# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 4 Geological Investigations  
Based on the shallow focal depth and QMTs that showed clear strike-slip focal mechanisms [8], surface failures were expected after the mainshock [3,10]. Immediate reports in the media enabled quick online investigation of surface failures and linear infrastructure damage that occurred along an approximately 30 km-long section of the sinistral NE-SW striking Sisak-Petrinja-Glina-Topusko Fault. A quick field inspection revealed that fresh fault planes in the outcrops on the Hrastovicka gora (Figure 5) appeared mostly along the longitudinal NW-SE-striking (Dinaric strike) Pokupsko-Kostajnica-Banja Luka Fault and showed clear dextral coseismic strike-slip displacements (Figures 5 and 6). A time-lapse analysis of the seismic sequence [11] showed that an approximately 20 km-long section of the Pokupsko Fault was (re)activated. The two subvertical, mutually perpendicular faults intersected near the epicentres. The historically important Pokupsko earthquake occurred in the vicinity [7] and was used by the famous Croatian geophysicist [PERSON] to discover the Mohorovic discontinuity. According to preliminary geological analyses of the official geological maps [12] and numerous field data that were published in the media, field prospecting results, available seismological data and preliminary geomorphological analyses of orthophoto images [13], the earthquake sequence that started in the Petrinja epicentral area with the M5 foreshock on 28 December 2020 is related to the active crustal fault system in the broader area of Petrinja (Figure 5). Due to the large amount of released energy during the movement of fault blocks and a relatively shallow focal depth, ruptures in crustal rocks were also manifested on the surface. Figure 4: Quick moment tensor solutions of the M6.2 mainshock on 29 December 2020 at 11:19 UTC (source: CSEM–EMSC [9]). Both left-lateral (sinistral) and right-lateral (dextral) coseismic surface movements were recognized along the fault lines in the wider epicentral area (Figure 6). The intersection of two faults (longitudinal and transverse faults) on the extension of the Dinarides (NW-SE) can be identified (Figures 5, 7 and 8). Both faults consist of multiple faults with horizontal strike-slip block movements. The transverse fault is a less known, nearly aseismic creeping fault, which is referred to as the sinistral Petrinja Fault. The longitudinal fault is the well-known regional dextral Pokupsko Fault. This fault system is a textbook example of deformations that occur in rock due to compressional stress along the generally north-south axis (Figure 4) and the pre-existing fault net (Figure 5). Figure 5: (a) A geological map of the Sisak, Petrinja and Glina areas in Croatia [14] with main faults (thick red lines) of the activated fault system highlighted, the activation of which caused earthquakes on 28 and 29 December 2020 (orange units = Paleogene (Pc, E), yellow units = Neogene (M), light yellow units = Quaternary loess (I) and colluvium (dpr), gray and light blue units = Quaternary alluvium (a, ap), light green units = palustrine): and (b) An interpreted orthophoto image [13] of the activated fault system that was captured a few years before the 2020 earthquakes. Numbers and blue triangles mark the positions of the field photographs that are shown in Figure 6: 1–6A, 2–6B, 3–6C, 4–6D. The inset shows sag ponds along the main creeping sinistral fault line, appearing on an active agricultural land (the orthophoto image is taken in 2018). Figure 6: **(a)** The dextral coseismic displacement along the NW-SE striking subvertical fault plane in an abandoned quarry SW of Zupic (WGS: 45.4229/16.2243): (**b**) Dextral coseismic displacement across a road west of Zupic. Note fresh sub-horizontal striations in the soil (WGS: 45.4239/16.2226).