Andrija Mohorovicic, a famous seismologist, was born on January 23, 1857. Few seismologists have as much name recognition as Mohorovicic, whose name appears in hundreds of papers each year, usually abbreviated as “Moho.” The Moho refers to Mohorovicic’s most famous contribution to the science of seismology, the Mohorovicic Discontinuity described below.

Seismology was not the first science in which Andrija made important contributions. His early career was in meteorology, an interest strongly influenced by his education. Mohorovicic grew up in the coastal town of Volosko, Croatia; his father was an anchor blacksmith. Andrija attended local schools until his university studies in mathematics and physics began in 1875 at the Faculty of Philosophy in Prague. After he received his degree, Andrija married a sailor’s daughter and taught secondary school in Zagreb, Osijek, and then Bakar.

His post at the Nautical School in Bakar proved to be career-defining. He taught meteorology at the school, and began to take a serious interest in the study - so serious that he established a meteorological station there in 1887. Within five years, he became head of the Meteorological Observatory in Zagreb, where he continued to make detailed observations on the velocities of clouds while teaching geophysics and astronomy at the secondary school. In 1893, he received his doctoral degree from the University of Zagreb for his work on clouds. The same year, he began teaching at the university, a career that lasted until 1918.

By 1901, Mohorovicic had achieved such recognition in the field of meteorology that he was appointed director of the meteorological services for Croatia and Slavonia. In this post, he extended the research of the organization to include seismology, to which he also turned his own interests. He obtained and installed a few seismographs around the country and thus very fortunately recorded a large earthquake in the Pokuplije region of Croatia, about 40 km southwest of Zagreb, which occurred on October 8, 1909. This earthquake provided the data that led to his famous discovery of the boundary between the earth’s crust and mantle, where the composition and density of the rocks changes so dramatically and abruptly that the velocity of seismic waves traveling across the boundary changes as well, causing them to refract, or bend, the way light refracts when it passes from air into water.

This boundary became known as the Mohorovicic Discontinuity, shortened to the Moho, and it marks the bottom of the earth’s crust. In Croatia, the Moho was measured at about 54 km below the surface, but it ranges from a depth of 5 km below the oceans to as much as 70 km beneath major mountain ranges. Geophysicists today work to measure the exact depth of the Moho at various locations around the world, as the thickness of the crust can help them understand the geologic processes acting in a given area.

Mohorovicic continued to make important contributions to seismology into the 1920’s, developing mathematical expressions to describe the travel of seismic waves, and building instruments to record earthquakes. Andrija Mohorovicic died in 1936, a month short of his 80^th birthday.

For more information on the Moho, please visit our module on  Earth Structure.

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