SUMMARY: Seismology in Croatia has deep roots that extend well into the 19th century. It developed at first almost exclusively as the result of efforts being made by Andrija Mohorovičić. In later years, the number of active seismologists varied, but never exceeded twelve, who were all affiliated with the Geophysical Institute in Zagreb. This small seismological community bore the responsibility to maintain high standards in seismological research and education, and to keep Croatian seismology recognizable in the world of geophysical science. Early Croatian seismology developed under strong influence of world-wide, especially European, achievements in that field. It was also strongly affected by the occurrence of several strong earthquakes on its territory, most notably the Zagreb earthquake of 1880, and the Pokupsko one of 1909. Studying the seismograms of the Pokupsko earthquake, Mohorovičić was able to prove the existence of the boundary layer – the Mohorovičić discontinuity – between the Earth's crust and the mantle. Mohorovičić's other contributions are also important (Mohorovičić's epicentrals, Mohoro-vičić's seismograph, Mohorovičić's travel-time curves (hodochrones)). After Mohorovičić retired the seismological research lost most of it's momentum, and for 20 years no seismological papers were published. After the World War II the Geophysical Institute is incorporated into the Faculty of Science and Mathematics of the Zagreb University, and the seismologists became more active. Most of the efforts are dedicated to construction of local travel-time curves based on empirical data, and to the problems related to locations of the earthquakes. In the second half of the century, especially in the last three decades, the scientific interest of Croatian seismologists broadens, and international cooperation is intensified. The research topics are no longer limited to investigations of local seismicity, new techniques and methods are introduced, and instrumentation is substantially improved. The most important subjects of study include improvement of earthquake location techniques, revision and updating of the Croatian Earthquake Catalog, quantification of earthquakes (especially Ms), seismotectonical considerations, historical seismology, determination of velocity distribution including anisotropy studies, attenuation of seismic waves (coda Q), earthquake statistics and prediction (CN algorithm), earthquake hazard and risk employing novel approaches to the subjects, and seismic zonation of Croatia.

Seismology in Croatia has deep roots that extend well into the 19th century. It developed at first almost exclusively as the result of efforts being made by Andrija Mohorovičić. In later years, the number of active seismologists varied, but never exceeded twelve, who were all affiliated with the Geophysical Institute in Zagreb. This small seismological community bore the responsibility to maintain high standards in seismological research and education, and to keep Croatian seismology recognizable in the world of geophysical science.

One of such events was the great Zagreb earthquake of 9 November 1880, with the hypocenter below the Zagreb Mt., after which the sporadic earthquake research efforts evolve into systematic ones. Immediately after the earthquake, the Yugoslav Academy of Science and Arts in Zagreb establishes the "Committee for observation of earth-quake-related phenomena" with the task to study Croatian earthquakes and methodically collect all related data. In the first volume of its Papers, the Academy publishes the extensive report on the Zagreb earthquake (Torbar, 1882), where the phenomena related to that event are not only described, but also explained. The Academy's Committee later also collected and published all available information on Croatian earthquakes from the period 361–1906 which was used as solid basis for scientific study of the natural phenomenon (Kišpatić 1895, Kišpatić 1907).
In January 1892, Andrija Mohorovičić becomes the director of the Meteorological Observatory in Zagreb (later the Geophysical Institute). In 1893 he initiates systematic collection of earthquake related data, which are regularly published since 1906 in the Seismic reports. Following the strong earthquake of 17 December 1901 near Zagreb, Kišpatić and Mohorovičić made every effort to have a seismic station set up in Zagreb. After the first seismoscopes and seismographs were obtained in the beginning of the 20th century, Mohorovičić realized the importance of accurate time-keeping. He therefore initiated regular observation of times of passage of stars through the local meridian, thus founding the Croatian time service.
The strong earthquake in the Kupa Valley (Pokupsko) on 8 October 1909 entered into history of seismology. Studying its seismograms, Mohorovičić was able to prove the existence of the boundary layer – Mohorovičić discontinuity – between the Earth's crust and the mantle (Mohorovičić, 1910), thus establishing himself as one of the greatest and most renowned Croatian scientists of all times. His studies of the Pokupsko earthquake also yielded the procedure of unique location of earthquake focus and the analytical expression for the increase of elastic wave velocity with depth (Mohorovičić's law). Based on theoretical considerations, Mohorovičić also predicted the shape of deep-focus earthquakes, which was confirmed by Wadati in 1928. Mohorovičić's other contributions are also important (Mohorovičić's epicentrals, Mohoro-vičić's seismograph, Mohorovičić's travel-time curves (hodochrones)). He worked actively until the late 1920-ies.
After Mohorovičić retired, the seismological research lost most of it's momentum, and for 20 years no seismological papers were published. The reasons for that should be searched for in the political turmoil and in the changes the country went through in those times. The seismographic observations in Zagreb, however, continued with practically no interruptions, and all seismograms are still available in the archive of the Geophysical Institute. After the World War II the Geophysical Institute is incorporated into the Faculty of Science and Mathematics of the Zagreb University, and the seismologists became more active. Most of the efforts are dedicated to construction of local travel-time curves based on empirical data, and to the problems related to locations of the earthquakes.

Following the damaging Makarska-Hvar earthquake of 1962, seismological research is intensified again. Croatian seismologists publish papers in cooperation with foreign colleagues that deal with location of earthquakes, seismicity of Central Adriatic and other Croatian regions, and analyze the problem of optimum distribution of seismic stations. The first seismic maps of Croatian territory are being published, as well as studies dealing with microzonation of cities and larger urban areas. Inclusion of Yugoslavia (and thus also Croatia) into the UNESCO project "Survey of the seismicity of the Balkan region" in 1970-ies provided Croatian seismologists with the opportunity to widen their scientific interest, and actively participate in international teams studying all aspects of seismicity of the countries in SE Europe. At that time instrumentation is significantly modernized (see below) and new seismic stations are opened. One of the main outcomes of that project is most certainly the Catalog of earthquakes and the collection of macroseismic maps from the investigated area, which served as basis for later compilation of Croatian earthquake catalog and the Croatian macroseismic digital database. In this period papers are also published on the seismotectonics of the Croatian region. The relation between magnitude, intensity and maximal acceleration is also investigated. The increased seismic activity of the Friuli region (N Italy) in 1976, as well as the great Montenegro earthquake of 1979 prompted macroseismic and neotectonic studies, analyses of the aftershock sequences, investigations of temporal and spatial variation of seismicity, research involving estimation of parameters of seismic forces, strong-motion analyses, etc.
The Croatian seismology in the last two decades of the 20th century is characterized by notable broadening of research subjects, which was aided by the advent of affordable computers. In 1985 the Croatian Seismological Survey is founded, that is responsible for collection, analysis and archiving of all kinds of seismological data. Topics related to seismicity research and microzonation studies are now accompanied by subjects of the more general nature – theoretical considerations of earthquake statistics, analyses of surface-wave dispersion, numerical algorithms for the earth-quake location... Seismological methods are used to infer geological and elastic properties of the crust and uppermost mantle of the Croatian territory, including anisotropy and vertical as well as lateral distribution of seismic velocities and coda-Q measurements. In the field of earthquake quantification, it has been shown that standard calibrating functions for Ms yield distance-dependent magnitudes, the depth correction for Ms is proposed, while local magnitudes were related to maximum observed intensities and strong-motion records. It has been shown that anomalous seismicity existed prior to several significant earthquakes in Croatia, and the CN algorithm for intermediate term earthquake prediction was successfully applied to the Dinarides area. Fault-plane solution analyses provided the insight into the properties of the tectonic stress field. Historical seismograms (e.g. Vicentini record of the San Francisco event of 1906 from Zagreb) were studied in some detail, and modern methods were used to infer the fine details of the magnification curves of the old mechanical seismographs. Based on the seismic zoning of the Croatian territory, seismic hazard elements were estimated also by deterministic modeling using synthetic accelerograms.
Croatian seismologists actively participated in a number of bilateral and multilateral international seismological projects.

The first seismological station on Croatian territory was opened in Pula in 1900, equipped at first with a Vicentini, and afterwards also with Conrad and Wiechert seismographs. The station was closed in 1918, after the collapse of the Austro-Hungarian empire, and it is not known what happened to the instruments and the seismogram archive. Besides Pula, the following stations also operated in the first half of the 20th century: Rijeka (1901–1918, seismograph  Vicentini), Sinj (1914–1924, seismograph Conrad), Šibenik (1926–1940, seismograph Conrad, temporary operation), Dubrovnik (1927–1929, seismograph Conrad), Zagreb (1906–).
The Zagreb (ZAG) seismological station was founded when the first seismological instrument – the Agamemnone electrical seismoscope – was acquired by the Academy in 1901. At the conference of directors of meteorological institutes, held in Innsbruck in 1905, Mohorovičić asked Dr. Konkoly from Budapest to lend him one of the Vicentini seismographs, which was installed in basement of the Geophysical Institute in Zagreb on April 6, 1906. The instrument was operational for 18 years. Unsatisfied with the instrument's performance, Mohorovičić purchased the Wiechert seismograph with a mass of 80 kg that recorded horizontal ground motion and installed it in Zagreb in 1908. Soon afterwards, in 1909, a new horizontal instrument was obtained with a mass of 1000 kg. The vertical Wiechert instrument (1200 kg) was acquired in 1932. These seismographs operated with almost no interruptions until 1983 when they were moved to the Institute’s new location on Horvatovac, Zagreb. These seismographs were restored and are now in an operating condition.
Electromagnetic seismographs (Sprengnether, SKM-3, Vegik) were obtained in early 1970-ies, in the framework of the UNESCO project "Survey of the seismicity of the Balkan region". At that time, the strong-motion network was installed too. The new instruments were used to open new permanent stations Puntijarka (PTJ, 1972) and Hvar (HVAR, 1973), and later also Dubrovnik (DBK, 1986) and Rijeka (RIY, 1988). In addition to permanent stations, a number of temporary ones were installed all over Croatian territory, some of which are still active.
The first 16-bit digital instrument was installed in Zagreb in 1989. The modern Croatian seismic network will be based on seven broad-band seismometers with 24-bit digitizers, which were obtained in 1999.

Seismology is taught at the Department of Geophysics of the Faculty of Science and Mathematics, University of Zagreb, at both the undergraduate and postgraduate levels. Students wishing to obtain seismological education study physics for the initial two years, general geophysics (including courses in meteorology, oceanography, seismology and other geophysical disciplines) during their third study year, while more specialized seismological subjects are parts of the curriculum of the fourth year. The undergraduate study is concluded by defense of the B. Sc. thesis. Good students normally continue their education at the postgraduate study, which ends with a M. Sc. or a Ph. D. degree.