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2.
“In the earth’s interior, where seismic waves travel invisibly and inaudibly, they can be followed only by mathematical equations.”
Dragutin Skoko, Mohorovicic’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 tame 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 dock 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.
What title would you give this essay?
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