comprehensive account of all
these body-wave studies on low-velocity layers and discontinuitie
in the upper mantle was published (21). Subsequently, she continued
such body-wave studies by examining the new evidence for the 400
km discontinuity (24, 26). She found it to be very clear in some
regions, but poorly recorded or absent in others.
One of the research centres she enjoyed
most was Lamont Geological Observatory (now Lamont-Doherty Earth
Observatory). It was then in its heyday of innovation in seismology
under the directorship of Maurice Ewing (1906-1974). Ewing wa
a close friend and admirer of Inge Lehmann, and he made possible
several of her visits to Lamont through Harry Oscar Wood award
of the Carnegie Institution of Washington. In a personal letter
to Lady Jeffreys (12 June 1974) from Denmark soon after Ewing'
death, Lehmann expressed her debt and affection:
I was profoundly shaken by the death
of Maurice Ewing. It is very sad that his activities should be
cut off so early. I admired him immensely, but to me he was not
only the great scientist, he was also a wonderful friend. He invited
me to come to Lamont in the first place, and it was due to hi
encouragement and support that I could continue to work for many
years under better conditions than I had ever known before. The
last chapter of my life became interesting and very pleasant.
Reviewing now the events of those 20 years, I feel more keenly
than ever what I owe to him. In Galveston [Ewing transferred in
1972 to the University of Texas] he was far away but his death
evoked in me a curious feeling of loneliness. No one here know
much about him, no one has any idea of what he has done for me-so
there is no one to talk to about him. It was very nice indeed
to get your letter.
Initially, her interest in the Lg phase
drew her to Lamont. She brought earthquake records from Europe
and determined travel times and other features of Lg. It was at
Lamont in 1960 that I first had an opportunity to discuss seismology
with Inge Lehmann, and I was at once struck by the deep seismological
knowledge that she had regarding seismograms and their wave patterns.
By now over 70 years of age, Lehmann became
greatly stimulated by the seismological challenges and opportunitie
of the VELA UNIFORM programme. With financial support from thi
source, she turned to probe the structure of the upper mantle,
using the seismic body waves recorded from underground nuclear
explosions (23, 25). These studies involved interpretations of
seismograms containing complicated reflected seismic phases. She
advocated the reading of such records by one person, ' [who] paid
attention to the shape of the curves so that it might be possible
to trace a phase from one station to another one, and in thi
way determine a time curve which was not otherwise observable'.
Her critical approach to science was exemplified by her pointing
out the dangers of reading phases where they are expected at be:
'If the readings are adapted to time curves already existing,
they are not very useful.' Professor Jack Oliver (personal communication),
who was closely associated with Lehmann at Lamont, has written:
Her work with nuclear explosion
was important, not so much because she found something new but
because her stature and her integrity provided respectability
and credibility for those whose studies she presented support
for. This was not a trivial point in the partly-political, a
opposed to purely-scientific, atmosphere during the time when
the nuclear test-ban treaty was a hot political topic. Lesser-known
seismologists could be suspected of warping results to support
a particular political point of view.
The 50th anniversary of the discovery of
the Earth's inner core in 1936 by Inge Lehmann was marked by a
symposium and special publications (Bolt 1987). Her proposal,
published in the Bureau Central Seismoloque International,
Travaux Scientifique (8) was intended as a way to avoid difficultie
that had arisen in interpreting the observed arrivals of seismic
core waves when only a unitary terrestrial core was assumed. Example
of the recorded wave forms available to her in 1936 and the simple
Earth model used in her argument are shown in figure 1.
Early seismological observations showed
an observational shadow of P wave arrivals
beyond epicentral distances of 105°
or so. Yet on the other side of the Earth, particularly at distance
near 140°, unidentified seismic wave onsets were seen from
earthquakes out to the antipodes. These unidentified waves were
of P type, but were delayed by 5 min after the times predicted
by the simple extrapolation of the Jeffreys-Bullen (1935) P travel-time
curve for 0° to 105°.
A unitary core was first proposed on seismological
evidence by R.D. Oldham to explain these features (Brush 1980).
Inside its outer boundary the seismological travel times entailed
a sharp drop in seismic P wave velocity. Rays in such a central
core are called PKP and their paths may be followed in figure
1. The ray that just grazes the core emerges at the surface a
a tangent to it (Ray 2) or is refracted back to beyond 180°
(Ray 2a). As these PKP rays become steeper they refract back to
shorter distances until at an angle of about 142° (Ray 3)
they cease their backward regression and there is an optical caustic.
Steeper and steeper rays then progress again in a normal way to
greater and greater distances (Rays 4 and 6), until at last the
diametrical ray passes to the antipodes.
The improved seismographs of the 1920
and 1930s enabled observers such as Lehmann to spot additional
waves in a distance gap between 105° and 142° that could
not be explained on the unitary-core model. (The arrows in the
figure on the east and vertical component seismograms at two northern
stations indicate PKP waves identified by Lehmann.) Various idea
were floated to account for the unexpected onsets, notably that
such seismic energy was due to diffraction of the seismic wave
at the 142° caustic. (Later, Harold Jeffreys (1939) demonstrated
that the diffraction hypothesis was theoretically untenable.)
As already mentioned, Inge Lehmann wa
in an excellent situation to become familiar with these seismic
wave phases because her far-flung Danish seismographic network,
as well as other European stations, was located at large epicentral
distances from energetic earthquake sources in the South Pacific.
For example, a damaging earthquake on 16
June 1929 in New Zealand was well-recorded by European seismographs.
Lehmann, in comparing a number of these recordings, could clearly
see onsets of PKP waves. Such evidence enabled her to make the
necessary imaginative jump (8):'
An explanation of the P3' wave (now
denoted PKIKP) is required, since now it can hardly be considered
probable that it is due to diffraction. A hypothesis will be here
suggested which seems to hold some probability, although it cannot
be proved from the data at hand.
The strength of Lehmann's argument was buttressed
by her ability to discard unessential information. She assumed
a simple two-shell Earth model, as shown in figure 1, with constant
seismic P velocities in the mantle (10 km s-t) and
in the core (8 km s-1). These were reasonable average
values for both shells. She then introduced a small central core,
again with a constant P velocity. Such simplifications entailed
straight seismic rays (chords) rather than curved paths, thu
permitting the calculation of the wave travel times by elementary
trigonometry. Lehmann then proceeded by successive adjustment
to show that a reasonable velocity (10 km s-1) and
radius of the inner core (1400 km) could be found that predicted
a travel-time curve close to the actual observations of the travel
times of the core waves (P3') in question (i.e. Ray
5 in figure 1).
In effect, Lehmann proved an existence
theorem: namely, a plausible tripartite Earth structure could
be found that explained the main features of the observed core
waves. However, she did not go on to solve the inverse problem;
that is, having proved the existence theorem, she did not use
her measurements of travel times to estimate statistically the
inner core parameters that satisfied them within the measurement
uncertainties.
This final step was done first two year
later by Gutenberg and Richter (1938) who inferred an inner core
radius of about 1200 km and a mean inner core P velocity of 11.2
km s-1. They argued, however, that, 'Both observed
amplitudes and travel times, suggest a rapid but continuous increase
in velocity in a particular narrow range ... within the core rather
than the discontinuity. Moreover, no reflective waves have been
found, such as would correspond to a discontinuity.' In a parallel
programme of inferring this interior terrestrial structure, Harold
Jeffreys (1939), after first giving a crucial test indicating
decisively that the diffraction interpretation for the P3'
phases was unacceptable, adopted the Lehmann inner-core hypothesi
and obtained a satisfactory numerical inversion of his PKP travel-time
dataset. But, unlike Gutenberg and Richter, he retained Lehmann'
sharp boundary. 'The opinion of Lehmann and Gutenberg and Richter
that PKP between 110-142° is refracted at an inner core i
therefore substantiated.' It is notable, however, that for over
20 years after this adoption of an inner core model, the rate
of increase of velocity at the inner core boundary remained highly
controversial.
Subsequently, independent arguments by
Birch (1940) and Bullen (1946) established that the rapid increase
in P velocity at the inner core boundary entailed a transition
from liquid to solid conditions, with a jump in shear wave velocity
from zero to about 3.1 km s-1, if the pressure-induced
gradient in incompressibility was to be plausible. It was not
until 1962 (Bolt 1962) that direct new evidence supporting Lehmann'
sharp boundary was advanced, and not until 1970 that high-angle
reflections (PKiKP) of seismic P waves incident on the inner core
were observed unequivocally on seismograms (Engdahl et al.
1970). Later it was shown (Bolt & Qamar 1970) that the
high-frequency reflections PKiKP (and possibly the doubly internally
reflected PKIIKP wave) at steep incident angles on the inner-core
boundary are evidence for jumps in both density and incompressibility
at the boundary. These inferences have been supported closely
by recent Earth model inversions using higher modes of terrestrial
eigen-vibrations (Dziewonski & Anderson 1981). An account
of the studies of the inner core up to the late 1980s can be found
in Bolt (1987). In this decade, the properties of the inner core
remain a strong attractor of seismological research. Lehmann would
no doubt have been delighted to assess the recent evidence for
significant anisotropy in the inner core (Tromp 1995) and for
its rotation relative to the outer core (Song & Richards 1996).
In 1986, during the special symposium at
the American Geophysical Union Annual Meeting in her honour (Bolt
1987), she wrote to me, 'I was, of course, aware that it was 50
years since I discovered the core but I did not pay much attention
to the fact. I see now that I will have to take the anniversary
more seriously.'
In the history of seismology, the life
of Inge Lehmann will be marked in very special ways. Her work
was a product of several qualities of a remarkable woman. She
was a dedicated and thorough observational seismologist. She had
a highly critical and independent mind and the capacity to work
out, in quantitative ways, the implications of alternative models.
She could cut through the complications to the core of the problem.
There can be little doubt that, as a lone woman working in seismology
at that time, she needed strong resources of character to sustain
her high motivation and curiosity.
Nils Groes has painted a memorable picture:
Inge's grown-up life was characterized
by hard work, tough grind, a magnificent scientific effort and,
finally, great academic appreciation... I remember Inge one Sunday
in her beloved garden on Søbakkevej; it was in the summer,
and she sat on the lawn at a big table, filled with cardboard
oatmeal boxes. In the boxes were cardboard cards with information
on earthquakes and the times for their registration all over the
world. This was before computer processing was available, but
the system was the same. With her cardboard cards and her oatmeal
boxes, Inge reglstered the velocity of propagation of the earthquake
to all parts of the globe. By means of this information, she deduced
new theories of the inner parts of the Earth.
He further comments that there were not
many Danish women who gained the international scientific celebrity
status of Inge Lehmann:
It was not easy for a woman to make
her way into the mathematical and scientific establishment in
the first half of the twentieth century. As she said, 'You should
know how many incompetent men I had to compete with-in vain.'
Inge was probably not always very diplomatic. Nevertheless, she
obtained great scientific results.
Lehmann always liked stimulating conversation
and intelligent debate. She saw the need for constructive criticism
and was impatient of the second-rate. She had a great physical
energy, enjoying many mountaineering and skiing trips to the Alp
and the mountains of Norway. She sometimes seemed to want to have
a little less dignified lifestyle than the one that fate had produced
for her, but at other times she was, to say the least, set in
her ways. Inge Lehmann had an enormous fund of knowledge and experience
about seismology from its early days in this century; in retrospect,
it is a great shame that more effort was not made to persuade
her to write up her reminiscences in more detail (28).
She participated in a reception held at
the Danish Geodetic Institute on her l00th birthday in 1988. A
large number of geophysicists from both sides of the Atlantic
participated. Professors C. Kisslinger and V. Keilis-Borok gave
speeches describing her achievements. Later she enjoyed relistening
to a tape-recording of them. The Danish seismologist, Dr Erik
Hjortenberg, her friend for many years, was invited to visit Inge
in her beloved summer cottage in 1990, and admired how she could
answer the phone as if nothing had changed, but, in fact, being
almost blind, she needed a lot of help to move around. There can
be no doubt that her almost 105 years were unusually stimulating
and creative; Nils Groes writes, 'One day at the hospital, Inge
told us that all day she had been thinking about her own life
and she was content. It had been a long and rich life full of
victories and good memories.' Those of us who were fortunate to
know her as a scientist and friend can only confirm her own assessment.
In writing this Memoir, I have benefited
from much advice and help from many friends and colleagues of
Inge Lehmann. Many people favoured me with reminiscences and anecdote
from their encounters and association with her. I am specially
grateful to Dr Erik Hjortenberg who contributed greatly to an
earlier memoir. I also thank Lady Jeffreys, Dr Jack Oliver and
Dr J. Taaghold for helpful information.
Inge Lehmann was one of the founders in 1936 of the Danish Geophysical Society and chaired the organization in 1941 and 1944. She was elected the first President of the European Seismological Federation in 1950, and served as a member of the Executive Committee of the International Association of Seismology and Physics of the Earth's Interior (Vice-President, 1963-67). She was elected Associate of the Royal Astronomical Society, London, in 1957, Honorary Fellow of the Royal Society of Edinburgh in 1959, and Foreign Member of the Royal Society, London, in 1969. Perhaps because she had no Ph.D., she was particularly proud of the honorary degrees of Doctor of Science at Columbia University in 1964 and D.Phil. of the University of Copenhagen in 1968. She received several honours and medals from international societies, including honorary membership of the European Geophysical Society in 1973, and the Bowie Medal of the American Geophysical Union in 1971, given for outstanding contributions to fundamental geophysic and unselfish cooperation in research. A unique honour was the receipt of the Danish Tagea Brandt award twice. A complete list of awards is given below.
Her name will continue to be honoured in
the geophysical world. In 1996 the American Geophysical Union
established the Inge Lehmann Medal. It will be given for 'outstanding
contributions toward the understanding of the structure, composition
and/or dynamics of the Earth's mantle and core'. It will be awarded
not more than every other year in odd-numbered years. It is of
interest that the A.G.U. chose to associate the medal with a much
broader geophysical research scope than Lehmann's own seminal
seismological discovery of a major Earth structure.
In her centennial year, two colleague
and I raised the question of the most appropriate way to name
a major structural discontinuity (Anderson et at 1987) in the
Earth after her. The inner-core boundary (ICB) is one of the three
first-order seismo-compositional discontinuities that divide the
Earth into crust, mantle, outer core and inner core. The other
two discontinuities are well known by names of their discoverers,
Andrija Mohorovicic and Beno Gutenberg. In this tradition, the
ICB should be called the Lehmann Discontinuity in honour of it
discoverer. The case was complicated because this title had already
appeared in the literature (e.g. Anderson 1979, 1981) informally
with respect to a discontinuity she inferred in the upper mantle
at a depth of about 220 km. As outlined above, Lehmann's work
on discontinuities in the upper mantle stems from 1953 and later
(15, 18), but she proposed (8) the inner-core model 17 years before.
In fact, the use of her name for structural features in the upper
mantle has been sparse. We thus felt it most fitting that precedence
should be accorded the earlier inference of the inner core, which
is a feature of more central importance in the dynamic Earth.
1936-48: Member of the Executive Committee
of the International Seismological Association.
1938: Tagea Brandt Award.
1951-54; 57-60: Member of the Executive
Committee of the International Association of Seismology and Physic
of the Earth's Interior.
1957: Associate Royal Astronomical Society,
London.
1959: Honorary Fellow Royal Society, Edinburgh.
1960: The Harry Oscar Wood Award in Seismology.
1963-67: Vice-President of the Executive
Committee of the International Association of Seismology and Physic
of the Earth's Interior.
1964: Doctor of Science (Sc.D.) h.c. Columbia University in the City of New York.
1964: Deutsche Geophysikalische Gesellschaft,
Emi-Wiechert Medal.
1965: Kgl. Danske Videnskabernes Selskab
(Royal Danish Academy of Sciences and Letters) Gold Medal.
1967: Tagea Brandt Award.
1968: Doctor of Philosophy (Dr.phil.) h.c.
University of Copenhagen.
1969: Foreign Member Royal Society, London.
1977: Medal of the Seismological Society
of America.
Anderson, D.L. 1979 The deep structure of
the continents. J. Geophys. 84, 7555.
Anderson, D.L. 1981 Discontinuities in the
mantle (Abstract). EOS 62, 1073.
Anderson, D.L., Bolt, B.A. & Morse,
S.A. 1987 The Lehmann discontinuity. EOS 68, 1593.
Birch, F. 1940 The alpha-gamma transformation
of iron at high pressure, and the problem of the Earth's magnetism.
Am. J. Sci. 235, 192.
Bolt, B.A. 1962 Gutenberg's early PKP observations.
Nature 196, 121.
Bolt, B.A. 1976 Nuclear explosions and
earthquakes. The parted veil. New York: W.H. Freeman.
Bolt, B.A. 1987 50 years of studies on the
inner core. EOS 68, 80.
Bolt, B.A. & Qamar A. 1970 An upper
bound to the density jump at the boundary of the Earth's inner
core. Nature 228, 148.
Brush, S.G. 1980 Discovery of the Earth'
core. Am. J. Phys. 48, 705.
Bullen, K.E. 1946 A hypothesis on compressibility
at pressures of the order of a million atmospheres. Nature
157, 405.
Dziewonski, A.M. & Anderson, D.L. 1981
Preliminary reference Earth model. Phys. Earth Planet Inter.
25, 297.
Engdahl, E.R., Flinn, E.A. & Romney,C.E
1970 Seismic waves reflected from the Earth's inner core. Nature
228, 852.
Gutenberg, B. 1914 Über Erdbebenwellen
VIIA. Nachr. Ges. Wiss. Göttingen Math. Physik. Kl, 166.
Gutenberg, B. & Richter, C.F. 1938 P'
and the Earth's core. Mon. Not. R. Astr Soc. Geophys. Suppl.
4, 363.
Jeffreys, H. 1939 The times of the core
waves. Mon. Not R. Astr Soc. Geophys. Suppl. 4,
594.
Jeffreys, H. & Bullen, K.B. 1935 Time
of transmission of earthquake waves. Bur Centr Seism. Internat.
A., Fasc. II, 202 pp.
Song, X.D. & Richards, P. 1996 Seismological
evidence for differential rotation of the Earth's inner core.
Nature 382, 221.
Tromp, J. 1995 Normal-mode splitting observation
from the great 1994 Bolivia and Kuril Islands earthquakes: Constraint
on the structure of the mantle and inner core. GSA Today
5, 137.
The following publications are those referred
to directly in the text. A full bibliography appears on the accompanying
microfiche, numbered as in the second column. A photocopy is available
from the Royal Society Library at cost. This list is based on
several personal files. Many incidental writings are not generally
available and abstracts have been excluded. Some of the former
are early Institute reports (in Danish) and the available abstract
list is very incomplete. Quotations of Inge Lehmann are from her
R.A.S. autobiographical notes unless otherwise attributed.
(1)/(4) 1929 ScPcS.
Beitr. Geophys. 23, 369-378.
(2)/(5) 1930 P' as read from the record
of the earthquake of June 16th 1929. Beitr. Geophys. 26,
402 412. Medd. Geod. Inst. Kobenhavn no. 1.
(3)/(6) 1930 A hammer for the Galitzen
vertical component pendulum. Beitr. Geophys. 26,
413-415.
(4)/(9) 1931 Die Bedeutung der Europäischen
Stationsgruppe für die Bestimmung von seismischen Laufzeitkurven.
Verhandl. Fünften Tagung
Balt. Geod Combustion, 192-212, Helsinki.
(5)/(10) 1932 Untersuchung der Europäischen
Registrierungen der Erdbeben vom 18.VIII.1928, 24.X.1930 und 13.XI.1925
(with G. Plett). Beitr.Geophys. 36, 38-77. Medd.
Geod. Inst. Kobenhavn no. 4.
(6)/(11) 1934 Transmission time for seismic
waves for epicentral distances around 20°. Medd. Geod. Inst.
Kobenhavn no. 5.
(7)/(12) 1935 Seismic time-curves for epicentral
distances around 80°. The Aleutian earthquake of 1929, July
7th. Publ. Bur Cent. Seism. Int. A 12 109-133.
(8)/(13) 1936 P', Publ. Bur Cent. Seism.
Int. A 14, 87-115.
(9)/(14) 1937 Seismic time-curves and depth
determination. Mon. Not R. Astr Soc. Geophys. Suppl. 4,
250-271.
(10)/(21) 1948 On two explosions in Danish
waters in the autumn of 1946. Geofis. Pur Appl. 12,
3-19.
(11)/(22) 1949 The reliability of the European
seismological stations, Medd. Geod. Inst. Kobenhavn, no. 22.
(12)/(23) 1949 Den mikroseismiske uro og
vejret. Naturens Verden 162-185. Kobenhavn.
(13)/(24) 1952 On the microseismic movement
recorded in Greenland and its relation to atmosphenc disturbances.
Ponteficiae Acad. Scient. Scripta Varia 23, 73-109.
(14)/(25) 1953 On the short-period surface
wave Lg and crustal structure. IUGG Newsletter 2,
248-250. Lamont Geolog. Obs. Contr. 73.
(15)/(26) P and S at distances smaller than
25°. Trans. Am. Geophys. Un. 34, 477-483.
(16)/(30) 1955 The times of P and S in
northeastern America. Annali Geofis. VIII, 351-370.
(17)/(35) 1957 On Lg as read in North American
records. Annali Geofs. X, 1-21. Lamont Geolog. Obs.
Contr. 222.
(18)/(38) 1959 Velocities of longitudinal
waves in the upper part of the Earth's mantle. Annali Geofis.
15, 93-118. Dom. Obs. Ottawa, XIX, 10.
(19)/(42) 1961 S and the structure of the
upper mantle. Geophys. Jl R. Astr Soc. 4. The
Earth today, dedicated to Sir Harold Jeffreys, 124-138. Lamont
Geolog. Obs. Contr. 449.
(20)/(43) 1962 The travel times of the longitudinal
waves of the Logan and Blanca atomic explosions and their velocitie
in the upper mantle. Bull. Seism. Soc. Am. 52, 519-525.
(21)/(44) Recent studies of body waves in
the mantle of the Earth. Q. Jl R. Astr. Soc. 3,
288-298.
(22)/(45) 1964 On the travel times of P
as determined from nuclear explosions. Bull. Seism. Soc. Am.
54, 123-139. Lamont Geolog. Obs. Contr. 674.
(23)/(50) 1967 On the travel times of P
as obtained from the nuclear explosions Bilby and Shoal. Phys.
Earth Planet Interiors 1, 14-23. Lamont Geolog. Obs.
Contr. 1097.
(24)/(53) 1968 Discontinuous velocity change
in the mantle as derived from seismic travel times and amplitudes.
Bur Centr. Seism. Int. A 24, 167-174.
(25)/(54) 1969 Travel times and amplitude
of the Salmon nuclear explosion. Bull. Seism. Soc. Am.
59, 959-966. Lamont Geolog. Obs. Contr. 1309.
(26)/(55) 1969 Discontinuous velocity change
in the mantle as derived from seismic travel times and amplitudes.
Publ. But: Cent. Seism. Int. A 24, 167-174.
(27)/(57) 1970 The 400 km discontinuity.
Geophys. Jl R. Astr Soc. 21, 359-372.
(28)/(58) 1987 Seismology in the days of
old. EOS 68, 33-35.