As early as January 2020, the old building of the Zagreb-Grič Observatory was accepted as an EPS Historic Site. The observatory was originally a meteorological station, founded at this site in 1861 and has been continuously operating ever since. In 1906, a seismological station was also established within the observatory. In 1908 Andrija Mohorovičić succeeded in acquiring new and improved seismographic equipment, making the Croatian observatory in Zagreb one of the most advanced in Europe. From the readings of data recorded on a destructive local earthquake the following year, Mohorovičić discovered the boundary between the Earth’s crust and its mantle in 1910. The inauguration of the memorial plaque took place on 23rd September 2022, in the presence of Dr. Luc Bergé, EPS president, members of Croatian Physical Society and Croatian Meteorological and Hydrological Service, representatives from Faculty of Science of University of Zagreb, from Croatian Academy of Science and Arts, from Rudjer Bošković Institute and Institute of Physics, from the City of Zagreb and from the media.

The welcome address on behalf of Croatian Physical Society was given by Prof. emeritus Ivica Picek, who had also made the nomination of the site. He expressed his belief that this recognition enabled to increase the visibility of the intellectual heritage of Mohorovičić, important as a breakthrough in the understanding of our planet.  He recalled that the ceremony had to be moved forward from 2020 due to the COVID-19 pandemic and the subsequent earthquakes that had made serious damage on the Grič 3 site building. A generous support by the Croatian Meteorological and Hydrological Service (DHMZ) has been essential to proceed with the final ceremony event.

Director General of Croatian Meteorological and Hydrological Service, Dr. Branka Ivančan-Picek, recalled the 75^th anniversary of establishment of the Croatian Meteorological and Hydrological Service at this very site. While DHMZ had to be displaced to a new temporary location, Grič 3 building will be restored as Croatian History Museum. Thereby the meteorological measurements will be preserved there, and Mohorovičić’s memorial office will be kept within the museum.  

President of Croatian Academy of Science and Arts, academician Velimir Neidhardt, noticed that the recent local earthquakes brought to light the importance of the criteria and rules which Mohorovičić introduced for building in seismic active regions. In this way Mohorovičić’s heritage became a vivid part of world’s science and our everyday lives.

President of EPS, Dr. Luc Bergé, recalled a memorable scientific biography of Andrija Mohorovičić. After introducing the public into the EPS projects and activities, he stressed that the Grič 3 site would be remembered as the first Croatian Historic Site of EPS.

On behalf of the Major of Zagreb, Ms Madeleine Wolf expressed a satisfaction that in year which marked the 165^th anniversary of Mohorovičić’s birth, besides installing Mohorovičić’s monument at the same location, now the EPS’s plaque placed him on the map of the major world discoveries.

After the unveiling ceremony, a lecture was organised in the building of National Hall of Croatian Academy of Science and Arts. Academician Mirko Orlić gave a talk on the life and work of Andrija Mohorovičić (Volosko, 1857 – Zagreb, 1936).  After study of mathematics and physics in Prague (1875 – 1879), for next twelve years Mohorovičić taught at high schools in Zagreb, Osijek and Bakar, where he established a meteorological station in 1887.  In 1891 he became a professor at the Main Technical School at Zagreb, and he was named director of the meteorological observatory there in 892. From 1901 Mohorovičić’s major scientific interest was seismology, spurred from 1906 by installation of a modern seismological station. Turning point in his career, that led to one of the important breakthroughs in seismology, was an earthquake on October 8, 1909, with epicentre near Pokupsko, 39 km south of Zagreb. Seismographs of that and subsequent earthquakes were shared to Mohorovičić from 30 seismographic stations all around Europe. The most distant one was 2,405 km away from the epicentre (Tbilisi, Georgia). From the seismographs Mohorovičić created ‘hodochrones’, a set of curves that graphically represent time passed from the earthquake event versus station distance from the epicentre. Analysis of ‘hodochrones’ showed that the earthquake onset times for stations being between 300 and 720 km away from the epicentre are represented by two longitudinal and two transverse waves rather than a single longitudinal and a single transverse wave. The stations being closer than 300 km and farther from 720 km recorded a single pair of longitudinal and transverse waves. This observation, published in 1910 in “Annual Report of Zagreb Meteorological Observatory for 1909”, has been the first report of such phenomenon at all. In the same publication, Mohorovičić set hypothesis of earthquake wave propagation within the Earth’s interior that had excellently explained observed data. Until the depth of 50 km, wave velocity smoothly increases with depth with inverse cubic law. At the depth of 50 km, there is “…a sudden change of material making up the interior of the Earth…” (cite from the “Annual Report…”). The “change of material” modern seismology recognizes as ‘Mohorovičić discontinuity’.

During the lecture we learned that all the important things that happened to Mohorovičić occurred on Fridays – including the present celebration. After the lecture, a reception arranged by DHMZ for some 60 guests was hosted in the building of National Hall of Croatian Academy of Science and Arts to perform and conclude this memorable Friday.

Unveiling of the plaque making Zagreb-Grič Observatory the first EPS Historic Site of Croatia.
From left to right: V. Neidhardt, B. Ivančan-Picek, M. Wolf, L. Bergé and Ivica Picek.