# Destructive M6.2 Petrinja Earthquake (Croatia) in 2020--Preliminary Multidisciplinary Research
## 9 Conclusions  
The strongest earthquake of the Petrinja (Croatia) 2020 seismic sequence on 29 December 2020 with a magnitude of 6.2 and an estimated intensity of VIII-IX EMS, was preceded the day before by a series of earthquakes that began with the M5 foreshock. It was felt throughout Croatia and in the surrounding countries, and it caused substantial damage and numerous ground failures within local site effects. Based on the shallow focal depth and clear strike-slip focal mechanisms, surface failures were expected after the mainshock. Immediate reports in the media enabled a quick online investigation of surface failures, which showed that linear infrastructure damage occurred along an approximately 30 km long section of the sinistral NE-SW striking Sisak-Petrinja-Glina-Topusko Fault. A quick field inspection showed that fresh fault planes in the bedrock appeared mostly along the longitudinal NW-SE striking (Dinaric strike) Pokupsko-Kostajmica-Banja Luka Fault and showed clear dextral coseismic strike-slip displacements. Simultaneously, an approximately 20 km long section of the Pokupsko Fault was (re)activated. The two subcritical, mutually perpendicular faults intersected near the epicentres. Based on preliminary geological and multiple SAR image analyses, the M6.2 earthquake was the result of the activation of complex fault systems, or the intersection of two main faults (longitudinal and transverse faults) on the strike of the Dinarides (NW-SE). Both faults consist of multiple faults with horizontal block movements (strike-slip). The fault system is a textbook example of major failure in the upper crust along the pre-existing fault net at the critical moment of likely centennial release of generally north-south oriented compressional strain that has accumulated in the crust due to continuous northward movement of the Adriatic microplate (Adria). The dextral Pokupsko-Banja Luka Fault could be one of the main inherited active faults between the crustal segments of Adria, while the sinistral Petrinja Fault could represent a reactivated Mesozoic transform fault that borders crustal fragments and played a role in the formation of the Dinarides fold-and-thrust belt of once greater Adria. According to a survey of damage to buildings that was conducted by structural engineers in the field, approximately 15% of buildings sustained DG 4 or DG 5 damage (very heavy damage or destruction), approximately 20% DG 3 and approximately 65% DG 1 to DG 2 (negligible to slight damage or moderate damage). Buildings of special or outstanding historical or cultural heritage significance or value, such as St. Nikola's church and St. Vid's Roman Catholic parish in the village of Zazina (17 km from Petrinja), which were built in the late 18 th century, mostly collapsed or became unserviceable during the earthquake. According to a preliminary analysis of earthquake ground motion, in the close epicentral area for the rock, the estimated PGA values from selected GMPE relations are 0.29 to 0.33 g, whereas those from RVT models are 0.32 and 0.44 g. These values substantially exceed the PSHA values for 95 and 475 yrp but are comparable with 0.33 g for 1000 yrp, which proves the significance of this earthquake within the known historical seismicity in this area. Based on all available data, we conclude that the resulting PGA\({}_{site}\) values were likely between 0.4 and 0.6 g, depending on the local site characteristics and the distance from the epicentre. The damage was observed to be more severe for two reasons: (a) underestimated PGA\({}_{ROCK}\) values and (b) unsatisfactory building regulations. Therefore, the necessity of the establishment of a new seismic hazard assessment, analysis/correction of construction regulations and geophysical and geological analysis of the area with the objective of assessing local soil conditions (as the main cause of amplification of input seismic excitation) is evident.