Historical Seismologist

Andrija Mohorovičić (1857–1936)—On the occasion of the 150th anniversary of his birth

Davorka Herak and Marijan Herak
Andrija Mohorovičić Geophysical Institute

Andrija Mohorovičić, a world-renowned seismologist and the greatest Croatian scientist of all time, was born 150 years ago on 23 January 1857 in Volosko near Rijeka, Croatia. Although his name is mostly associated with the discovery of the crustmantle boundary, his other achievements in seismology, as well as those in other geophysical disciplines, certainly deserve to be remembered.

Mohorovičić enrolled in Prague University to study mathematics and physics in 1875. The very solid foundations in science that he obtained from professors such as Ernst Mach and Heinrich Durège are clearly reflected in his later work and scientific attitude. An overview of his days in Prague is given by Orlić (1998). After graduation he taught at high schools in Zagreb and Osijek and at the Nautical School in Bakar. In 1892 he became the director of the Meteorological Observatory in Zagreb. The following year Mohorovičić obtained the doctorate of philosophy at Zagreb University, where he was later elected an adjunct associate university professor to teach courses on geophysics and astronomy. He became a corresponding member of the Yugoslav Academy of Science and Arts in 1893 and a full member in 1898. Mohorovičić retired in 1922. An excellent and thorough account of his life and scientific accomplishments was published in a monograph by Skoko and Mokrović (1982, 1998) in Croatian and English.

At the beginning of his career Mohorovičić focused on meteorology. His scientific interests lay in the explanation of various meteorological phenomena—atmospheric dynamics and observations of rare events (e.g., a tornado near Novska or the whirlwind near Čazma). He also studied the climate of Zagreb and the decay of temperature with altitude. Looking to learn more about the atmospheric circulation but lacking proper instruments, he built his own nephoscope, a camera obscura-like instrument for observation of clouds. These observations formed the basis for his dissertation. As only recently rediscovered (Grubišić and Orlić 2007), Mohorovičić was the first person to describe atmospheric rotors with a horizontal axis, which he observed during bora-wind episodes in the northern Adriatic. The paper about it appeared in 1889 in one of the leading meteorological journals of the time and was immediately translated into English. It somehow disappeared from reference lists in the 1930s.

The following quotation, from 1901, perhaps best illustrates the clarity of Mohorovičić’s vision:

The ultimate goal of a meteorologist is to set up differential equations of the movements of the air and to obtain, as their integral, the general atmospheric circulation, and as particular integrals the cyclones, anticyclones, tornados, and thunderstorms.

A perfect description of today’s weather forecast, at the very beginning of the 20th century it was beyond even science fiction. Mohorovičić is also recognized for the high standards he set for professional meteorology and for the unification of the meteorological service in Croatia. He was the first in Croatia to publish weather forecasts in daily papers.

About the turn of the century Mohorovičić’s scientific interest turned almost exclusively to seismology, although he continued to spend a tremendous amount of time on routine meteorological observatory duties. It is indeed remarkable for a scientist who decided to turn his career upside-down in his mid-forties by starting research from scratch in a new, almost nonexistent field in his country, to subsequently achieve such an international reputation. The reason for this dramatic change is not known—one can only speculate that intense seismic activity around the Croatian capital in the late 19th century ignited the spark in his curious mind. The formal background was also set, as an Earthquake Committee of the Yugoslav Academy was established in 1880 when Zagreb was devastated by a large earthquake, and Mohorovičić later became actively involved. He founded the Zagreb seismological station in 1906, when he installed the Vicentini-Konkoly seismograph in the basement of the Meteorological Observatory (and on seismogram number 9 he recorded the great San Francisco earthquake!). Soon he realized that better instruments were needed, and he purchased Wiechert horizontal seismographs, which became operational in 1908 and 1909. He wrote at the time, “with this we are on the level with all the better observatories in Central Europe.”

The meticulous analyses of recordings of the Kupa Valley earthquake of 8 October 1909 (Mohorovičić 1910a, b, c) made by these instruments, together with seismograms collected from all over Europe, enabled him to prove the existence of the crustmantle boundary, which later became known as the Mohorovičić discontinuity, popularly known as the Moho. This unveiling of one of the big secrets of the Earth’s interior places him among the founding fathers of modern seismology. It was in line with how he saw the essence of this young scientific discipline:

The goal of seismology is to study the interior of the Earth, and to continue where the geologist stops; it has in modern seismographs a sort of binoculars that enables us to look into the largest of depths.

This discovery, recognized as one of the milestones of science in the beginning of the 20th century, is also the most important scientific contribution ever published in a Croatian journal. The paper itself was unusual, at least by today’s standards—published bilingually (Croatian and German in the same volume) in the Observatory bulletin, it had a title that was not informative at all, was rather lengthy (56 pages), and was written in a narrative form stating all of Mohorovičić’s dilemmas, observations, ideas, and trials. The number of topics covered and the number of important contributions in the paper were remarkable. In this one paper one finds: the macroseismic case study of the Kupa Valley earthquake of 1909; the proof of the existence of the discontinuity together with a complete theory of propagation of seismic waves in the new Earth model; inverted model parameters (P- and S-velocities, interface depth, exponential laws of velocity increase with depth within the crust and in the upper mantle—Mohorovičić’s law); the first empirical and theoretical hodochrones for the phases he introduced at local and regional distances (Pg, Sg, Pn, Sn, together with a large number of crustal reflections, including converted phases!); instructions on how to estimate epicentral distance based on onset times of a multitude of phases on the seismograms; new procedures for determination of focal depth; prediction about what the seismograms of (then unknown) deep earthquakes should look like; a suggestion that the “maximum phase” (Lg or Airy phase) is closely related to channeled post-critically reflected S waves within the crust even for distant events. His train of thought closely follows Feynman’s (1967) “recipe” for the creation of a new physical law—guess it, compute consequences, compare them to nature and observations, if they agree it is right, if they disagree it is wrong (paraphrased). This trial-and-error approach is even today often the method of choice in solving inverse problems, one of the first of which in seismology was solved by Mohorovičić in 1910.

There are also other, less-known achievements of his, including an elegant method of location of epicenters (Mohorovičić’s epicentrals, 1915–1918), a method to determine total friction in mechanical seismographs (1917, 1918, 1924), a novel seismograph design (1917, 1918, unfortunately never realized), a number of improved travel-time curves (1914a, b, c), etc. Mohorovičić was also among the first to recognize the importance of the seismic-resistant design of buildings. In a series of lectures for the Croatian Society of Engineers and Architects, as early as 1909 (published in 1911) he attempts to “explain how the Earth trembles, and how these tremors affect buildings, and draw attention to some principles that both architects and building contractors should follow.” Furthermore: “In order to study earthquake effects on buildings, we must first accurately represent the shaking of soil beneath the building, as well as the forces this shaking exerts, and then we must study how these forces affect the building as a whole and its individual parts.” He went on to analyze the effects of building resonance and computed accelerations for various ratios of the period of dominant shaking and the building eigen-period. Quite ahead of his time, he set some of the basic principles of earthquake-resistant design and warned against erecting heavy buildings on soft ground and steep slopes and suggested firm joints between the basic building skeleton, the beams, and the walls, etc. He was also the first one to statistically compute expected exposure of buildings in Zagreb to earthquakes, and he tried to persuade entrepreneurs “to consider the earthquake hazard and spend more, in order to make buildings more resistant and safe.” Out of six basic principles of conceptual design for aseismic building construction as outlined in the current Eurocode-8, Mohorovičić advocated five of them already at the beginning of the 20th century!

Andrija Mohorovičić was a very careful, thorough, and diligent scientist who enjoyed the search for explanations of observations in theory, but never favored theory over observations. A true erudite, he spoke—besides his mother tongue—English, German, Italian, French, and Czech (in addition to Latin and Greek). He published about 40 papers, of which he was always the only author. This speaks not of his vanity but rather of hard conditions he had to work in, resulting in a persistent shortage of coworkers.

His thoughts and ideas were truly visionary, often decades before his time (harvesting wind energy, hail suppression, models of the Earth and of the atmosphere, deep earthquakes, earthquake- resistant design, etc.). In 1970 one of the craters on the dark side of the moon was named after him, as was the asteroid No. 8422 in 1996. The crust-mantle boundary on the moon as well as on Mars is also known as the Mohorovičić discontinuity. Andrija Mohorovičić is one of only very few Croatian scientists of an international reputation who spent his whole career in his homeland. He is recognized as the founder of the Zagreb seismological school, the Croatian Seismological and Meteorological Surveys, and the public time service. Owing to the tradition he initiated, the University of Zagreb is among the few in the world awarding a degree in seismology at the undergraduate as well as the graduate level.

Throughout the year 2007, the Department of Geophysics of the Faculty of Science in Zagreb organized a series of events to mark the sesquicentennial of Mohorovičić’s birth. Among the commemorations and many lectures about his life and scientific achievements three events stand out: the unveiling of the newly erected bust in front of the house in which he was born and lived during his childhood in Volosko, the painting contest for schoolchildren with geophysics and his achievements as the motif, and the issuing of a postage stamp in his honor. The stamp, reproduced here on the cover of this issue of SRL, was promoted in Zagreb on 23 April 2007 and subsequently in Rijeka four days later. To have two promotions is quite exceptional— the second one was organized because of the high interest from his native region. Both events were accompanied by excellent philatelic exhibitions dedicated to meteorology and geophysics (by D. Poje, M. Vučetić, E. Hernitz, and T. Gregl in Zagreb) and to seismology and earthquakes (in Rijeka, presented by P. Suhadolc from Trieste University).

On the occasion of the centenary of the Zagreb seismological station, the Andrija Mohorovičić Memorial Rooms were opened in the premises of the Department of Geophysics in Zagreb. There we display Mohorovičić’s original instruments— Wiechert seismographs, observatory clocks, chronometers, astronomical instruments, microbarograph and barometers—all restored and in prefect condition; and his furniture, documents, correspondence, papers, manuscripts (including complete derivations leading to the discovery of the crust-mantle boundary), photographs, posters, etc. The Memorial Rooms may be visited online at http://www.gfz.hr/sobe-en/index.htm.

REFERENCES

Feynman, R. (1967). The Character of Physical Law. Cambridge, MA: MIT Press, 174 pps.

Grubišić, V., and M. Orlić (2007). Early observations of rotor clouds by Andrija Mohorovičić. Bulletin of the American Meteorological Society 88, 693–700.

Mohorovičić, A. (1910a). Potres od 8. X 1909. Godišnje izvješće Zagrebačkog meteorološkog opservatorija za godinu 1909. 9/4, 1–56.

———. (1910b). Das Beben vom 8. X. 1909., Jahrbuch des meteorologischen Observatoriums in Zagreb (Agram) für das Jahr 1909. 9/4, 63 pps.

———. (1910c). Earthquake of 8 October 1909 (translation), Geofizika 9, 1992, 3–55.

———. (1911). Djelovanje potresa na zgrade. Vijesti Hrvatskoga društva inžinira i arhitekta 32, 17–18, 33–35, 51–53, 69–72, 85–86, 103– 105, 112–116, 126–129, 139–142.

———. (1914a). Hodograf prvih longitudinalnih valova potresa (emersio undarum primarum). Rad JAZU 204, 1–20.

———. (1914b). Hodograph der ersten longitudinalen Wellen eines Bebens (emersio undarum primarum). Bulletin International de l’Académie Yougoslave des Sciences et des Beaux-Arts B2, 139–157.

———. (1914c). Hodograph der normalen P-Wellen für eine mittlere Herdtiefe der Erdbeben. Beilage zu den seismischen Aufzeichnungen, Kgl. Landesanstalt für Meteorologie und Geodynamik, Zagreb, 4 pps.

———. (1915–1918). Die Bestimmung des Epizentrums eines Nahbebens. Gerlands Beiträge zur Geophysik 14, 199–205.

———. (1917). Principi konstrukcije sizmografa i prijedlog za konstrukciju nova sizmografa za horizontalne komponente gibanja zemlje. Rad JAZU 217, 114–150.

———. (1918). Prinzipien für die Konstruktion eines neuen Seismographen. Bulletin International de l’Académie Yougoslave des Sciences et des Beaux-Arts B9/10, 6–20.

———. (1924). A critical review of the seismic instruments used today and of the organization of seismic service. Bulletin of the Seismological Society of America 14 (1), 38–59.

Orlić, M. (1998). Studentski dani Andrije Mohorovičića u Pragu. Geofizika 15, 119–123.

Skoko, D., and J. Mokrović (1982). Andrija Mohorovičić. 1st ed. Zagreb: Školska knjiga, 147 pps.

Skoko, D., and J. Mokrović (1998). Andrija Mohorovičić. 2nd ed. Zagreb: Školska knjiga, 111 pps.

Andrija Mohorovičić Geophysical Institute
Department of Geophysics, Faculty of Science
University of Zagreb, Croatia
herak [at] irb.hr
(D. H., M. H.)

HOME