2. "In the earth's interior, where seismic waves
travel invisibly and inaudibly, they can be followed only by mathematical
Dragutin Skoko, Mohorivicic's biographer
The boundary separating Earth's crust from its upper mantle is called the Mohorovicic discontinuity, or the Moho, for short, in honor of the Croatian seismologist Andrija Mohorovicic. In 1909, he used data on the travel time of earthquake waves to demonstrate that their velocity changes at about 50 km beneath the surface. Others later refined the study of crust and upper mantle and applied new methods, but Mohorovicic paved their way.
Mohorovicic's father, also named Andrija (or Andrew), was a maker of anchors. The young Mohorovicic loved the sea, and married a sea captain's daughter. He taught for nine years at the Royal Nautical School in Baka. After becoming director of the Meteorological Observatory in Zagreb, in 1891, he studied and wrote primarily about clouds, rainstorms, and high winds. After a severe earthquake in 1901, however, Mohorovicic and his colleagues petitioned their government to establish a seismic station in Zagreb. In 1910, Mohorovicic published his account of the earthquake of November 9, 1880. In it, he plotted a now standard transit time graph--arrival time versus epicenter distance to recording station--using the data for 29 stations that ranged to a distance of 2,400 km from the epicenter.
After plotting data for a large number of earthquakes over a wide area, he had begun to notice that the P wave arrivals required two curves on his graph. Because it was not possible to have P waves traveling in the same medium at different velocities, and the earlier P arrivals were only seen at some distance from the epicenter, he reasoned that the two different arrival times represented two different phases of P waves traveling different paths. After working out the refraction equations and tests to determine optimal values for the depth of the focus, the ray paths of the two P waves, the corresponding two S phases, and their reflection paths, he concluded that at approximately 50 km there must be an abrupt change in the material that composes the interior of the Earth, because he observed an abrupt change in the velocity of the earthquake waves. Although this conclusion was not accepted immediately, Beno Gutenberg was able to confirm it with his own research as early as 1915.
Even after earthquakes became one of his primary interests, as chief of the observatory, Mohorovicic was responsible for recording all the meteorological data for Croatia and Slovenia--precipitation, tornadoes, whirlwinds, thunderstorms, and more--with only an occasional assistant. He was responsible for all the mathematics involved in keeping records and for answering hundreds of letters and requests for assistance, as well as teaching classes at the University. He was patient and precise in his collection and analysis of data, but he loved good scientific instruments, and was frequently frustrated at the inadequacy of the instruments available and the difficulty of obtaining new ones. An accurate clock was particularly important to his research, because in studying earthquakes, an error of one second in the time of arrival means an error of 5.6 km in estimating the length of its travel, making it impossible to accurately locate the focus of an earthquake. By 1913 he had finally obtained a crystal clock with a radio receiver that allowed him to synchronize with the Paris Observatory, but in 1914, during World War I, the army commandeered it for military use. When the clock was returned to him after the war, he also received a new radio receiver that took two railroad cars to transport.
Mohorovicic published a paper in 1909 on the effect of earthquakes on buildings that described periods of oscillation (see lesson 4.3). In this he was at least 50 years ahead of the times both in his own country and elsewhere. Croatia's first national Provisional Engineering Standards for Construction in Seismic Areas were published in 1964.
During his lifetime, Mohorovicic maintained contacts with seismologists all over the world. He retired in 1922, but remained active until shortly before his death in 1936. His only grandchild, Andre', remembers that he was always good natured--a kind and peaceful man. Mohorovicic, like Alfred Wegener, received the honor of having his name given to a crater on the dark side of the moon.
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