# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 4 Geological Investigations
Insets show the fault details with marked displacement and the fault surface projection on the lower hemisphere; (**c**) Sinistral displacement of a bridge NE of Glina (WGS: 45.3697/16.1183); and (**d**) Sinistral displacement on the road W of Petrinja (WGS: 45.4449/16.2590). The locations of the field photographs are presented in Figure 5.  
An orthophoto image (which was captured a few years before the 2020 earthquakes) of a section of the Petrinja Fault that strikes NE-SW and crosses the Kupa Valley (Figure 5) shows typical strike-slip sag ponds, which were created due to probably continuous movements along the creeping Petrinja Fault. The sag ponds are clear evidence of the fault  
Figure 8: Line of sight (LOS) displacements for (**a**) Sentinel-1A ascending orbit T146, which has an amplitude of 74 cm and (**b**) Sentinel-1B descending orbit T124, which has an amplitude of 53 cm. Figure 7: Interferogram of the (**a**) Sentinel-1A ascending orbit T146 and (**b**) Sentinel 1-B descending orbit T124. Each colour fringe corresponds to 2.8 cm of ground displacement (corresponding to a displacement of half of a wavelength of 5.6 cm, since the origin of the data is C-band radar). activity (creeping) during years before the 2020 earthquake [13]. Interestingly, although the Pokupsko Fault has clearer geomorphological expression, the coseismic block movements were likely larger along the Petrinja Fault (Figures 7 and 8). Seismological data clearly show that the Pokupsko Fault was/is characterized by stronger seismicity, while the Petrinja Fault is nearly aseismic (Figure 1b). Nevertheless, foreshocks could occur along the Petrinja Fault, while most of the aftershocks were evidently related to the 20 km-long activated section of the Pokupsko Fault. While the former is part of the probably deep crustal Sisak-Petrinja-Glina-Topusko Fault line, the latter has an even longer and clearer geomorphological and geological expression and could be defined as the Pokupsko-Kostajnica-Banja Luka Fault line (Figure 1a). Nevertheless, the mainshock was located strictly on the Petrinja Fault line (surface projection). Interestingly, the hypocentre of the mainshock was twice as shallow (6 km) as the strongest (first) foreshock (12 km). Furthermore, thermal and mineral water and hydrogen sulfide (H\({}_{2}\)S) springs occur along Pokupsko fault line [14]. The Pokupsko Fault produced most of its seismicity during the 2020 sequence along an approximately 20 km-long section characterized probably by the transgressional (positive flower) structure of the Hrastovicka gora (Figure 5). Similar but broader structures could be recognized on the NW extension of the fault (Vukomericke gorice), while transgressional structures (pull-apart or negative flower) could be recognized on the geological maps on the SE extension of the fault in the Mecencani and Kukuruzari areas [15] The later area was characterized by extreme secondary postseismic near-surface effects, namely suffusion and opening of almost a hundred sinkholes during the first months after the first foreshock [16]. This phenomenon could be related to sulfuric acid speleogenesis, since the water table is close to the surface in the area and close to contact with the underlying Miocene limestones and the late Quaternary alluvial cover that was deposited by the Sunja River [15]. Thus, we suppose that after the 2020 event, there was increased circulation of H\({}_{2}\)S along the fault line. H\({}_{2}\)S could reacted with oxygen at the water table and above, and caused increased corrosion of the Miocene limestones [17]. In combination with possible coseismic centimeterblock movements in the Mecencani pull-apart structure, collapses of the karsified limestones led to the suffusion of the overlying alluvium and the soil.