Beneath the Balkans there is a dense web that creates devastating earthquakes – that branch almost reaches Croatia!

Earthquake in Croatia, Photo: Tanjug / AP

In recent days, in the context of the recent earthquake that severely damaged Petrinja, the 1969 Banja Luka earthquake is often mentioned alongside human casualties. One of the questions is whether activity on the same fault destroyed Banja Luka more than 50 years ago, and now Petrinja.

The answer to that was given by prof. Dr. Marijana Herak from the Department of Geophysics of the Zagreb Faculty of Sciences.

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“Banja Luka was not destroyed by the relocation of the Pokupski fault, but it is a fault of the same large regional fault structure that runs from Jastrebarsko, through Pokupski and Glina, in Bosnia and Herzegovina and further southeast towards Serbia and Durmitor in Croatia. Usually certain sections of such formations are named after them. – so it is with the Pokupski fault. Then Banja Luka was hit by an earthquake that occurred in the Banja Luka section of the same fault composition “, notes prof. Herak.

Six years ago, prof. Herak and his associates also published an article on the subject of the Banja Luka earthquake in the Journal of Geodynamics. The article presents the results of the analysis of geological, geomorphological and geophysical data that indicate an active regime of compressive stresses in the marginal area of ​​the Dinarides and the Pannonian Basin, which caused an earthquake of magnitude 6.4 in the Banja Luka area on 27 December. October 1969.

This earthquake started with a previous strong shock on the night of October 26 at 2.55 am, and the earthquake continued until 8.53 am, and then the main earthquake occurred at 4.35 pm, similar to what happened in Petrinja, where the main earthquake it was also preceded by a weaker one. .

The epicenter of the Banja Luka earthquake was 20 km below the city. Fifteen people died, 1,117 were slightly and seriously injured, the material damage was enormous and some 86,000 houses were destroyed.

However, based on data from the last 100 years, earthquakes were recorded in the area of ​​that city in 1888, as well as in 1935 and 1981. Of course, the one in 1969 was the strongest and most deadly.

earthquake, Photo: shutterstock

Dr. Josip Stipčević, from the Department of Geophysics at the Faculty of Natural Sciences and Mathematics in Zagreb, recalls how the recent earthquake in the vicinity of Petrinja caused extensive damage and had a relatively unusual sequence of events, where the main earthquake was preceded by strongest earthquakes the day before.

“Following the development of events similar to Banja Luka in 1969, and the fact that all Petrinja earthquakes occurred in the fault zone, which according to some research can be traced to Banja Luka, the question of connection arises. between these earthquake zones. There were many questions, can Petrinja earthquakes cause reactivation of the Zagreb earthquake zone? I think it’s good to look at the connection between the fault zones that extend along the northwest line- southeast, and extend from Banja Luka to the confluence of the Kupa and Sava rivers, “says the geophysicist. from the Faculty of Sciences. According to what is currently known, the failure mechanisms in these areas are of a very different character.

“The Banja Luka earthquake had a compression mechanism in which one wing of the fault rises above the other, and all the earthquakes at Petrinja occurred on a fault that is perpendicular to the surface and where the fault wings slid side by side. As in the Petrinja earthquake, the main one The 1969 earthquake in Banja Luka was preceded by a strong 6.0 magnitude earthquake the day before, although increased seismic activity before the main series It had been going on for several months in the area around the epicenter.

Research has shown that the previous major and strongest earthquakes in that series are in the fault zone, which runs roughly in the northwest-southeast direction and passes from the south side of Kozara, and continues in the direction of Hrvatska Kostajnica .

On the other hand, the fault zone where the Petrinja earthquakes occurred can be practically traced to the front of the western entrance to Hrvatska Kostajnica. Thus, the logical question arises as to whether there is a connection between these fault formations despite their significant difference in seismic mechanisms. More importantly, if there is a connection, these fault systems ‘communicate’, that is. whether it is possible to expect the transfer of tectonic stress from one fault composition to another, “said assistant professor Josip Stipčević, PhD.