The active tectonics of the entire area of Croatia, including the wider epicentral area of Pokupsko-Petrinja-Sisak, is driven by the continuous movement of the Adriatic lithospheric microplate (Adria) to the north. Therefore, in the upper parts of the Earth’s crust,  strain occurs at the contact of the Dinarides and the Pannonian Basin. When the strain reaches a critical level, individual faults from that system are (re)activated. There is a sudden movement of blocks of the crust measuring from several hundred to thousands of cubic kilometers. As a result, a massive amount of energy is released, and earthquakes occur.

According to preliminary geological analyzes of Croatian Geological Survey’s scientists and experts, based on geological maps, numerous field data published in the media, field prospecting, available seismological (https://www.pmf.unizg.hr/geof/seizmoloska_sluzba/izvjesca_o_potresima), and preliminary satellite data (https://www.geof.unizg.hr), the earthquake that hit Petrinja and its surroundings on the 28th of December, 2020 activated the fault system in the underground of the broader area of Sisak, Petrinja, and Glina (figures below).

Geological map of Petrinja and Sisak with highlighted main faults from the activated fault systems that caused earthquakes on 28th and 29^th of December, 2020. Data on magnitudes and epicenters according to https://www.pmf.unizg.hr/geof/seizmoloska_sluzba/izvjesca_o_potresima .

Basic geological map of the Republic of Croatia 1:100.000 – Sheet Sisak

It is evident that there is an intersection of longitudinal and transverse faults to the strike of the Dinarides. Both fault systems consist of multiple faults with horizontal block movement (strike-slip). One is the lesser-known fault, which on this occasion is marked as the sinistral Petrinja fault, and the other is the better-known dextral Pokupsko fault. Such a fault system is a textbook example of deformations that occur in rock due to compression stress along the north-south axis within the existing fault net.

Both fault systems are shown on the Basic Geological Map of the Republic of Croatia 1: 100,000, sheet Sisak, prepared by the Department of Geology – CGS (Pikija, 1987), and on the Basic Geological Map of the Republic of Croatia 1: 300,000 (CGS, 2009). All geological maps covering the territory of the Republic of Croatia are available on request at the Croatian Geological Survey’s website www.hgi-cgs.hr.

Due to a large amount of released energy during the movement of fault blocks, ruptures in the rocks were manifested on the surface of the terrain. That is why in the wider epicentral area along fault lines, we find various surface manifestations of this movement and vibration of the terrain. Those include open cracks and paraclasses, fluid spills, sand volcanoes due to liquefaction in the Kupa and Sava river plains, deformations of the surface of the terrain and linear infrastructural objects, as well as numerous others. Those are unusual phenomena for our area, which can also cause various secondary geohazard events.

More data and details on the activated fault system will be available upon completing field research by the Croatian Geological Survey’s teams.

Dr. sc. Tvrtko Korbar, scientific advisor of the Croatian Geological Survey